TF-100 Chlorine testing

[kmdigital]

In another post somewhere, I've read that when testing higher levels of chlorine, where 0.5 ppm accuracy isn't needed, that you could use 5 ml of tap water and 5 ml of pool water, and each drop should be equal to 1 ppm instead of 0.5. However, in practice, it doesn't work that way at all. When I test it (using well water, no CL in it), it doesn't turn very pink and 3 or 4 drops clears it up. However, actual CL level is 7.5, so it seems that it's actually multiply the drops times 2.

My CYA is high, and need to replace most of the water, but will wait to do that till spring. I know CL is high, and I don't really want/need 0.5 ppm accuracy, and was looking to save some reagent. What am I doing wrong?

 

[Patrick_B]

If you ran a 5ml sample of pool water the result would be 1:1. When you cut it in half for a dilution to 10 ml you would double the result. I differ on diluting the FC test and prefer not too on this one. Not a big deal for CH or CYA, but in my experience FC more skewed with dilution. If you are only at 7.5 FC, I suggest using the 10mL sample. A 20mL sample is overkill for a pool, but I firmly believe that 10 is just right. We are only talking 15 drops for a 7.5 FC level... so it's not that much and well worth it.

 

[duraleigh]

I saw that post about a 5 ml sample.

A 2 oz bottle of R-0870 has roughly 1500 drops in it and costs about $10.00.

That works out to about 7/10 of a penny per drop.

So your net savings on using 5 ml instead of 10 ml in a pool with 5 ppm FC works out to about 3.5 Cents if my math is right.

I am not sure how well that 3.5 cents fits into someone else's budget but I plan on staying with the 10 ml sample as suggested by the folks who developed the test.

 

[Patrick_B]

I saw that post about a 5 ml sample.

A 2 oz bottle of R-0870 has roughly 1500 drops in it and costs about $10.00.

That works out to about 7/10 of a penny per drop.

So your net savings on using 5 ml instead of 10 ml in a pool with 5 ppm FC works out to about 3.5 Cents if my math is right.

I am not sure how well that 3.5 cents fits into someone else's budget but I plan on staying with the 10 ml sample as suggested by the folks who developed the test.

 

[chem geek]

In another post somewhere, I've read that when testing higher levels of chlorine, where 0.5 ppm accuracy isn't needed, that you could use 5 ml of tap water and 5 ml of pool water, and each drop should be equal to 1 ppm instead of 0.5.

What you wrote is not correct. You do not dilute the water sample. If you use a 5 ml water sample undiluted, then each drop is 1 ppm FC. If you dilute 1:1 as you wrote, then each drop would be 2 ppm (or put another way, each drop is 1 ppm but you need to double the result due to the dilution). For very high shock levels one can use a 5 ml sample size except for the OCLT that needs more accuracy where the 10 ml sample size should be used. Otherwise, there's no reason not to use the 10 ml sample size.

 

[kmdigital]

In another post somewhere, I've read that when testing higher levels of chlorine, where 0.5 ppm accuracy isn't needed, that you could use 5 ml of tap water and 5 ml of pool water, and each drop should be equal to 1 ppm instead of 0.5.

What you wrote is not correct. You do not dilute the water sample. If you use a 5 ml water sample undiluted, then each drop is 1 ppm FC. If you dilute 1:1 as you wrote, then each drop would be 2 ppm (or put another way, each drop is 1 ppm but you need to double the result due to the dilution). For very high shock levels one can use a 5 ml sample size except for the OCLT that needs more accuracy where the 10 ml sample size should be used. Otherwise, there's no reason not to use the 10 ml sample size.

Ahh, okay, that's the problem. Thanks for clearing that up, and it actually worked as you said. Since I'm running high FC for now, I like the idea of using a 5 ml sample, as that's plenty of accuracy for 12 - 14 ppm of FC. It happened to be a bit low when I tested it at the 7.5, but it had been a week since I added any chlorine and it was very hot earlier this month. Takes a lot of drops to measure 12 - 14 when each is only 0.5 ppm. Thanks, you definitely are a major asset to this forum! :-D

 

[jblizzle]

The only time the 5ml makes some sense is when SLAMing and FC up over 30ppm. 12ppm is not high enough to me to consider the lower accuracy of the smaller sample.