CH drop as a proxy for CYA reduction?

[rodmitch]

I'm new here so it's quite possible I've missed something that's been discussed many times. Anyway, I'm incrementally draining my pool for various dumb reasons and it's darkish when I leave for work and darkish by the time I get home and have dinner all of which adds up to: there are lots of requirements for a good CYA test. Outside, bright sun at your back, waist level, good eyesight and judgment. I haven't been able to perform a decent CYA test for the week that I've had the test kit. My initial, good-as-I-could CYA test was 160 (50% dilution).

However, I know my CH was 820, I've done a few mini-drain/refills on the pool, now the CH is 625ish. That's a 24% drop so it stands to reason that my CYA has dropped by 24% as well, right?

But that doesn't account for the fact the the new water has CH of 80 and CYA of 0 so it's not exactly apples to apples. I can plug the numbers into the PoolCalulator, including the 80, and it says if I want to take my CH from 820 to 625 I should replace 26% of the water. So if I fiddle with the CYA numbers a 26% drain gets me from 160 to 118.

Anything wrong with my logic here?

 

[jblizzle]

I don't think it is quite that simple. I think what you described would be true if you had done one 1 drain and refill of 26%, but I think the fact that you did it in multiple steps may not make the CYA calculation true.

Lets say you did a 10% and then 10% and then 10%
Doing the math that would lower the CH to 620ppm (pretty close actually to what you got)
For the CYA you would have 160*.9*.9*.9 = 117ppm .... hmmm well that is pretty darn close to your estimate, so maybe you logic does work

 

[Patrick_B]

Nothing wrong with the logic, but there are other factors. Evaporation, sample precision, and testing error. Also, CYA is not a precision test anyway. Very close, and more than adequate, but not extremely precise. To be blunt, 24% VS 26% is splitting hairs for pool chemistry talking about CH levels.

 

[duraleigh]

Every one does things a bit differently but if that was my pool, I would do larger drain increments and get your CYA to a manageable level of around 50 which will also bring your CH down into a more reasonable range.

That math would be an approx 33% drain followed by another approx 33% drain and your pool will be much easier to get along with.

 

[Patrick_B]

As always..duraleigh is keeping it simple.

 

[Isaac-1]

The other factor here is what is the CH of your fill water, unlike CYA which is a pool only chemical you will have CH in your fill water, also the amount may vary depending on the source of your water. (if your on a municipal water system, your home may be fed by multiple sources each with varying amounts of CH, which may change hour by hour as demand and balancing of the supply changes.)

Ike

 

[Patrick_B]

I agree it could Isaac, but he is basing this on 80 CH.

 

[jblizzle]

I agree it could Isaac, but he is basing this on 80 CH.

Which I did not realize was possible in the calculator.

 

[rodmitch]

I think I'm accounting for the incremental drains by keeping the original numbers as the reference point rather than the incremental numbers.

I don't think I'm under any illusions about accuracy. The 820 is +/- 10. The 625 is +/- 25 because I used the less drops/less accurate test. But I'm just looking for a ballpark idea of how my CYA is going down so I can lower my FC levels accordingly. I'm trying to be conservative while aiming for the high side of the FC range while not using more than I actually need to.

I'm hoping to do a more dramatic drain/fill this weekend. I know the incremental method uses more water and chlorine due to draining some of what is added each time.

I agree it could Isaac, but he is basing this on 80 CH.

Which I did not realize was possible in the calculator.

Yeah, I was thinking about how to account for it when I found it in the calculator. I thought it was pretty cool that it was there!