Doubting my CYA reading

[Vandergraff]

Using a TF100 I have been reading my CYA at around 45 ppm.

Every is going well- I adjust to about 6ppm FC each evening and the next evening FC is down to about 3ppm - then I adjust back to 6ppm.

Anyway I thought I would double check the CYA reading by buying the 50 ppm CYA sample.

Well big surprise - with the sample I would have read it at 70 - 100 ppm - evidently it really means the black dot TOTALLY disappears in direct bright sun. Even using the method of standing with the (bight California) sun behind me the block dot seems to disappear well above 50 ppm with the sample.

Testing my pool water in direct bright sunlight the block dot only totally disappears at close to 20 ppm - (with the sun behind me it is closer to 40 ppm).

If the pool is really 20 ppm then I probably should add some CYA to get to 30 ppm and can set lower chlorine targets. However I would have thought I would lose more FC each day - the pool gets direct sun for most of the day. Other readings are OK PH 7.5 TA 80-90 CH 220.

With the pool water clear and at 85F - air temp at ~100F and family and friends coming over during the holiday weekend I am reluctant to change anything for now. However does it seem possible that with a CYA of 20 ppm I would only lose half the FC each day in direct bright CA sun?

If I do need to get the CYA up I have trichlor tablets I can use.

 

[duraleigh]

I would not try to fix what isn't broken. You seem to have everything in equilibrium and losing pretty close to the expected amount of chlorine.

You could certainly tweak the CYA up if you wanted, but I am not sure the cost savings in chlorine is at all worth it.

 

[Vandergraff]

OK now I am convinced my CYA is under 30 - probably 20 or lower.

I have tried the CYA test from the TF100 kit again and the black dot totally disappears at ~20. Last few days FC has been 2 ppm in each evening (after boosting to 6ppm the previous evening). Getting through about 75 fl oz of 12% chlorine each day in ~16000 gallon pool (air temp ~ 100F and pool heating by solar to ~ 87F each evening). I also have some test strips which say CYA is low (I know don't rely on test strips - but they seem to confirm other readings).

So I will use some tricolor tablets (in a floater) to raise CYA. The tablets weigh 5 1/2 OZ - if I add 3 to 16000 gallon pool how much will this raise the CYA?

 

[anonapersona]

So I will use some tricolor tablets (in a floater) to raise CYA. The tablets weigh 5 1/2 OZ - if I add 3 to 16000 gallon pool how much will this raise the CYA?

Use the Pool Calculator, at the bottom, Effects of adding Chemicals.

 

[Vandergraff]

Thanks - hadn't seen that at the bottom of the Pool calculator.

It says 16.5 oz of trichlor will raise FC by 6.9, raise CYA by 4.2 and lower PH 3.7 in a 16500 gallon pool. I assume that is once the trichlor tablets are fully dissolved - which makes sense for CYA and PH.

A little surprised that 3 tablets would only raise FC by 6.9.

 

[Vandergraff]

I am really doubting my CYA reading now.

As above I bought the TFTestkits 50 ppm CYA Standard Solution.

Today I tested in bright sunlight.

With the 50 ppm reference solution I get the following results - all with the tube held at arms length/waist level

- Back to sun so shading the tube - black dot completely disappears at 90 - 100 ppm!
- Holding the tube in direct sun - black dot completely disappears at 60 - 70 ppm!

Using my pool water

- Back to sun so shading the tube - black dot completely disappears at ~30 ppm!
- Holding the tube in direct sun - black dot completely disappears at ~20 ppm!

I thought the advice on here (from Jason and others) was to test in bright sun your back to sun - if I do this I read the 50 ppm standard solution at 90 - 100 ppm.

I asked my wife to confirm my results - she also saw the black dot disappear at around the same point in both direct sun and with her back to the sun.

I have also looked the advice here http://www.taylortechnologies.com/Chemi ... ntentID=36

I have added some trichlor (tablets and powder) in the last few days to add some CYA.

My FC has been ~3 ppm at the end of each day for the last few days - even in bright direct sun all day. I am adjusting to ~6 ppm each evening (normally with liquid chlorine but with trichlor power last night). The trichlor tabs in the pool probably expalin why the FC is now back to 3ppm at the end of the day vs 2ppm a few days ago. It has been hot and sunny for the last week with air temps in the 90 - 100s and water temp around 86 at the end of the day,

What is clear is my pool water is CYA reading much lower than the 50 ppm standard solution - but frankly I have lost confidence in the standard solution accuracy and the validity of the readings I am seeing. Without confidence in test it is hard to know what to believe and do next.

 

[duraleigh]

Without confidence in test it is hard to know what to believe and do next.

I'm not sure what you are trying to accomplish. You have a well balanced pool that is losing about a normal amount of FC and your CYA is a little low. What is your goal?

 

[Vandergraff]

It is true that my the pool looks good - but I would like to reduce my chlorine usage a bit. It is pretty high and my CYA seems to be low - but I just don't know how low.

I would like to get a good idea of what my CYA level is so I can adjust as needed.

 

[duraleigh]

Calculate a dosage that will increase your CYA by 20ppm and judge your FC consumption for a week or so.

 

[Vandergraff]

I am in the process of raising the CYA by 10 to see how it goes.

I wish there was a way to measure CYA that I could have confidence in.

 

[duraleigh]

Since the accuracy of the CYA test is plus or minus 15ppm, you may have a difficult time perceiving the difference in a 10ppm increase.