Tds And When To Dump The Water?

[OttawaGreg]

All,

Well I am into month four of the Dychlor/Bleach method, although I've started to use some tabs (litho-hypo) lately. Anyhow, the water looks and feels great. pH, TA, are fine but I know my CYA is high.. likely around 100. I brought a sample in for testing and they said TDS was 3500. That's high all right, but I am not getting any foaming and the water is clear. They recommend dumping the water once you're over 2500, which is about 3 months in.

Should I change the water or keep it going? Any opinions?

Also, would an accurate CYA reading help in determining this?

thanks in advance,

Greg

[OttawaGreg]

All,

Well I am into month four of the Dychlor/Bleach method, although I've started to use some tabs (litho-hypo) lately. Anyhow, the water looks and feels great. pH, TA, are fine but I know my CYA is high.. likely around 100. I brought a sample in for testing and they said TDS was 3500. That's high all right, but I am not getting any foaming and the water is clear. They recommend dumping the water once you're over 2500, which is about 3 months in.

Should I change the water or keep it going? Any opinions?

Also, would an accurate CYA reading help in determining this?

thanks in advance,

Greg

That should read - "I am into month five (5) of Dychlor/Bleach method..."

Greg

[quantumchromodynamics]

How did your Cyanuric acid level get so high? The dichlor/bleach method stops adding cyanuric acid at 30 ppm.

What is your calcium level?

[Hillbilly Hot Tub]

Also, what was your fill water at for TDS? Some fill water is high to start.

[chem geek]

As noted in this thread Greg shocked the spa with Dichlor and bleach after the water went south when the chlorine got to zero for too long. The shocking with Dichlor added additional CYA, I suspect.

Greg, as far as TDS goes, it depends on whether that number is correct. The 50 ppm Borates will increase TDS by around 280 ppm. So your initial starting TDS was probably around 700 ppm if the CH was 250 ppm and the TA 60 ppm and Borates 50 ppm and CYA 30 ppm. With one person-hour of soaking per day in 350 gallons, so 7 ppm FC added each day, this would add around 340 ppm salt per month. So even after 5 months I wouldn't expect the TDS to be much above 2400 ppm. The shocking you did probably raised the TDS some, but I suspect that their 3500 ppm is wrong on the high side.

Anyway, if you've been using around 7 ppm FC per day in 350 gallons because you've been soaking for one person-hour each day, then your water is getting near the time for a change, though you could probably go another month. Just keep in mind that getting the TDS above 1500-2000 ppm increases corrosion rates, though salt pools are at 3000 ppm (but this is not as usual for spas). Remember the classic industry rule for a water change:

Water Replacement Interval (WRI) = (1/3) x (Spa Size in U.S. Gallons) / (Number of Persons Per Day)

The assumption is probably for a 20 minute soak so one person-hour every day would result in a WRI in 350 gallons of 39 days. With Dichlor-then-bleach, one can usually go twice as long so that would be around 80 days or nearly 3 months. That is for one person-hour per day in 350 gallons. What is your actual tub size and usage (how many people soaking for how long every day)?

[quantumchromodynamics]

All,

Well I am into month four of the Dychlor/Bleach method, although I've started to use some tabs (litho-hypo) lately.

Lithium hypochlorite tabs? Are you sure? What brand?

Lithium hypochlorite will add to the TDS fairly rapidly due to the relatively high amount of TDS components relative to the actual chlorine supplied.

BioGuard Lithium Hypochlorite

[OttawaGreg]

UPDATE:

Thanks for all the replies. Here is a little history.

On a new fill (October 31, 2009) our water from the tap is;

pH: 7.5, TA: 80, CH: 30, and TDS: 200. Using the usual chemicals I balanced the water to CH: 180, pH: 7.6, and TA: 90.

I was using Dychlor for a few days and took various readings, they were all way too low, zero or just above (and not the 30 I was aiming for). Then, time passed quickly and all of a sudden CYA was 60. Oops.. so I switched to bleach and kept the tub running for a few months. Just bleach with the occasional MPS.

As Chem Geek noted, the water went south early Jan. from neglect. A family emergency had us out of town and the water was a light shade of brown on return. I have brought worse back from the grave so I shocked the *&!^*& out of it and it responded well. Recently we were not going to use the tub for a few days (or a week?) so I dug out the tabs [spaGuard Smart Tabs - Stabilized CH - Trichloro=S-Tetro] and threw them in on a very slow release. I used the tub here and there and topped up with bleach. I had a few itchy feelings so I thought I'd get it tested for TDS.

I was expecting it to be on the high side but not 3500! Her first two readings were 2000, 2500, then two more consecutive readings of 3500. I guess from the incidents I had, the shock treatments, and the tabs use I should expect this.

The thing is, the water is right back to it's usual balanced self and there is no foaming. I treat foaming as the number one sign if a high TDS. So if the water looks great, stays balanced, but has a very high TDS do I still need to dump the water? I gather CYA is likely around 100. I don't have much left in my drop test kit but could probably manage one more CYA test to verify this.

The weather all this week is unseasonably warm so I could easily do a refill on the weekend. I probably should... just looking for some opinions.

Greg

[OttawaGreg]

How did your Cyanuric acid level get so high? The dichlor/bleach method stops adding cyanuric acid at 30 ppm.

What is your calcium level?

On my new water fill (October 31 last year) I accidentally got the CYA to 60 instead of the 30 I was aiming for. It was zero for the longest while then all of a sudden 60. I lost track of time. I guess Dychlor for two weeks instead of one. It went to 100 in January after the water going south and me shocking it back to life.

So, I started high (60) and went up to 100. Likely still around that mark.

CH was brought to 180 and never budged.

We did use our tub quite allot this past few months. I was usually adding about 250-300 ml bleach after each use (say for one hour (1-2 persons) 4-5 nights a week)

Greg

[chem geek]

Greg,

You are getting a lot of use out of your tub so have probably been adding a lot of bleach (about 9 fluid ounces) after each soak. I thought you had an ozonator so am surprised that you need to add as much chlorine as you have been doing -- a functioning ozonator would normally handle some of the bather load letting you use less chlorine when you are frequently using the tub (though it uses more chlorine when not using the tub).

Anyway, with such high bleach or lithium hypochlorite equivalent amounts, the salt and therefore TDS levels could be as high as they say, but the TDS will be mostly salt. Nevertheless, you do have a high bather load in your spa -- 1-2 people for an hour is considered high (unless your spa is unusually large). Good to hear that your water is lasting so well.

You should consider changing the water soon -- not a major rush, but as I noted the high TDS means higher conductivity so weaker metals exposed to that water could corrode faster.

Richard

[simonc]

You should consider changing the water soon -- not a major rush, but as I noted the high TDS means higher conductivity so weaker metals exposed to that water could corrode faster.

Richard

Chem Geek (Richard) -

Thanks for discussing this topic. I have a few questions:

1. How do you measure TDS? I don't think it's part of my Taylor K-2006 test kit.

2. Is it important to test for TDS? TDS seems not to be discussed very much in this forum.

3. Is salt the major component of hot tub TDS for chlorine based systems? If so, a high TDS would imply a lot of salt. So why not just test for salt directly instead of TDS? (I don't know how to test for salt either.)

Just trying to learn ...

- Simon

[quantumchromodynamics]

TDS is a measure of all dissolved solids in the water. Things like sodium, chloride, potassium, calcium, lithium, iron, copper, magnesium, sulfates, nitrates, carbonates, bicarbonates, phosphates, borates, cyanuric acid etc.

The most accurate way to measure TDS is to evaporate all of the water and weigh the leftover solids.

There are drop test kits such as the Taylor K-1764, and there are conductivity testers such as the Taylor K-6542

The K-1766 will test specifically for chloride. Using both the TDS meter and the specific tests for chloride, calcium etc will give you the best idea of the water makeup.

Most Total Dissolved Solids will be salt.

The TDS does affect the CSI, so it should be used in the CSI calculations if it is higher than normal.

Typically, it should be measured once or twice a year just to keep an eye on it but it should not be a problem for most people.

[chem geek]

In my own pool I just use the Taylor K-1766 salt (chloride) test since most of the TDS is salt. I can add an adjustment for the CH, TA and CYA, but it's not a big deal. I only wanted a rough idea of the salt level.

The conductivity meter needs its reading adjusted for the type of contents in the water but since conductivity is part of what accelerates corrosion, just measuring conductivity itself is reasonable.

[quantumchromodynamics]

Greg, the tablets that you are using are trichlor, not lithium hypochlorite, right?

It seems like you were identifying them as lithium hypochlorite.

Trichlor contains a lot of cyanuric acid.

Primarily due to the high cyanuric acid level, I think that you should change out the water.

[simonc]

Chem Geek & QCD -

Thanks for explaining TDS.

I don't think I'm going to worry about it for my hot tub. The solution to high TDS in tubs seems to be ... when in doubt about TDS, just drain the tub. In swimming pools, I can see measuring TDS is more important because draining can be a lot of work and expensive.

I'm not going to get another test kit/device. My wife thinks I play with the chemicals in the test kit too much already. It's sort of fun, actually. I'm old enough to remember "Chemistry Sets" that actually contained things that by today's standards would be too dangerous and way too much fun. No ... I don't want my tub to blow up, but ... it would be cool.

- Simon

[quantumchromodynamics]

The poolcalculator has a place for salt when calculating the CSI.

The Taylor Watergram slide rule uses about 1,000 ppm as the basic TDS level.

For every 1,000 ppm of TDS above 1,000 ppm, you can reduce the CSI by 0.1.

The Taylor Watergram Digital has an entry for TDS.

Watergram calculators

Typically, you would take a water sample to the pool store to have them do a TDS test.

It should be measured once or twice a year just to keep an eye on it but it should not be a problem for most people.

[OttawaGreg]

Greg,

You are getting a lot of use out of your tub so have probably been adding a lot of bleach (about 9 fluid ounces) after each soak. I thought you had an ozonator so am surprised that you need to add as much chlorine as you have been doing -- a functioning ozonator would normally handle some of the bather load letting you use less chlorine when you are frequently using the tub (though it uses more chlorine when not using the tub).

Anyway, with such high bleach or lithium hypochlorite equivalent amounts, the salt and therefore TDS levels could be as high as they say, but the TDS will be mostly salt. Nevertheless, you do have a high bather load in your spa -- 1-2 people for an hour is considered high (unless your spa is unusually large). Good to hear that your water is lasting so well.

You should consider changing the water soon -- not a major rush, but as I noted the high TDS means higher conductivity so weaker metals exposed to that water could corrode faster.

Richard

Richard,

I do have a (functioning) ozinator. I've never tested it but believe it is working. There was a kink in the line at the start but I fixed that and can see the bubbles. I think a source of the salt is likely sweat. When I work out at the gym I am soaking wet as I perspire allot.. more then most people. Sweat is alkaine.. so perhaps this is playing a big part in my problem.

Out tub is a small one at 280 gallons (1050 litres).

I'm glad to have had four+ months on this fill and will change the water this weekend as the weather is warm. This time I'll be sure to stop the Dychlor when CYA is lower.

[OttawaGreg]

Greg, the tablets that you are using are trichlor, not lithium hypochlorite, right?

It seems like you were identifying them as lithium hypochlorite.

Trichlor contains a lot of cyanuric acid.

Primarily due to the high cyanuric acid level, I think that you should change out the water.

Yes, my mistake. I thought they were lithium but when you asked I got the bottle and read it. I don't use them much, actually at all. I just pulled them out this past week as we were not going to be using the tub and I get tired of lifting the lid to add bleach when I'm not going in. After having the water go bad I am more nervous now, and as such have been maintaining a higher level of residual CL.

Yup, a water change is scheduled for this weekend. It will be a very warm 10 deg. Celsius!

[chem geek]

Well it's a near miracle you've been able to go as long as you have without changing the water given your bather load and smaller tub. There's no way you would have gone that long with Dichlor-only. You would have been adding over 200 ppm CYA each month in that case.

Yes, sweating more would result in a higher bather load and need more chlorine. Even so, it just seems that your ozonator either isn't working or is awfully weak. Bubbles don't mean it's working -- it just mean that air is getting pumped through the system, but the corona discharge (or possibly UV) part of the system may be defective. If you can capture any of those bubbles in an jar filled with water that you invert, then you can remove the jar and smell those bubbles to see if you can detect the typical ozone "electrical smell".

[OttawaGreg]

Well it's a near miracle you've been able to go as long as you have without changing the water given your bather load and smaller tub. There's no way you would have gone that long with Dichlor-only. You would have been adding over 200 ppm CYA each month in that case.

Yes, sweating more would result in a higher bather load and need more chlorine. Even so, it just seems that your ozonator either isn't working or is awfully weak. Bubbles don't mean it's working -- it just mean that air is getting pumped through the system, but the corona discharge (or possibly UV) part of the system may be defective. If you can capture any of those bubbles in an jar filled with water that you invert, then you can remove the jar and smell those bubbles to see if you can detect the typical ozone "electrical smell".

Another to-do for this weekend. Submerge a container and have the ozinator bubbles displace the water, lift to the surface and smell the ozone. I believe that is how it is tested right? For some reason I have yet to test mine.. it is brand new so it better be working. The guy who installed it had a nice kink in the tube that I had to chop off and reconnect.

[quantumchromodynamics]

The Myron L 512T5D Poolmeter is calibrated for TDS using 1,000 ppm of their "442" Standard Solution, which consists of 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride.

It is also calibrated for salt in the 0 to 5,000 ppm range.

Water conductivity is measured in MicroSiemens per Centimeter (µS/cm)). To get ppm TDS or salt, you multiply by the appropriate conversion factor. The conversion factor for salt is about 0.5. The conversion factor for TDS is about 0.65 to 0.85.

Example: Seawater has a conductivity of about 54,000 µS/cm. That works out to about 35,000 ppm TDS or about 28,000 ppm salt.

Typical Seawater contains

Chloride...........18,000 ppm

Sodium.............10,000 ppm

Sulfate................2,500 ppm

Magnesium........1,000 ppm

Calcium................400 ppm

Potassium.............400 ppm

Your TDS level of 3,500 will be about equal to a salt level of 2,300 ppm, so you would use 2,300 ppm in the salt field when using the pool calculator.

Here is a chart showing some conductivity measurements and the corresponding levels of TDS or salt.

It's possible that the person who tested your water for TDS used the conversion factor for salt instead of TDS.

It's important to have them report both salt and TDS readings so that you can have a more accurate measure of your water quality.

You should also ask them when the meter was last calibrated. You could even ask them to use their reference sample to show you that their test is accurate. They should have a reference sample to periodically calibrate the meter.

It is also important that the meter be rinsed before and after every test to ensure accuracy. If the meter is not rinsed, then the residue from previous tests could accumulate and skew the reading to the high side.

The Myron L 512T5D Pool meter that comes with the Taylor K-6542 can be set to read as ppm TDS or ppm Salt.

[simonc]

Your TDS level of 3,500 will be about equal to a salt level of 2,300 ppm, so you would use 2,300 ppm in the salt field when using the pool calculator.

QCD -

I don't want to get a meter or salt test kit, but can I estimate how much salt based on how much chlorine I've added?

Since I use a gallon jug of 6% bleach as my chlorine source, I know how many jugs I've gone through in a fill. My salt level should mostly result from how much bleach I've added since the fill was new. Therefore ... I should be able to roughly estimate my salt ppms based on the quantity of bleach I've used. Correct?

So ... approximately how many ppms of salt does a 500 gallon tub have after using up a gallon of 6% bleach. (I assume this is an easy calculation for someone like yourself who knows what they are doing with chemical calculations. If it's a difficult calculation, please don't worry about it. I can live without knowing my salt ppm level.)

(Just a note on "jug size" ... pardon the expression ... The Clorox I buy at the supermarket is comes in 3 quart jugs. The ones I buy at Costco come in the larger one gallon size.)

Thanks,

- Simon

[quantumchromodynamics]

So ... approximately how many ppms of salt does a 500 gallon tub have after using up a gallon of 6% bleach.

Cl₂ + 2 NaOH → NaCl + NaClO+ H₂O

1 liter of bleach adds:

66 grams of sodium hypochlorite (NaOCl)

51.814 grams of sodium chloride

62.865 grams of chlorine (Measured as Cl₂

For every 10 ppm of chlorine added, there is 8.2421 ppm of salt added + 8.2421 ppm of salt from the sodium hypochlorite for a total of 16.4842 ppm.

1 liter of bleach added to 1,000 liters of water = 103.628 ppm salt.

1 ounce of 6 % bleach added to 100 gallons of water will add 8.0959 ppm of salt.

1 ounce of 6 % bleach added to 100 gallons of water will add 4.9113 ppm of chlorine.

128/5 X 8.0959 = 207 ppm salt added.

Note this assumes that one mole of salt is created per mole of sodium hypochlorite.

____________________________________________________________

Alternative analysis:

The specific gravity of Clorox is given as 1.1, which means that there is about 100 grams of solute dissolved in the water increasing the mass of the water from 1,000 grams per liter to 1,100 grams per liter.

The ingredients of Clorox are:

Water, Sodium hypochlorite, Sodium chloride, Sodium carbonate, Sodium hydroxide and Sodium polyacrylate.

The mass of sodium hypochlorite is given as 6 %, so that means that 66 grams of the solute is sodium hypochlorite. That would leave 34 grams for the rest of the solutes.

If we assume that 1 liter of bleach added to 1,000 liters of water = 82 ppm salt, then 1 ounce of 6 % bleach added to 100 gallons of water will add 6.4 ppm of salt and that for every 10 ppm chlorine added, there will be 13 ppm salt added.

128/5 X 6.4 = 164 ppm salt added.

Since I'm not sure how much salt is in Clorox, I am going to give the range of between 164 ppm and 207 ppm of added salt when 1 gallon of bleach is added to 500 gallons of water.

You can go to your local pool store to have them test your salt and TDS for free.

[Edit] I think that you could use the following as a good approximation

1 ounce of 6 % bleach added to 100 gallons of water will add 6.4 ppm of salt

or

1 ounce of 6 % bleach added to 100 gallons of water will add 7.8 ppm of TDS. [End edit]

Clorox Ingredients

MSDS

[chem geek]

QCD,

How do you get that the TDS is higher than the salt? Also, in your second set of calculations you should get the same rule that for every 10 ppm FC you get about 16 ppm of salt. This is also 16 ppm TDS as salt is all that remains after the chlorine gets used up. You've got the NaCl from the initial addition and the NaOCl becomes NaCl plus oxygen gas. The oxygen gas does not remain in the water -- perhaps that is what you were counting towards TDS, but shouldn't.

Richard

[quantumchromodynamics]

In addition to salt, there is some carbonate and some polyacrylate that count towards the TDS, although I'm not sure how much.

Since the bleach has a density of 1,100 grams per liter, then there can only be 100 grams of dissolved solutes.

There are 66 grams of sodium hypochlorite (NaOCl), which produce 51.814 grams of sodium chloride.

That leaves 34 grams of other solutes. If we assume that 32 of those grams are salt, then we have a total salt added of 83.814 grams.

83.814 grams of salt added to 1,000 liters of water will add 83.814 ppm of salt.

1 liter of bleach added to 1,000 liters of water will add 62.865 ppm of chlorine.

83.814/62.865 = 1.3332

10 ppm of chlorine will add 13.332 ppm of salt.

The TDS added will be 85.814/62.865 = 1.3651 ppm salt added per 1 ppm of chlorine.

1 ounce of 6 % bleach added to 100 gallons of water will add 6.70 ppm of TDS.

1 ounce of 6 % bleach added to 100 gallons of water will add 6.55 ppm of salt.

[chem geek]

You are counting the oxygen in the sodium hypochlorite, NaOCl, towards the TDS, but as I wrote earlier when chlorine gets used up either by breakdown in sunlight:

2OCl^- + hv ---> O₂(g) + 2Cl^-

Hypochlorite Ion + UV light ---> Oxygen Gas + Chloride Ion

OR

2HOCl + hv ---> O₂(g) + 2H⁺ + 2Cl^-

Hypochlorous Acid + UV light ---> Oxygen Gas + Hydrogen Ion + Chloride Ion

or by oxidation of ammonia

2NH₃ + 3HOCl ---> N₂(g) + 3H₂O + 3H⁺ + 3Cl^-

Ammonia + Hypochlorous Acid ---> Nitrogen Gas + Water + Hydrogen Ion + Chloride Ion

or by oxidation of urea

(NH₂)₂CO + 3HOCl --> N₂(g) + CO₂ + 3H⁺ + 3Cl^- + 2H₂O

Urea + Hypochlorous Acid --> Nitrogen Gas + Carbon Dioxide + Hydrogen Ion + Chloride Ion + Water

you can see when chlorine gets consumed/used, the oxygen atom in sodium hypochlorite ends up as either oxygen gas, or in carbon dioxide or in water, none of which count towards TDS (assuming the carbon dioxide in water outgasses which in spas is the most likely outcome). So the TDS from bleach or chlorinating liquid is really just the resulting salt level.

As for the amount of salt, you can see that for every hypochlorite ion there are two chloride ion that result after consumption/usage since there is one from the initial creation from chlorine gas added to a solution of lye:

Cl₂(g) + 2Na⁺ + 2OH^- ---> 2Na⁺ + OCl^- + Cl^-

Since the OCl^- ultimately results in chloride ion (with the oxygen going to oxygen gas, carbon dioxide or water), the net result is that every mole of sodium hypochlorite results in two moles of sodium chloride (salt) -- one that was already there as salt from the above equation and another that is created when the hypochlorite gets consumed/used.

Chlorine is measured in ppm Cl₂ while salt is measured in ppm NaCl so this means that 1 ppm chlorine results in (2 moles salt per 1 mole chlorine) * 58.443/(2*35.4532) = 1.648 ppm salt. So 10 ppm FC results in 16.5 ppm salt. As noted above, the TDS increases by this same amount. There is a small amount of TDS increase from the "excess lye" and even less from the polyacrylate that is presumably in very low concentration (relative to hypochlorite concentration) so these can generally be ignored. For example, the amount of "excess lye" in 6% Clorox Bleach that has a pH of 11.9 is around 0.06% by weight (compared to 6% by weight of sodium hypochlorite).