Question About Ppm

[rckymtntim]

Hi All,

I have a question about ppm. I understand the concept that 1mg is 1ppm of a liter. My issue is how much 6% Clorox bleach do I need to add to raise my FC levels. When I add a 1/2 cup of bleach to my 165 gallon spa it seems to raise my FC by about 4.4. Does this sound about right? This seems that 1 oz of bleach will raise my FC by about 1.1ppm.

[quantumchromodynamics]

1 gram in 1,000 liters = 1 ppm.

Bleach contains about 5.7 % chlorine by weight. It has a density of 1.08 grams per milliliter. 1.08 X .057 = 0.06156 grams of chlorine per milliliter of Clorox. 1/0.06156 = 16.2 milliliters = 1 gram of chlorine as CL₂. Therefore, 16.2 milliliters of regular 6% Clorox bleach added to 1,000 liters = 1 ppm.

16.2 milliliters = 0.5478 fl oz. (fluid ounces)

1,000 liters = 264.2 gallons

(0.5478)(165)/264.2 = 0.342

Therefore, 0.342 ounces per 165 gallons = 1.0 ppm.

1/2 cup = 4 ounces. 4/0.342 = 11.7 ppm.

1/2 cup should be raising your FC by 11.7 ppm.

You can use thepoolcalculator to calculate the correct amount of chemicals to add to achieve a desired level. You can use the "Effects of adding chemicals" section of the pool calculator to determine the effect of adding a certain amount of a particular chemical.

(Other references show 5.84 % by weight and 1.1 grams per milliliter. Which equals 0.06424 grams of chlorine per milliliter of Clorox Ultra Germicidal Bleach)

Some bleach is only 3 %, so you have to be careful to only get the 6 % bleach. That could explain why you are not getting the expected result.

Are you sure about the volume of your tub? 165 gallons is somewhat smaller than normal.

If you have a high chlorine demand, that could be using up chlorine and reducing your measured amount.

What chlorine test are you using? Are you measuring free and combined chlorine?

____________________________________________

Clorox is 6.00 % sodium hypochlorite by weight. Chlorine is measured in units of chlorine gas. Therefore, you have to convert the percent sodium hypochlorite to percent chlorine.

NaOCl = 74.442 grams per mole. Cl₂ is 70.906 grams per mole. (70.906)/74.442 = .9525.

0.9525 x 6.00 % = 5.715 %.

Clorox is 5.715 percent chlorine. Specific gravity = 1.1.

0.05715 x 1.1 = 0.062865 grams of chlorine per milliliter of Clorox.

[chem geek]

As QCD noted, use The Pool Calculator to calculate dosages. 1/2 cup (4 fluid ounces) of 6% bleach in 165 gallons would raise the FC by 11.7 ppm. 1/2 cup of 6% bleach in 450 gallons would raise the FC by 4.3 ppm. 1 ppm = 1 milligram per liter (mg/L) = 1 gram per 1000 liters (g/L). [EDIT] I fixed this; I incorrectly wrote 1000 grams per liter originally. [END-EDIT]

Are you adding the chlorine after a soak and are you then measuring the chlorine level some time after that? If so, then realize that most of the chlorine will combine with ammonia (and urea) from your sweat and urine so it will lower FC (Free Chlorine) and form Combined Chlorine (CC). Over the next hour or so, the remaining FC will break down the CC so that the CC will drop towards zero and the FC will drop somewhat as well.

So if you wanted to measure the amount of FC from chlorine in your spa, you wouldn't do that by measuring after a soak. You could instead do this by adding some chlorine a day after your soak when there is still residual FC left, but don't take too long to wait to measure the chlorine -- 10 minutes or so with the circulation pump running should be sufficient.

[rckymtntim]

1 gram in 1,000 liters = 1 ppm.

Bleach contains about 5.7 % chlorine by weight. It has a density of 1.08 grams per milliliter. 1.08 X .057 = 0.06156 grams of chlorine per milliliter of Clorox. 1/0.06156 = 16.2 milliliters = 1 gram of chlorine as CL₂. Therefore, 16.2 milliliters of regular 6% Clorox bleach added to 1,000 liters = 1 ppm.

16.2 milliliters = 0.5478 fl oz. (fluid ounces)

1,000 liters = 264.2 gallons

(0.5478)(165)/264.2 = 0.342

Therefore, 0.342 ounces per 165 gallons = 1.0 ppm.

1/2 cup = 4 ounces. 4/0.342 = 11.7 ppm.

1/2 cup should be raising your FC by 11.7 ppm.

You can use thepoolcalculator to calculate the correct amount of chemicals to add to achieve a desired level. You can use the "Effects of adding chemicals" section of the pool calculator to determine the effect of adding a certain amount of a particular chemical.

(Other references show 5.84 % by weight and 1.1 grams per milliliter. Which equals 0.06424 grams of chlorine per milliliter of Clorox Ultra Germicidal Bleach)

Some bleach is only 3 %, so you have to be careful to only get the 6 % bleach. That could explain why you are not getting the expected result.

Are you sure about the volume of your tub? 165 gallons is somewhat smaller than normal.

If you have a high chlorine demand, that could be using up chlorine and reducing your measured amount.

What chlorine test are you using? Are you measuring free and combined chlorine?

____________________________________________

Clorox is 6.00 % sodium hypochlorite by weight. Chlorine is measured in units of chlorine gas. Therefore, you have to convert the percent sodium hypochlorite to percent chlorine.

NaOCl = 74.442 grams per mole. Cl₂ is 70.906 grams per mole. (70.906)/74.442 = .9525.

0.9525 x 6.00 % = 5.715 %.

Clorox is 5.715 percent chlorine. Specific gravity = 1.1.

0.05715 x 1.1 = 0.062865 grams of chlorine per milliliter of Clorox.

As QCD noted, use The Pool Calculator to calculate dosages. 1/2 cup (4 fluid ounces) of 6% bleach in 165 gallons would raise the FC by 11.7 ppm. 1/2 cup of 6% bleach in 450 gallons would raise the FC by 4.3 ppm. 1 ppm = 1 milligram per liter (mg/L) = 1 gram per 1000 liters (g/L). [EDIT] I fixed this; I incorrectly wrote 1000 grams per liter originally. [END-EDIT]

Are you adding the chlorine after a soak and are you then measuring the chlorine level some time after that? If so, then realize that most of the chlorine will combine with ammonia (and urea) from your sweat and urine so it will lower FC (Free Chlorine) and form Combined Chlorine (CC). Over the next hour or so, the remaining FC will break down the CC so that the CC will drop towards zero and the FC will drop somewhat as well.

So if you wanted to measure the amount of FC from chlorine in your spa, you wouldn't do that by measuring after a soak. You could instead do this by adding some chlorine a day after your soak when there is still residual FC left, but don't take too long to wait to measure the chlorine -- 10 minutes or so with the circulation pump running should be sufficient.

Thank you both for the replies.

Quantum, yes it is a Marquis Spirit spa that is a 2-3 person spa and the specs say 165 gallons. And, I use clorox 6% bleach.

I am currently working out of town for 10 days at a time. When I am home for 4 days I check my FC and CC daily. While I am gone my wife adds a 1/2 cup of bleach every other day. She does not use the tub while I'm gone. When we use the tub I put in 1/2 cup of bleach after each use. I check the next day and my FC is usually around 4ppm. I won't add any bleach at that point but will add a 1/2 cup after use that night. I check my CC when I get home and it's usually 0.2-0.4. CD is about 35%.

[chem geek]

Does the spa have an ozonator? If it does, then that would explain the higher chlorine demand (CD), at least on a percentage basis. If not, then I would check your filters if it's been more than a month using the spa. 1/2 cup (4 fluid ounces) of 6% bleach in 165 gallons would be 11.7 ppm FC which is quite a lot when not using the tub (when using the tub, without an ozonator, then the FC and then CC will get very high after a soak since the same bather waste from 1-2 people is in a smaller volume of water). If the CD is really around 35%, then when not using the tub you would only need to add around 1/2 ounce (1 tablespoon) of bleach to maintain 4 ppm. So something isn't right in the numbers.

Some others have reported seeing higher CC when their chlorine levels got very high so that might be what is going on with your CC as well. Again, the numbers you are giving aren't adding up. You say that 1/2 cup of bleach raises the FC by 4.4 ppm, but that is inconsistent with a true 6% bleach and a 165 gallon spa size.