Testing CYA

[marchyman]

I started maintaining my pool last August. Since then I've checked CYA levels 3 times.

Aug 31: 50
Oct 5: 40
Jan 4: 60

The variability surprises me, especially since the only chemicals I've added since the Oct reading are 10% liquid chlorine and 14.5% muriatic acid. My first thought is that I'm doing it wrong, but how hard can it be to see/not see a black dot? Are there other factors that would cause CYA to rise?

 

[dereksanders]

It’s one of the toughest tests and at best is never completely accurate. It’s particularly difficult to get the same reading at different times of the year, or even during different times of day.

If you have a significantly lower water level through evaporation the reading could be nominally higher. The only way to truly raise the CYA is to add stabilizer. I’m assuming you haven’t used pucks since your profile shows LC.

 

[Rocket J Squirrel]

I believe the temperature of the pool water can affect the CYA reading. My understanding is that colder water results in a lower reading.

Try putting the sample in your pocket to absorb body warmth while you run the other tests. Do the CYA test last and see whether that provides more consistent results.

 

[marchyman]

Water level is "full'. I haven't had to add any since semi closing the pool, i.e. turning off the heater and turning down the pump speed. I measure a minimum of chlorine and pH once a week to keep levels correct as it is always possible the hot tub will be used. Oh... the pool is covered. This time of year the cover is only pulled back for a quick cleaning when I check chemistry. Pucks have not been used since I took over pool maintenance.

The weather is colder, but the CYA reading is higher. And it's SF Bay Area cold, not midwest cold. Water temp was 52 today, ambient was about 55 °F.

I'm beginning to wonder if I didn't give the bottle of water/reagent a 30 second shake before using it to take the October measurement.

 

[PoolStored]

Take a sample, put the bottle in warm water to warm the sample to room temperature, then do a full panel of tests and post results. Temperature doesn't matter for most tests, but does matter for CYA test (Precipitation rate is slower but solubility decreases with this test in cold water).

 

[Bperry]

I started maintaining my pool last August. Since then I've checked CYA levels 3 times.

Aug 31: 50
Oct 5: 40
Jan 4: 60

The variability surprises me, especially since the only chemicals I've added since the Oct reading are 10% liquid chlorine and 14.5% muriatic acid. My first thought is that I'm doing it wrong, but how hard can it be to see/not see a black dot? Are there other factors that would cause CYA to rise?

The error tolerance on the test is at least +\- 10ppm so those results are in line with that.

 

[marchyman]

Take a sample, put the bottle in warm water to warm the sample to room temperature, then do a full panel of tests and post results.

I let the water temp of the sample hit 69 °F before testing. Today's numbers are FC: 7, CC: 0, pH: 7.5, TA: 100, CYA: 60, CH: 350, CSI: 0.11 calculated. That is the same as yesterday save for FC because I added chlorine. I like that the results are consistent.

My pH rises slowly. I wait until it hits 8 before adding acid to bring it down to 7.5. Eventually my TA will get in range. When I took over maintenance it was 120+