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Nitro's Approach To Water Maintaince

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Well, after losing some posts about balancing water, I decided to organize my thoughts and put them all down in one place. And of course save it on my own system incase anything happens to the forums again. So here we go.

Introduction

There are two main things to consider when maintaining your hot tub water, Water Balance and Sanitation. There are a few different methods to sanitize your hot tub, Bromine and Chlorine being the most popular. However, I will only cover Chlorine, because that's what I know the best, thanks to Richard (Chem Geek). It is based on his famous Dichlor/Bleach method. If you're interested in Bromine, here is an excellent writeup. However, Water Balance is related to all sanitation methods, so this could be helpful to anyone.

I will assume you have a fresh fill of water in your tub. If you don't and you're having problems, or it's close to the time to refill, you're better off just refilling your tub and starting over. Hot tub water should be changed every 3-5 months depending on usage. However, if your tub has serious problems, or you bought a used tub etc., you may want to decontaminate your tub. Hillbilly Hot Tub has laid out the process nicely here.

In order to really maintain your water properly you need to test it. Test strips are ok for getting rough estimates, but IMO will make it more difficult to maintain your tub. It would be difficult for me to keep my tub as well tuned as it is by just using test strips. For a novice, I can see it being really difficult, especially if you have Well Water. Do yourself a favor and get a Drop Test kit such as the Taylor K-2006.

One more note. Before I got my hot tub, I knew nothing about water chemistry. After I started studying hot tub water chemistry, I realized there was a lot more to keeping a hot tub water safe then I anticipated. However, after learning it, I realized it doesn't have to be difficult or time consuming. This may get a little technical, but once you learn it, you'll have no trouble keeping your water balanced/sanitized.

Water Balance

Water can be either Acidic, Balanced or Alkaline. If your water is not balanced, it could cause all kinds of problems. If it's too Acidic it could cause corrosion. If it's too Alkaline it could cause scaling. Both of which will reducing the effectiveness of your sanitizer. If your water is not balanced your water is not safe for you, or your tub. So it's really critical to get this correct.

There are four main parameters to keeping your water balanced. Water Temperature, Calcium Hardness (CH), Total Alkalinity (TA) and Potens Hydrogen (pH). Assuming your water temp is around 100 degrees, we'll just focus on CH, TA and pH. The best way to test these is with a Drop Test kit, like the one mentioned above. IMO Test Strips are just not accurate enough to test these, especially for a beginner.

Calcium Hardness: CH is basically the amount of Calcium in your water. The lower your CH, the higher your TA and pH need to be to have balanced water. The higher your CH, the lower your TA and pH need to be. The first thing you should do is test your CH in your tub. If you haven't filled your tub yet, test your tap water for CH using a drop test kit. If you have a water softener in your house, it will remove most of the Calcium in your water. That means you may get a different reading from your indoor sink then from the outdoor hose. Check both to make sure. If you have really bad well water i.e. lots of metals etc., you may want to fill your tub using the water softener water, then just add Calcium afterward. If your water is city treated water, you may be ok straight from the hose. Also, consider getting a pre-filter that attaches to your hose, and filters out some of the metals.

Here's my recommendation. If your CH is much below 100 ppm, you should add calcium to bring it up to 130-150 range. If your CH is over 100, you can leave it alone and just make a note of what it is. The CH level will be used to determine your TA/pH levels. To raise Calcium you add Calcium Chloride, aka Calcium Increaser. You can find it at most Pool/Spa supply stores. The good news is you only need to add Calcium once per water change, because it will stay relatively the same until your next water change.

Potens Hydrogen: pH is the measure of acidity in your water. It's on a scale of 0 to 14, 7 being neutral. The human eye has a pH of 7.5, so the ideal range in Pools/Spas is 7.4-7.6 with a min of 7.2 and max of 7.8. However, and this is important, just because you have a pH of 7.5, doesn't mean your water is balanced. If your CH is too low, you could be corroding your hot tub heater. If your TA is too high, you could be forming scale in your tub. Both of which will reduce your sanitizer effectiveness. So it is very important to have balanced water along with an ideal pH.

Total Alkalinity: Alkalinity is a pH Buffer. High levels of TA will not allow pH to change from additions of acid or base. However, a high TA requires a low pH to have balanced water. On the other hand, very low levels of TA will allow the pH to change with very little acid. A very low TA level could cause your pH to drop to low levels, very fast with little acid. Also, the lower your TA, the higher your pH needs to be to have balanced water. So as you can see, a very low TA can become very unstable.

However, pH will have a tenancy to rise with aeration (i.e. use of jets and air), more so if your TA is high. Although, as long as you're not adding Acid (or anything with a lower pH) to your tub, the pH will not usually drop. Therefore, in hot tubs the problem is normally pH rise (or Drift), because of all the aeration. So the trick is to get the TA high enough to not create an unstable situation, and low enough to not allow pH to rise too much.

Let me repeat the last sentence, because it's the single most important thing to keeping your water balanced. The key to having balanced water, without pH drift, is having the correct TA level. If you find your pH rises too high (>8.0) after using your tub, your TA is too high, and needs to be lowered. If you find your pH is too low and/or your water is continually acidic, your TA is too low and needs to be raised. By fine tuning your TA, you can get your pH perfectly balanced, that rarely needs adjustment.

SO, what's a good TA then? That depends on your CH. If your CH is around 150 ppm, your TA would need to be 80 ppm for your water to be balanced with an ideal pH of 7.6. How did I come up with those numbers you ask? Easy, I went to The Pool Calculator website, and plugged in a Temp of 100, CH of 150, pH of 7.6 and adjusted TA until the CSI was close to zero. This website allows you to calculate your Saturation Index, based on the four Main parameters, Temp, CH, TA and pH. If CSI is less than zero your acidic, if it's greater than zero your alkaline and of it's close to zero your balanced.

OK, how do we adjust TA then? If TA is too low, you just add Baking Soda to raise it. However, if TA is high, it's little more involved. You'll need Acid (Dry or Muriatic). Depending on how high your TA is will depend on how long it will take you. Plan on it taking around an hour to decrease TA by 100 ppm. So if your TA is 300 ppm, plan it taking around 2-3 hours. First, uncover your tub and turn on all your jets, air, blowers, waterfalls etc. Test your pH. When it's greater than 7.8, add enough acid to bring it down to 7.0. Keep aerating until your pH is 7.8 again (about 30 min), then add more acid and repeat. Every time you add acid you're lowering your pH and TA. When you get your TA tuned perfectly, your pH will rise to a level (i.e. ~7.6) and stop, then you know you're at your ideal TA level. If your pH is rising too high (>8.0), bring your TA down a little more. If your pH doesn't rise enough from aeration (after an hour or more), you over shot it and need to add a little Baking Soda to raise your TA. After a few days/weeks of monitoring it, you'll get your TA tuned perfectly. You may need to add a little Dry Acid once a week or two, but your water should be well balanced at that point.

I wouldn't go much below 50 ppm TA, because that could become unstable if you add any kind of acid. Also, if you have a very high TA, above 200, you may need to add more acid in the beginning to get your pH down. Remember, TA is a pH buffer, so the higher the TA, the more acid it takes to bring the pH down. It takes approximately 8 oz total of Dry Acid to bring TA down 100 ppm in a 350 Gal tub. However, the important thing is not to add too much acid all at once, causing your pH to drop below 7.0. That's why you should divide up the dosages, while aerating constantly. Also, if you lower your TA to 50-60, you will need to raise you CH to compensate.

Lastly, there is one more thing I haven't mentioned that you can add to your tub that will help buffer your pH, Borates. The easiest way to add Borates to your tub is to get a product called Gentle Spa. It is pH balanced, so there is no need to add acid to counter balance it. The ideal amount is ~50 ppm, and only needs to be added once per water change. That will help keep your pH from drifting from aeration. Not to mention, actually make your water feel silkier. I highly recommend it.

BTW, I'm purposely not giving the exact amounts of Calcium, Baking Soda or Acid to use, because The Pool Calculator will calculate that all for you. Just enter your tub size at the top, enter the current level in the left column, then enter your goal amount in the right. It will tell you the amount and what you need to add. For instance, if you enter tub size of 320 gal, a pH of 7.8 in the left column and 7.0 in the right, it calculates .4 oz or 2 tbs of Dry Acid. All you need to balance your water perfectly is Calcium Chloride, Baking Soda, Dry Acid and The Pool Calculator.

Sanitation

I'm only going to discuss Chlorine here, because I don't know enough about Bromine to comment on it. This is based on Chem Geeks Dichlor/Bleach method. I have found this method to be very easy and safe to use. I have yet to have a problem. I don't want to get into pros and cons, because that's better suited for it's own thread. If you don't know his method, this should explain it to you. If you do know it, maybe this will help you execute it better. I'm not going to get too technical. I'm just going to discuss everything that's important to keep your tub sanitized easily. If you want more details, search through Chem Geek's posts. You'll have plenty to read. Disclaimer: I'm not trying to convince you to use this method. I'm just explaining it. Please weigh all options before making a decision.

There are really two main things happening when your pool/tub is being sanitized. Organic waste is being oxidized, and bacteria/viruses are being killed. Chlorine does both, very well. That's why it's the most popular sanitizer in pools and spas. However, in order for chlorine to be effective there needs to be a Free Chlorine (FC) level in your tub at all times. If it drops to zero, bacteria will multiple within hours. So the number one rule with using chlorine as a sanitizer is, never let it drop to Zero for any length of time. Now lets get started on the procedure.

The first thing we are going to use to sanitize our tub is Dichlor. Dichlor is actually two things, Chlorine and Cyanuric Acid (CYA). CYA is a chlorine stabilizer, which slows down dissipation from UV rays (sunlight). It also acts as a buffer to hold chlorine in reserve. Having CYA in your tub will stop the chlorine from dissipating as fast while waiting for something to do. However, CYA also cuts down on the ability for the chlorine to sanitize. So in other words, the higher the CYA level, the longer the FC will wait in the tub, but the less effective the FC is. With no CYA in the tub, using bleach would be too strong and not practical to use. So, there needs to be a happy medium. Luckily Chem Geek has come up with one for us. He suggests a CYA of ~20 ppm. Personally I use a CYA of 30 ppm for reasons I won't get into here. But I think Chem Geek would agree anywhere between 20-30 is fine. Where the problem comes in is when CYA gets too high (above 100) after weeks of using Dichlor. Then your FC needs to be really high to get the same sanitation effectiveness. That's why we switch to bleach after we get our CYA to 20-30. Bleach doesn't add anymore CYA.

So, I'm going to assume our target CYA is 30, but feel free to use 20 and adjust as necessary. I find that it's easier to keep track of how much Dichlor you're adding to the tub and calculate the CYA, rather than testing CYA with the drop test kit until you get a 30 ppm reading. For every 10 ppm of Dichlor you add to your tub, you add 9 ppm CYA. So roughly 34 ppm Dichlor will give you 30 CYA. On a fresh fill, and after your water is balanced by the above method, shock your tub with Dichlor to 10 ppm FC. Then over the course of the next days, depending on bather load, add 24 ppm more FC using Dichlor. You want to keep your FC normally between 3-6 ppm everyday. This means you will be testing your FC EVERYDAY.

For example: On one day you test your FC and it's 1 ppm. You'll then add 5 ppm FC to bring it to 6. The next day you test FC and it's 3. You then add 3 ppm FC. You will do this until you add a total of 34 ppm FC to your tub using Dichlor. After that, you then switch to using bleach (Clorox Regular Unscented). Important: if you're not willing to test your tub everyday, at least for the first month or two, this method is not for you. I'd recommend another sanitation method

When you use your tub (this applies to when you are still using Dichlor or after you switch to bleach), the rule of thumb is, you'll use approximately 7 ppm FC per person per hour in a 350 gal tub. Now this will depend on a lot of things, water temp, cleanliness of the users etc. If it's just two people, after taking showers, using no swimsuits, with the temp at 98 you may only need 2 ppm FC/person/hour. However, if you have 6 people in their clothes, drinking (spilling) beer, with a temp at 102 causing everyone to sweat, you may need 20 ppm FC/person/hour. So this is where you may need to make a judgment call and/or experiment. Remember, the Chlorine has to do two things, Oxidize Waste, and Kill Bacteria. If your tub is being subjected to a lot of waste (sweat, beer etc), your FC will be used up and not be able to kill bacteria. Not good! So I'll give you two scenarios and how you might go about running an experiment. Remember, all this up front work is only necessary in the beginning, until you learn how much FC is being used in your tub. Once you know, it becomes easy to maintain. The rule of thumb is, assuming CYA is 20-30 ppm, FC should normally be between 3-6, with a min of 1 and shock to 12 once a week.

We'll go with the easy one first, two people, showered, no suits etc. (Note: Some people prefer to use the tub with very little FC. That's your choice. I'm just telling you how to determine how much FC you're using in a safe manner). Test your FC and add Chlorine to get to 5 ppm FC. After one hour of soaking test FC again. If you find you have no FC after an hour, next time start with 6 or 7. If you find you still have 3 ppm FC, next time it's safe to start with 3-4 ppm FC. Get it? The trick is to have the lowest amount of FC without ever dropping to zero. If you plan on staying in the tub longer than an hour, either start out with more FC, or add some during the soak. However, you don't want to get into the tub with the FC much above 8, unless of course you like a strong smell of chlorine. I got in once with FC at 10, and it was a little too much for me, and I like the smell of Chlorine.

On the other extreme. Full tub of people drinking, sweating etc. Do the same as above, but start out with more FC, maybe 6-8 ppm. Check the FC in an hour (or even a half). If you have no FC, you need to add some and check more often. If you still have FC, add some, and check again in the same time frame. It's the same as above, but you're just using more FC, more often. However, having the FC drop to zero for a short time with your lover, is not the same as with 4-5 other people, if you get my drift. So it's much more important to keep FC above zero at all times during a Hot Tub Party.

If you find you're having trouble keeping FC in your tub during party's, and/or you smell a foul odor during your soaks, you can add Potassium Monopersulfate (MPS or Non-Chlorine Shock) before (and/or during) your soak to help the Chlorine Oxidize waste. The only thing I would say is, go easy with MPS. Use only the amount needed and not much more. It is acidic and will lower your TA/pH if you use a lot. Start out with 1-2 tbs in addition to your normal FC, and see if that cuts down on the FC usage. Up it to 3-4 tbs, or add another 1-2 tbs during the soak if needed. MPS will help oxidize waste. However, it will not kill bacteria, so you still need FC. Also, you can use MPS as a shock once a week to help oxidize leftover waste, and reduce Combined Chlorine (spent FC).

After a month or two you will learn a routine, and be able to add Chlorine less often (i.e. every 2-3 days). How often you need to add Chlorine will depend on your Chlorine Demand (CD). The lower the CD, the less often you need to add Chlorine. The key is to keep CD as low as possible. HERE is a link that describes Chlorine Demand in detail, how to measure it and how to keep it low.

Wow if you made it this far, you did pretty good. It seems like a lot to learn, but it's really not. I'll summarize it here.

Summary

Water Balance

If your CH is much below 100 ppm, raise it to ~150 ppm. Otherwise, make a note of it.

Adjust your TA to ~80 ppm to start with then fine tune from there. Use Baking Soda to raise it, or Acid/Aeration method to lower it. It takes ~8 oz Dry Acid to lower TA by 100 ppm in a 350 Gal Tub. Don't add all at once! If you lower your TA, you may need to raise your CH to compensate.

pH should be between 7.2-7.8. Aerate to Raise pH. Acid to lower it.

If pH creeps up too much (>8.0), lower TA. If pH is to low, and doesn't raise enough by aeration, raise TA.

Add ~50 ppm Borate. (Borox/Acid or Gentle Spa)

Calcium, Baking Soda, Dry Acid and Borates is all you need to balance your water.

Use The Pool Calculator to calculate exact amounts to add.

Sanitation

On a fresh fill use Dichlor until you get to 20-30 ppm CYA. (10 ppm FC = 9 ppm CYA)

Then switch to Clorox Unscented 6% bleach.

Never let FC drop to Zero for any length of time. Keep it between 3-6 ppm normally, min of 1 ppm, and shock to 12 ppm once a week.

Use MPS if needed before hot tub party's and/or once a week to help oxidation. Don't use more than needed, because it's acidic and may lower your TA/pH.

Check your Chlorine Demand, and keep it low.

Dichlor, Bleach and MPS is all you need to keep your tub sanitized.

Use The Pool Calculator to calculate exact amounts to add.

Lastly, get a drop test kit such as the Taylor K-2006. Also get FC test strips. They come in handy for a quick FC estimate, during party's etc. The only two things you need to check regularly are FC and pH. Check them everyday and after soaks. Adjust TA as needed. If you have your water well balanced, you'll rarely need to adjust your TA. You'll just be adding chlorine everyday, and after soaks. Shock once a week, and/or after high bather loads. Use MPS if you have party's, and/or don't want to use as much chlorine. Keep Chlorine Demand low, and you can add Chlorine every 2-3 days. Easy as can be, and your tub is always ready.

I hope that helps you to understand Water Maintenance a little better. It helped me just to write. If I missed anything important please let me know, I'll be happy to add it.

Happy tubbing! :)

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An excellent writeup! Just to note: the 7 ppm FC per person per hour guideline is for a 350 gallon tub. The other way to say it is that the guideline is 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach (e.g. Clorox regular unscented) or 7 teaspoons of MPS non-chlorine shock per person per hour -- that quantity of sanitizer is independent of tub size. This guideline is conservative for fairly hot tubs (near 104F) and as you point out the actual amount depends a lot on the cleanliness of the bathers, how much they sweat, what else they dump into the tub, etc.

I would also add that with the Dichlor then bleach method, even after adjusting TA, some may find the pH to rise too much, especially if they have an ozonator. The Borates will help slow the pH rise, but the same amount of acid will be needed to lower the pH back down to the desired range.

Richard

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Well, after losing some posts about balancing water, I decided to organize my thoughts and put them all down in one place. And of course save it on my own system incase anything happens to the forums again. So here we go.

Introduction

There are two main things to consider when maintaining your hot tub water, Water Balance and Sanitation. There are a few different methods to sanitize your hot tub, Bromine and Chlorine being the most popular. However, I will only cover Chlorine, because that's what I know the best, thanks to Richard (Chem Geek). It is based on his famous Dichlor/Bleach method. However, Water Balance is related to all sanitation methods, so it could be helpful to anyone.

I will assume you have a fresh fill of water in your tub. If you don't and you're having problems, or it's close to the time to refill, you're better off just refilling your tub and starting over. Hot tub water should be changed every 3-5 months depending on usage. However, if your tub has serious problems, or you bought a used tub etc., you may want to decontaminate your tub. Hillbilly Hot Tub has laid out the process nicely here.

In order to really maintain your water properly you need to test it. Test strips are ok for getting rough estimates, but IMO will make it more difficult to maintain your tub. It would be difficult for me to keep my tub as well tuned as it is by just using test strips. For a novice, I can see it being really difficult, especially if you have Well Water. Do yourself a favor and get a Drop Test kit such as the Taylor K-2006.

One more note. Before I got my hot tub, I knew nothing about water chemistry. After I started studying hot tub water chemistry, I realized there was a lot more to keeping a hot tub water safe then I anticipated. However, after learning it, I realized it doesn't have to be difficult or time consuming. This may get a little technical, but once you learn it, you'll have no trouble keeping your water balanced/sanitized.

Water Balance

Water can be either Acidic, Balanced or Alkaline. If your water is not balanced, it could cause all kinds of problems. If it's too Acidic it could cause corrosion. If it's too Alkaline it could cause scaling. Both of which will reducing the effectiveness of your sanitizer. If your water is not balanced your water is not safe for you, or your tub. So it's really critical to get this correct.

There are four main parameters to keeping your water balanced. Water Temperature, Calcium Hardness (CH), Total Alkalinity (TA) and Potens Hydrogen (pH). Assuming your water temp is around 100 degrees, we'll just focus on CH, TA and pH. The best way to test these is with a Drop Test kit, like the one mentioned above. IMO Test Strips are just not accurate enough to test these, especially for a beginner.

Calcium Hardness: CH is basically the amount of Calcium in your water. The lower your CH, the higher your TA and pH need to be to have balanced water. The higher your CH, the lower your TA and pH need to be. The first thing you should do is test your CH in your tub. If you haven't filled your tub yet, test your tap water for CH using a drop test kit. If you have a water softener in your house, it will remove most of the Calcium in your water. That means you may get a different reading from your indoor sink then from the outdoor hose. Check both to make sure. If you have really bad well water i.e. lots of metals etc., you may want to fill your tub using the water softener water, then just add Calcium afterward. If your water is city treated water, you may be ok straight from the hose. Also, consider getting a pre-filter that attaches to your hose, and filters out some of the metals.

Here's my recommendation. If your CH is much below 100 ppm, you should add calcium to bring it up to 130-150 range. If your CH is over 100, you can leave it alone and just make a note of what it is. The CH level will be used to determine your TA/pH levels. To raise Calcium you add Calcium Chloride, aka Calcium Increaser. You can find it at most Pool/Spa supply stores. The good news is you only need to add Calcium once per water change, because it will stay relatively the same until your next water change.

Potens Hydrogen: pH is the measure of acidity in your water. It's on a scale of 0 to 14, 7 being neutral. The human eye has a pH of 7.5, so the ideal range in Pools/Spas is 7.4-7.6 with a min of 7.2 and max of 7.8. However, and this is important, just because you have a pH of 7.5, doesn't mean your water is balanced. If your CH is too low, you could be corroding your hot tub heater. If your TA is too high, you could be forming scale in your tub. Both of which will reduce your sanitizer effectiveness. So it is very important to have balanced water along with an ideal pH.

Total Alkalinity: Alkalinity is a pH Buffer. High levels of TA will not allow pH to change from additions of acid or base. However, a high TA requires a low pH to have balanced water. On the other hand, very low levels of TA will allow the pH to change with very little acid. A very low TA level could cause your pH to drop to low levels, very fast with little acid. Also, the lower your TA, the higher your pH needs to be to have balanced water. So as you can see, a very low TA can become very unstable.

However, pH will have a tenancy to rise with aeration (i.e. use of jets and air), more so if your TA is high. Although, as long as you're not adding Acid (or anything with a lower pH) to your tub, the pH will not usually drop. Therefore, in hot tubs the problem is normally pH rise (or Drift), because of all the aeration. So the trick is to get the TA high enough to not create an unstable situation, and low enough to not allow pH to rise too much.

Let me repeat the last sentence, because it's the single most important thing to keeping your water balanced. The key to having balanced water, without pH drift, is having the correct TA level. If you find your pH rises too high (>8.0) after using your tub, your TA is too high, and needs to be lowered. If you find your pH is too low and/or your water is continually acidic, your TA is too low and needs to be raised. By fine tuning your TA, you can get your pH perfectly balanced, that rarely needs adjustment.

SO, what's a good TA then? That depends on your CH. If your CH is around 150 ppm, your TA would need to be 80 ppm for your water to be balanced with an ideal pH of 7.6. How did I come up with those numbers you ask? Easy, I went to The Pool Calculator website, and plugged in a Temp of 100, CH of 150, pH of 7.6 and adjusted TA until the CSI was close to zero. This website allows you to calculate your Saturation Index, based on the four Main parameters, Temp, CH, TA and pH. If CSI is less than zero your acidic, if it's greater than zero your alkaline and of it's close to zero your balanced.

OK, how do we adjust TA then? If TA is too low, you just add Baking Soda to raise it. However, if TA is high, it's little more involved. You'll need Acid (Dry or Muriatic). Depending on how high your TA is will depend on how long it will take you. Plan on it taking around an hour to decrease TA by 100 ppm. So if your TA is 300 ppm, plan it taking around 2-3 hours. First, uncover your tub and turn on all your jets, air, blowers, waterfalls etc. Test your pH. When it's greater than 7.8, add enough acid to bring it down to 7.0. Keep aerating until your pH is 7.8 again, then add more acid and repeat. Everytime you add acid you're lowering your pH and TA. When you get your TA tuned perfectly, your pH will rise to a level (i.e. ~7.6) and stop, then you know you're at your ideal TA level. If your pH is rising too high (>8.0), bring your TA down a little more. If your pH doesn't rise enough from aeration (after an hour or more), you over shot it and need to add a little Baking Soda to raise your TA. After a few days/weeks of monitoring it, you'll get your TA tuned perfectly. You may need to add a little Dry Acid once a week or two, but your water should be well balanced at that point.

Lastly, there is one more thing I haven't mentioned that you can add to your tub that will help buffer your pH, Borates. The easiest way to add Borates to your tub is to get a product called Gentle Spa. It is pH balanced, so there is no need to add acid to counter balance it. The ideal amount is ~50 ppm, and only needs to be added once per water change. That will help keep your pH from drifting from aeration. Not to mention, actually make your water feel silkier. I highly recommend it.

BTW, I'm purposely not giving the exact amounts of Calcium, Baking Soda or Acid to use, because The Pool Calculator will calculate that all for you. Just enter your tub size at the top, enter the current level in the left column, then enter your goal amount in the right. It will tell you the amount and what you need to add. For instance, if you enter tub size of 320 gal, a pH of 7.8 in the left column and 7.0 in the right, it calculates .4 oz or 2 tbs of Dry Acid. All you need to balance your water perfectly is Calcium Chloride, Baking Soda, Dry Acid and The Pool Calculator.

Sanitation

I'm only going to discuss Chlorine here, because I don't know enough about Bromine to comment on it. This is based on Chem Geeks Dichlor/Bleach method. I have found this method to be very easy and safe to use. I have yet to have a problem. I don't want to get into pros and cons, because that's better suited for it's own thread. If you don't know his method, this should explain it to you. If you do know it, maybe this will help you execute it better. I'm not going to get too technical. I'm just going to discuss everything that's important to keep your tub sanitized easily. If you want more details, search through Chem Geek's posts. You'll have plenty to read. Disclaimer: I'm not trying to convince you to use this method. I'm just explaining it. Please weigh all options before making a decision.

There are really two main things happening when your pool/tub is being sanitized. Organic waste is being oxidized, and bacteria/viruses are being killed. Chlorine does both, very well. That's why it's the most popular sanitizer in pools and spas. However, in order for chlorine to be effective there needs to be a Free Chlorine (FC) level in your tub at all times. If it drops to zero, bacteria will multiple within hours. So the number one rule with using chlorine as a sanitizer is, never let it drop to Zero for any length of time. Now lets get started on the procedure.

The first thing we are going to use to sanitize our tub is Dichlor. Dichlor is actually two things, Chlorine and Cyanuric Acid (CYA). CYA is a chlorine stabilizer, which slows down dissipation from UV rays (sunlight). It also acts as a buffer to hold chlorine in reserve. Having CYA in your tub will stop the chlorine from dissipating as fast while waiting for something to do. However, CYA also cuts down on the ability for the chlorine to sanitize. So in other words, the higher the CYA level, the longer the FC will wait in the tub, but the less effective the FC is. With no CYA in the tub, using bleach would be too strong and not practical to use. So, there needs to be a happy medium. Luckily Chem Geek has come up with one for us. He suggests a CYA of ~20 ppm. Personally I use a CYA of 30 ppm for reasons I won't get into here. But I think Chem Geek would agree anywhere between 20-30 is fine. Where the problem comes in is when CYA gets too high (above 100) after weeks of using Dichlor. Then your FC needs to be really high to get the same sanitation effectiveness. That's why we switch to bleach after we get our CYA to 20-30. Bleach doesn't add anymore CYA.

So, I'm going to assume our target CYA is 30, but feel free to use 20 and adjust as necessary. I find that it's easier to keep track of how much Dichlor you're adding to the tub and calculate the CYA, rather than testing CYA with the drop test kit until you get a 30 ppm reading. For every 10 ppm of Dichlor you add to your tub, you add 9 ppm CYA. So roughly 34 ppm Dichlor will give you 30 CYA. On a fresh fill, and after your water is balanced by the above method, shock your tub with Dichlor to 10 ppm FC. Then over the course of the next days, depending on bather load, add 24 ppm more FC using Dichlor. You want to keep your FC normally between 3-6 ppm everyday. This means you will be testing your FC EVERYDAY.

For example: On one day you test your FC and it's 1 ppm. You'll then add 5 ppm FC to bring it to 6. The next day you test FC and it's 3. You then add 3 ppm FC. You will do this until you add a total of 34 ppm FC to your tub using Dichlor. After that, you then switch to using bleach (Clorox Regular Unscented). Important: if you're not willing to test your tub everyday, at least for the first month or two, this method is not for you. I'd recommend another sanitation method

When you use your tub (this applies to when you are still using Dichlor or after you switch to bleach), the rule of thumb is, you'll use approximately 7 ppm FC per person per hour. Now this will depend on a lot of things, water temp, cleanliness of the users etc. If it's just two people, after taking showers, using no swimsuits, with the temp at 98 you may only need 2 ppm FC/person/hour. However, if you have 6 people in their clothes, drinking (spilling) beer, with a temp at 102 causing everyone to sweat, you may need 20 ppm FC/person/hour. So this is where you may need to judgment call and/or experiment. Remember, the Chlorine has to do two things, Oxidize Waste, and Kill Bacteria. If your tub is being subjected to a lot of waste (sweat, beer etc), your FC will be used up and not be able to kill bacteria. Not good! So I'll give you two scenarios and how you might go about running an experiment. Remember, all this up front work is only necessary in the beginning, until you learn how much FC is being used in your tub. Once you know, it becomes easy to maintain. The rule of thumb is, assuming CYA is 20-30 ppm, FC should normally be between 3-6, with a min of 1 and shock to 12 once a week.

We'll go with the easy one first, two people, showered, no suits etc. (Note: Some people prefer to use the tub with very little FC. That's your choice. I'm just telling you how to determine how much FC you're using in a safe manner). Test your FC and add Chlorine to get to 5 ppm FC. After one hour of soaking test FC again. If you find you have no FC after an hour, next time start with 6 or 7. If you find you still have 3 ppm FC, next time it's safe to start with 3-4 ppm FC. Get it? The trick is to have the lowest amount of FC without ever dropping to zero. If you plan on staying in the tub longer than an hour, either start out with more FC, or add some during the soak. However, you don't want to get into the tub with the FC much above 8, unless of course you like a strong smell of chlorine. I got in once with FC at 10, and it was a little too much for me, and I like the smell of Chlorine.

On the other extreme. Full tub of people drinking, sweating etc. Do the same as above, but start out with more FC, maybe 6-8 ppm. Check the FC in an hour (or even a half). If you have no FC, you need to add some and check more often. If you still have FC, add some, and check again in the same time frame. It's the same as above, but you're just using more FC, more often. However, having the FC drop to zero for a short time with your lover, is not the same as with 4-5 other people, if you get my drift. So it's much more important to keep FC above zero at all times during a Hot Tub Party.

If you find you're having trouble keeping FC in your tub during party's, and/or you smell a foul odor during your soaks, you can add Potassium Monopersulfate (MPS or Non-Chlorine Shock) before (and/or during) your soak to help the Chlorine Oxidize waste. The only thing I would say is, go easy with MPS. Use only the amount needed and not much more. It is acidic and will lower your TA/pH if you use a lot. Start out with 1-2 tbs in addition to your normal FC, and see if that cuts down on the FC usage. Up it to 3-4 tbs, or add another 1-2 tbs during the soak if needed. MPS will help oxidize waste. However, it will not kill bacteria, so you still need FC. Also, you can use MPS as a shock once a week to help oxidize leftover waste, and reduce Combined Chlorine (spent FC).

Wow if you made it this far, you did pretty good. It seems like a lot to learn, but it's really not. I'll summarize it here.

Summary

Water Balance

If your CH is much below 100 ppm, raise it to ~150 ppm. Otherwise, make a note of it.

Adjust your TA to ~80 ppm to start with then fine tune from there. Use Baking Soda to raise it, or Acid/Aeration method to lower it.

pH should be between 7.2-7.8. Aerate to Raise pH. Acid to lower it.

If pH creeps up too much (>8.0), lower TA. If pH is to low, and doesn't raise enough by aeration, raise TA.

Add ~50 ppm Borate. (Borox/Acid or Gentle Spa)

Calcium, Baking Soda, Dry Acid and Borates is all you need to balance your water.

Use The Pool Calculator to calculate exact amounts to add.

Sanitation

On a fresh fill use Dichlor until you get to 20-30 ppm CYA. (10 ppm FC = 9 ppm CYA)

Then switch to Clorox Unscented 6% bleach.

Never let FC drop to Zero for any length of time. Keep it between 3-6 ppm normally, min of 1 ppm, and shock to 12 ppm once a week.

Use MPS if needed before hot tub party's and/or once a week to help oxidation. Don't use more than needed, because it's acidic and may lower your TA/pH.

Dichlor, Bleach and MPS is all you need to keep your tub sanitized.

Use The Pool Calculator to calculate exact amounts to add.

Lastly, get a drop test kit such as the Taylor K-2006. Also get FC test strips. They come in handy for a quick FC estimate, during party's etc. The only two things you need to check regularly are FC and pH. Check them everyday and after soaks. Adjust TA as needed. If you have your water well balanced, you'll rarely need to adjust your TA. You'll just be adding chlorine everyday, and after soaks. Shock once a week, and/or after high bather loads. Use MPS if you have party's, and/or don't want to use as much chlorine. Easy as can be, and your tub is always ready.

I hope that helps you to understand Water Maintenance a little better. It helped me just to write. If I missed anything important please let me know, I'll be happy to add it.

Happy tubbing! :)

Nitro, thanks so much for the information. I have also saved this, and will also print it off. I am not able to retrieve any of the earlier posts!!! NOT GOOD! My computer shut down while I was on this site and hasn't worked since. Very upsetting for a new chemist!!! LOL . By the way , tomorrow is the official re-fill day!!! yippee

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Wow - this is great information. I'm struggling after 3 months to figure out a water maintenance plan for our new spa. I travel alot for 2-3 day stints so we decided on the Bromine water maintenance technique. Cant seem to go more than 6 weeks before the water gets cloudy or foams and leaves that brown residue on the sides of the spa. Anyone have a quick write up covering best practices for this method?

thanks!

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An excellent writeup!

Thanks Richard.

Just to note: the 7 ppm FC per person per hour guideline is for a 350 gallon tub. The other way to say it is that the guideline is 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach (e.g. Clorox regular unscented) or 7 teaspoons of MPS non-chlorine shock per person per hour -- that quantity of sanitizer is independent of tub size. This guideline is conservative for fairly hot tubs (near 104F) and as you point out the actual amount depends a lot on the cleanliness of the bathers, how much they sweat, what else they dump into the tub, etc.

I added the 350 gal tub to the 7 ppm FC/person/hour guideline. My main point there is that it can vary a lot, depending on other factors. I figure the best thing to do is tell people to test it for themselves, so they get a good idea how much they're using in their tub. I guess what might be confusing is if someone plans on having 4 people soak for 2 hours, it wouldn't be a good idea for them to add 56 ppm FC before they get in. :lol: BTW, how did you come up with that figure?

I would also add that with the Dichlor then bleach method, even after adjusting TA, some may find the pH to rise too much, especially if they have an ozonator. The Borates will help slow the pH rise, but the same amount of acid will be needed to lower the pH back down to the desired range.

I did mention they may have to add acid once a week to keep the pH in range. However, I might be lucky, but I have yet to add acid in over three weeks, and my pH has hovered between 7.6 and 7.8. I have used MPS once a week, which does help keep the pH down. I don't have an ozonator though. I wonder if it is possible to adjust TA perfectly to keep pH in range for a month or longer. To me it seems possible. What do you think?

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Nitro, thanks so much for the information. I have also saved this, and will also print it off. I am not able to retrieve any of the earlier posts!!! NOT GOOD! My computer shut down while I was on this site and hasn't worked since. Very upsetting for a new chemist!!! LOL . By the way , tomorrow is the official re-fill day!!! yippee

I hope I covered everything we talked about. Good luck!

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Wow - this is great information. I'm struggling after 3 months to figure out a water maintenance plan for our new spa. I travel alot for 2-3 day stints so we decided on the Bromine water maintenance technique. Cant seem to go more than 6 weeks before the water gets cloudy or foams and leaves that brown residue on the sides of the spa. Anyone have a quick write up covering best practices for this method?

I don't know enough about Bromine to comment, because I never used it. However here's an excellent writeup about it.

You may also look into the Spa Pilot, which is a Salt Water Chlorine Generator. I never used it, but from what I hear, it will keep your water sanitized for days at a time.

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BTW, how did you come up with that figure?

I wonder if it is possible to adjust TA perfectly to keep pH in range for a month or longer. To me it seems possible. What do you think?

I came up with the 7 ppm FC in 350 gallons on the basis of the high-end chlorine usage from daily soakers on this forum. Though the amounts varied, there seemed to be a fairly consistent high-end limit so that's what I came up with as something conservative as a starting point.

Though one could possibly have a very low TA to maintain pH when there is a lot of aeration, this is dangerous since any additions of acids or bases from whatever external sources would make the pH swing a lot. If one uses borates as an additional pH buffer, then this risk is lowered, though I still wouldn't go much lower than 50 ppm TA in any event. Though Borates are a good pH buffer with capacity against a rise in pH, they are limited in capacity against a drop in pH and the carbonate buffer needs to be there for that -- just in case.

Richard

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Though one could possibly have a very low TA to maintain pH when there is a lot of aeration, this is dangerous since any additions of acids or bases from whatever external sources would make the pH swing a lot. If one uses borates as an additional pH buffer, then this risk is lowered, though I still wouldn't go much lower than 50 ppm TA in any event. Though Borates are a good pH buffer with capacity against a rise in pH, they are limited in capacity against a drop in pH and the carbonate buffer needs to be there for that -- just in case.

My TA has been around 60, and like I said I've had a pretty rock solid pH. Although, as I mentioned I've been using MPS about once a week (3 times) which does seem to help keep the pH in check. I wonder if using MPS once a week alone, is enough to keep the pH down on a long term basis. It's been over three weeks for me.

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This post needs to be stickied to the top of the page!!

[/quote

Calling Chem geek and Nitro!!

Hi there.

Well, today we have re-filled the hot tub. I think that I have followed the correct procedures, but.... I put in 8 TBS of calcuim increaser, plus 2 TBS of Dichlor. I aerated for 17 in. Needless to say the water is still quite cold. I tested. Here are the results: pH 8.0 , CH 175, TA 100, FC 10.0.

My question for now is this- when testing the CH is says the water should turn blue. Does this mean a 'pretty vibrant blue', or more of a 'violet lightish blue?' If so,(violet sort of blue) the CH is perhaps about 150.

Should I adjust the TA to 80, or wait a bit and see what happens as the water heats up?

Also, I went to the store to get SpaGuard Plus, but got the their brand name's version of SpaGuard Plus. There were no ingredients on either bottle (the store actually had an empty bottle of the SpaGuard Plus beside their version) but the instructions and purpose were the same.

When exactly should I add this to the hot tub. I know it should be before I switch to Purox, which will more likely be Friday, so perhaps I should add it on Thursday? I should follow the directions on the bottle?

Hmmm, I guess I have asked too many questions again!!!

By the way, I had to take my lap top in to get fixed. I am a little worried about using this site. It did cause my computer to shut down. I tried to contact the webmaster, but alas, it was not allowed.

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Laura,

You can lower the pH a bit using pH Down. Don't worry about the TA yet, especially since you're still using Dichlor. Just see how the pH goes and add acid to lower it as needed -- over time the TA will slowly drop and if you want to accelerate that process later you can do so (i.e. aerate more and add more acid, as Nitro described for the TA lowering procedure).

You can see what the Calcium Hardness (CH) test should look like here. To prevent a fading endpoint, you can add some drops of titrant to your sample before you add the calcium buffer. Just add the number of drops of titrant you add to your total number of such titrant drops (the 20 drops of buffer never count towards this).

It's not critical when you add the SpaGuard Plus, but I'd add it the day before you switch to Purox.

I don't think this particular website had anything to do with your computer crash, though this site did have some sort of problem where it lost a week's worth of data so perhaps you were online when that happened and it did something to your browser -- that shouldn't have hurt your system, however. There are viruses your computer can get from visiting certain websites that exploit flaws in a browser, but if you keep your system up to date with automatic updates, you should be avoiding most such problems.

Richard

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Laura,

You can lower the pH a bit using pH Down. Don't worry about the TA yet, especially since you're still using Dichlor. Just see how the pH goes and add acid to lower it as needed -- over time the TA will slowly drop and if you want to accelerate that process later you can do so (i.e. aerate more and add more acid, as Nitro described for the TA lowering procedure).

You can see what the Calcium Hardness (CH) test should look like here. To prevent a fading endpoint, you can add some drops of titrant to your sample before you add the calcium buffer. Just add the number of drops of titrant you add to your total number of such titrant drops (the 20 drops of buffer never count towards this).

It's not critical when you add the SpaGuard Plus, but I'd add it the day before you switch to Purox.

I don't think this particular website had anything to do with your computer crash, though this site did have some sort of problem where it lost a week's worth of data so perhaps you were online when that happened and it did something to your browser -- that shouldn't have hurt your system, however. There are viruses your computer can get from visiting certain websites that exploit flaws in a browser, but if you keep your system up to date with automatic updates, you should be avoiding most such problems.

Richard

Thank you Richard!!! I was trying to guess as to what you would say, and I came up with lowering the pH!!! I did not do it though....waited for advice. I will do it now.

I went to the CH link you sent and it looks as though I did indeed do the test correctly.

I was planning on adding 2 more TBS of Dichlor tomorrow, then 2 more TBS on Thurs. then begin using Purox Friday, (CYA should be around 30 by then) but if I need to add more Dichlor when we use the tub tomorrow, this would move my switch date to Purox to possibly Thursday correct?

I know it sounds strange about the lap top, and really doesn't make sense, but it still is maddening as hell!! It happened around 4:45 on Nov.8. Did you notice when this site had the troubles? My computer is still in the shop.

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I was planning on adding 2 more TBS of Dichlor tomorrow, then 2 more TBS on Thurs. then begin using Purox Friday, (CYA should be around 30 by then) but if I need to add more Dichlor when we use the tub tomorrow, this would move my switch date to Purox to possibly Thursday correct?

I know it sounds strange about the lap top, and really doesn't make sense, but it still is maddening as hell!! It happened around 4:45 on Nov.8. Did you notice when this site had the troubles? My computer is still in the shop.

If you add a lot more Dichlor earlier then you could switch to Purox earlier, but I wouldn't worry too much about that -- getting it exactly right isn't a big deal. If you switch on Friday I'm sure it will be fine. So long as your CYA is roughly in the 20-50 ppm range, that should be OK.

It seems that the site went down on Saturday, November 8th and after it came up the next day about one week's worth of posts were gone. They probably had some sort of crash and restored what they had from their last (apparently weekly) backup.

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I was planning on adding 2 more TBS of Dichlor tomorrow, then 2 more TBS on Thurs. then begin using Purox Friday, (CYA should be around 30 by then) but if I need to add more Dichlor when we use the tub tomorrow, this would move my switch date to Purox to possibly Thursday correct?

I know it sounds strange about the lap top, and really doesn't make sense, but it still is maddening as hell!! It happened around 4:45 on Nov.8. Did you notice when this site had the troubles? My computer is still in the shop.

If you add a lot more Dichlor earlier then you could switch to Purox earlier, but I wouldn't worry too much about that -- getting it exactly right isn't a big deal. If you switch on Friday I'm sure it will be fine. So long as your CYA is roughly in the 20-50 ppm range, that should be OK.

It seems that the site went down on Saturday, November 8th and after it came up the next day about one week's worth of posts were gone. They probably had some sort of crash and restored what they had from their last (apparently weekly) backup.

Hi Richard:

I put in one tbs pH down last night.

This morning I added 2 tbs Dichlor

At about 3:00 today I tested... pH was 7.6 or 7.7( it was in between the two colours)

FC 11

TA 100

Should I lower the TA now, and if so , how?

Also, someone is coming over at 7:30 tonight for a hot tub, is the FC too high?

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I was planning on adding 2 more TBS of Dichlor tomorrow, then 2 more TBS on Thurs. then begin using Purox Friday, (CYA should be around 30 by then) but if I need to add more Dichlor when we use the tub tomorrow, this would move my switch date to Purox to possibly Thursday correct?

I know it sounds strange about the lap top, and really doesn't make sense, but it still is maddening as hell!! It happened around 4:45 on Nov.8. Did you notice when this site had the troubles? My computer is still in the shop.

If you add a lot more Dichlor earlier then you could switch to Purox earlier, but I wouldn't worry too much about that -- getting it exactly right isn't a big deal. If you switch on Friday I'm sure it will be fine. So long as your CYA is roughly in the 20-50 ppm range, that should be OK.

It seems that the site went down on Saturday, November 8th and after it came up the next day about one week's worth of posts were gone. They probably had some sort of crash and restored what they had from their last (apparently weekly) backup.

Hi Richard:

I put in one tbs pH down last night.

This morning I added 2 tbs Dichlor

At about 3:00 today I tested... pH was 7.6 or 7.7( it was in between the two colours)

FC 11

TA 100

Should I lower the TA now, and if so , how?

Also, someone is coming over at 7:30 tonight for a hot tub, is the FC too high?

corection...I put on one teaspoon pH down last night NOT one TBS

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Read what Nitro had to say about lowering the TA. The procedure is also described in this post where you first add acid to lower the pH to around 7.0, then aerate the water (usually by running the jets), then add more acid when the pH rises and continue to aerate, then repeat (add acid to lower the pH and TA, aerate which makes the pH rise, etc.). When the TA reaches the desired level, then just aerate to get the pH up to around 7.5.

I estimate that to get from your TA of 100 down to 80 in 380 gallons will take cumulatively about 10 teaspoons of dry acid, obviously not added all at once. By the way, adding one teaspoon of dry acid when your TA is at 100 will only lower your pH from 7.6 to about 7.5. If you want to lower the pH from 7.5 to around 7.0, it will take about 2 tablespoons (6 teaspoons). So it will only take around two cycles of acid addition with aeration to get your TA down to 80.

Though the FC is on the high side, it's OK if you want to use the tub. You've already got some CYA in the water and the FC will drop when you use the tub. It's not any higher in active chlorine than when you used the tub on the first day.

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>> I estimate that to get from your TA of 100 down to 80 in 380 gallons will take cumulatively about 10 teaspoons of dry acid, obviously not added all at once.

Easy there. Small additions and retest. From experience I would probably only use maybe 3 tsp in my 280 gal spa to get the TA from 100 to 80, and that's with 50ppm boron.

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>> I estimate that to get from your TA of 100 down to 80 in 380 gallons will take cumulatively about 10 teaspoons of dry acid, obviously not added all at once.

Easy there. Small additions and retest. From experience I would probably only use maybe 3 tsp in my 280 gal spa to get the TA from 100 to 80, and that's with 50ppm boron.

That's fine advice. Always better to go lower in dosage and retest to see where you are. As far as having Borates, it doesn't change the amount of acid it takes to lower the TA. It only lessens the pH swing when the acid gets added. For a smaller 280 gallon spa such as yours, the estimate is 7 teaspoons of dry acid so if you used less and saw that drop, I suspect it's test rounding since the reading is +/- 10 ppm. The relationship of quantity of acid to TA drop is a fixed one -- it's the effect on pH that is far harder to calculate.

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I estimate that to get from your TA of 100 down to 80 in 380 gallons will take cumulatively about 10 teaspoons of dry acid, obviously not added all at once.

Just curious, how did you come up with 10 tsp? I'm reading the Taylor manual and it says, to decrease TA 20 ppm in a 400 gal tub, use 2.75 oz Dry Acid (93.2% Sodium Bisulfate). That comes to around 15 tsp (5 TBS) in a 380 gal tub.

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>> I estimate that to get from your TA of 100 down to 80 in 380 gallons will take cumulatively about 10 teaspoons of dry acid, obviously not added all at once.

Easy there. Small additions and retest. From experience I would probably only use maybe 3 tsp in my 280 gal spa to get the TA from 100 to 80, and that's with 50ppm boron.

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>> I estimate that to get from your TA of 100 down to 80 in 380 gallons will take cumulatively about 10 teaspoons of dry acid, obviously not added all at once.

Easy there. Small additions and retest. From experience I would probably only use maybe 3 tsp in my 280 gal spa to get the TA from 100 to 80, and that's with 50ppm boron.

That's fine advice. Always better to go lower in dosage and retest to see where you are. As far as having Borates, it doesn't change the amount of acid it takes to lower the TA. It only lessens the pH swing when the acid gets added. For a smaller 280 gallon spa such as yours, the estimate is 7 teaspoons of dry acid so if you used less and saw that drop, I suspect it's test rounding since the reading is +/- 10 ppm. The relationship of quantity of acid to TA drop is a fixed one -- it's the effect on pH that is far harder to calculate.

I find with really high TA, >200, it takes a lot of acid to move the pH even a little. My friends tub had a TA of 430. I added a total of 5 cups of dry acid, 1 cup every 30 mins to get the TA down to 80. I tested pH constantly and it never dropped below 7.0. Kids don't try that at home!

I think that's the reason why so many people have trouble getting their TA/pH down when TA is high. They add the "recommended" amount of acid, but they don't see any movement in pH. Then they start to doubt it's even working. That's what happened to my friend. Even though he kept adding acid (one ounce at a time), he never saw his pH move. He had know idea he needed to add 40 oz total.

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>> I estimate that to get from your TA of 100 down to 80 in 380 gallons will take cumulatively about 10 teaspoons of dry acid, obviously not added all at once.

Easy there. Small additions and retest. From experience I would probably only use maybe 3 tsp in my 280 gal spa to get the TA from 100 to 80, and that's with 50ppm boron.

To decrease TA from 100 to 80 in a 280 gal tub you're looking at a total of ~12 tsp (4 TBS) Dry Acid.

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In a 280 gallon tub, lowering the TA by 20 ppm requires 1.92 ounces weight of dry acid (sodium bisulfate 93.2%). I assume the density of this dry acid is 1.44 g/ml (I measured that from a Spa Down product). This means that it takes 1.28 fluid ounces volume which is 7.7 teaspoons. I think you may be assuming a density of 1 since that would give 11.5 teaspoons.

The Taylor numbers are correct in that it takes 2.75 ounces weight of dry acid to lower the TA by 20 ppm in a 400 gallon tub. Again, it's the density that must be taken into account to convert from weight to volume.

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