Discrepancy in CYA measurements

[cschnurr]

I took cover off the pool last weekend. I had the pump bearings & seal replaced, and then ran the pump for a couple days Yesterday, the water looked OK a bit murky but I could see the vacuum on the bottom. Water temperature is only 50F.

I got new CYA and Cl reagants for my Taylor K-2005C kit and took measurements.
TC=0
CC=0
pH=7.8-8.0
TA=90
CH=200
CYA=40

Salt =2500 using AquaChek strips from last year
Date on lid which I assume is Expiry date 2008/04.

Thinking everything looks pretty good, I went to the pool store to buy some liquid Chlorine. I took a water sample along for validation of my measurements.
I had them measure CYA and salt.
Their measurements were
CYA >100
Salt 1500
I had them test twice as I was surprised by the results

I came home and retested a couple times even with a syringe to measure the 7ml in case my bottle was contaminated. Each time I got CYA=40-50ppm.

So I added acid to bring down the pH ( just in case I was off scale in my pH reading) and added Chlorine to bring it up to 15ppm (shock level at CYA=40). I let the pump run 24 hours straight. It rained yesterday adding over an 1” of rainwater to the pool. Water is now clear but not sparkling.

This morning I took another water sample, and brought it into the house to ‘warm up’ in case there is a temperature effect. My CYA test still measured 40, pH dropped to 7.4 as expected, and Chlorine level did not change overnight.

I have had a SWG since the pool was installed and have never used pucks. The target CYA of the SWG is 60-80ppm, last year, I overshot slightly in the spring but my measurements (with old reagents) matched the same poolstore measurements at 80-85ppm. At the end of the season my measurements were CYA =80, Salt =3100. The pool was covered with a mesh safety cover for the winter.

Is there any explanation for the difference in measurements in either salt or CYA?
Historically, I’ve trusted my measurements more than the pool store, but I can’t ignore a discrepancy this large as this is the first test I’ve done with the new reagents and will determine whether or not to add stabilizer.

 

[JasonLion]

The AquaChek strips go bad when they get old, they are serious about the expiration date. Not that I automatically believe the pool store salt test either.

CYA can vanish over the winter but it will never go up. Your CYA level can't possibly be 100 if it was in the 80s when you closed the pool (unless you added some).

Pool store testing is very inconsistent. Some stores are good some of the time, very few stores are good all of the time. I would believe your CYA number over theirs and get new salt test strips.