contradicting CH numbers

[pikeman]

Three weeks ago I refilled my pool after a restoration with Wet Edge Satin Matrix. It looks great and the water is great. I had my first swim at 72f - brrrr!
Anyhow, I bought a fresh test kit, a TF100. I wanted a fresh start.
My numbers are:
FC=2.0
CC=0
PH=7.6
TA=170
CH=400 or 240?
CYA=20

I am a bit low on the CYA but the guy who initially dosed the CYA said since I use some trichlor pucks in addition to bleach it out to work out. I think I need another ten points because my 24-hour test with FC has it dropping a full point. We have had bright sunny weather. It is way too soon to be consuming FC at that rate.
The CH is absurd. I went to the pool store and the kid measured 240. He was using the same taylor reagents with a 10ml sample.He had one of those fancy spinner mixers but that shouldn't make this kind of difference.
I repeated the test three times - even tested the city water. I even had my wife read the instructions and witness the procedure.

What's going on here?

Thanks.
Pete

 

[Richard320]

A speedstir can make a big difference in the CH test. Mine dropped 200 ppm just by switching.

 

[smackdab]

Are you saying the CH test isn't accurate without a speedstir?

 

[Saturn94]

Some time ago I remember reading a post where someone was getting different FC readings using the speedstir vs manual mixing. I don't remember if they ever arrived at a good explanation.

 

[Texas Splash]

It's just that the CH is one of those tests that requires 3 agents and a lot of drops. Holding and spinning is not the most efficient when fumbling around for those things and sometimes it can effect the transition of colors. Sometimes with my speed stir, I will on occasion use the 25 ml measuring option from the Extended Test Kit notes to validate my test with more precision. It's just a great tool for this purpose, and don't want to rule-out that it may play a part in the actual reading.

 

[smackdab]

I agree it can be a little difficult to do the drops and and swirl by hand, but I think I do a pretty good job at it. Now I'm wandering just what my CH is going to read if I used a speedstir. A difference of going from 400 to 240 or dropping by 200 ppm doesn't seem to be worth a hoot to me. If that is the case, then pool store testing appears to be just as accurate as my testing. So if I add the drops faster and swirl quicker, my CH will actually go down and this will be more precise?

 

[JoyfulNoise]

In a lot of these tests, drop rate matters. The CH test isn't perfect and, if you transition to blue and wait a few minutes, the indicator will go pink again. Swirling by hand slows down your drop count rate and gives time for the calcium indicator to go back to purple. A SpeedStir allows you to drop titrant at a faster and more consistent rate (~ 1 drop/sec is a good measure) and that improves the test. Lighting also makes a huge difference too. I find bright outdoor lighting to work the best.

I also use the modified test approach where you split your R-0012 drops. Add R-0010, then about half your expected number of R-0012 drops, then the R-0011L indicator, then the remaining drops of R-0012 until the blue endpoint is reached. It helps a lot when your CH is as high as mine (850ppm as of today).

 

[pikeman]

I agree it can be a little difficult to do the drops and and swirl by hand, but I think I do a pretty good job at it. Now I'm wandering just what my CH is going to read if I used a speedstir. A difference of going from 400 to 240 or dropping by 200 ppm doesn't seem to be worth a hoot to me. If that is the case, then pool store testing appears to be just as accurate as my testing. So if I add the drops faster and swirl quicker, my CH will actually go down and this will be more precise?

The store guy was very deft as far as his drop rate. It was quick and precise. He'd probably done it a thousand times. I'm going to try it with swirling and making sure the mix is more thorough and see if it makes a differance. I'm also going to see what Taylor tech support has to say.
Pete

 

[duraleigh]

It appears we are jumping to conclusions here. If I read your post correctly, you got 400 THREE times doing your own CH test and the Pool Store got 240, is that correct?

If that is correct, you are assuming the pool store test of 240 is more accurate than your THREE tests of 400 ppm. Is that right?

 

[Patrick_B]

The store guy was very deft as far as his drop rate. It was quick and precise. He'd probably done it a thousand times. I'm going to try it with swirling and making sure the mix is more thorough and see if it makes a differance. I'm also going to see what Taylor tech support has to say.
Pete

Three at 400 is your answer.

I've run thousands of Titrations in my career, and if you got three like this, I'd trust you with my pool CH test over the PS with his 240 any day of the week. Trust your own results, especially when they are so consistent.

I also want to stress that all the Titrations can be run with equal accuracy by hand or with a stir bar setup like the Speedstir. It's far easier, and a good deal quicker with a stirrer no question but it can be trying to reach the endpoint stirring just by hand.

 

[duraleigh]

I am hoping for a response here from pikeman. It may be a different scenario Than what I think, but it might be another case of not trusting your own testing.

We see time and time again that the pool owner is best at testing his own pool. He cares the most.