Taylor reading confusion

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Taylor reading confusion

Postby Guest » Thu 21 Aug, 2008 05:54

Ok so here is the deal.

1.)Say you had zero TA how much would you add to a 25000 gal pool to get it in a recommended range?

I added 40lb and now on my test strip it reads 180 but the taylor reaches an endpoint at 19 drops (green to red)which is 190ppm? That just seems wrong to me

2.) I checked the CYA level of my pool which was 70 then I drained 1/4 of my pool refilled now the reading is 120. What the heck is going on here.

Now all I here around here is the Taylor test kit is great but this thing is driving me nuts. Now I know the strips are a big no-no around here but the chlorine and PH are very close. Like I said the TA reads 180 and the CYA says 0.

So here is the breakdown

TAYLOR

PH 7.4
TC 4
TA endpoint at 19 drops (190ppm)
CYA 120

Test strip

PH 7.4
TC 4
TA 180
CYA 0


chem geek
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Postby chem geek » Thu 21 Aug, 2008 10:05

40 pounds of Arm & Hammer Baking Soda or Alkalinity Up in 25,000 gallons would raise the TA by about 120 ppm. The precise amount of TA rise depends on the starting pH. You say you had zero TA to start with, but if the TA test immediately turned red, then that not only means you had zero (or negative) TA, but that your starting pH was 4.5 or below.

Both the test strip and the Taylor kit are giving you roughly the same result for TA so most likely the initial TA was not really zero. If it were around 60 ppm initially, then your result would make sense. Did you measure the TA of your water before you added the 40 lb. and it immediately turned red?

As for the CYA test, it doesn't have a marking for 120 and the markings get very close together at higher CYA since it's a logarithmic scale as you can see here . The test is subjective so prone to more error at high CYA levels, though doing the test consistently under the same lighting conditions usually gives you results +/- 10 ppm. Nevertheless, the CYA should not rise when diluting the pool water and certainly not as much as you saw. Test the tap water and you should find the CYA is zero (< 30 on the test -- you can fill the tube and the black dot is clearly visible and the water appears clear).

Richard
Guest

Postby Guest » Fri 22 Aug, 2008 05:27

Ok so I tested the tap water and of course it was zero CYA but I still don't understand the reason for the pool water to increase in CYA after refilling it.

I still don't understand this TA and now Calcium Hardness tests. I don't want to go crazy with chemicals because I'm closing it soon but want it at a good level before I winterize it.

Both TA and CH tests give me endpoints (color change) at 19 drops that equals 190ppm which factors out to around:

57lb of CH
66lb of TA

That just seems sick to me. Other then chlorine I haven't added anything in the last 2 weeks but levels are still the same as the post above. If it wasen't for the test strips I'm either paranoid from the different readings or they are actually stopping me from overloading the pool with chems. Can reagents be too old for some tests? Confused.
chem geek
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Postby chem geek » Fri 22 Aug, 2008 10:39

After draining part of the pool, was any sort of chlorine shock added? Some fast-dissolving shock is Dichlor where for every 10 ppm FC it adds 9 ppm CYA (with Trichlor pucks/tabs or powder, for every 10 ppm FC it adds 6 ppm CYA; with Cal-Hypo for every 10 ppm FC it adds 7 ppm CH).

I honestly don't have an answer for the CYA rise other then some chemical addition (Dichlor or Trichlor). How did you get to 120 ppm anyway? Was that an estimate with the black dot disappearing just below the 100 ppm line? There is no line below 100 ppm so all one really knows is that the CYA is > 100 ppm. With Trichlor or Dichlor, it doesn't take long to increase the CYA level -- did you test the CYA just before you did the partial drain or was it a while before (a month or more earlier) and then again after the partial drain?

On the TA, I already told you that a reading of zero TA would mean your pH was very low below 4.5 since that is when the indicator on the test would go immediately to red before adding any titrating drops. That is very strange and usually only happens in pools using Trichlor as their source of chlorine since it's very acidic (and the pool owner not testing and adjusting pH regularly). You said "say you had zero TA" so did you actually measure zero by having the sample turn red immediately?

For large chemical additions, one usually adds half the required amount and retests, just to be sure, at least until one gets a decent feel for their pool and knows how it responds (especially if multiple parameters are changing such as both pH and TA). It is certainly possible for reagents to be too old -- generally though they are good for 2 years though the manufacturer recommends replacing some of them every year. I don't understand your point about the test strips preventing you from going over since you got virtually the same result on the test strip as with the drop test for TA. It's the CYA test where the test strip is recording zero so is quite different.

As for having a TA of 190 and a CH of 190, that's not horrible as the saturation index at a pH of 7.5 is 0.14 in that case. If the pool were to be uncovered, then the pH may have a tendency to rise due to the high TA. The only risk is if the pH went way up then scaling could occur, but with the water getting cold and if the pool gets covered I think the outgassing of carbon dioxide that would cause a rise in pH won't happen very much (i.e. the pH should rise only very slowly). You could periodically check the pH (if possible) and lower it if it gets to 7.7, for example.

Richard
Guest

Postby Guest » Fri 22 Aug, 2008 20:04

Oh I think I understand now :slaps head:

I thought you wanted to get a blue to red change of the bat meaning a 190ppm would require you to add that amount of chemicals to the pool. Which gave me that 66lb of TA. Somehow I thought you wanted an endpoint with the fewest reagent drops possible. Wow that mistake would have been huge.

So you really are saying my TA is 190 meaning I should use around 3 gallons of muriatic acid to drop it to 120. Well this explains a lot of why I was so nervous about the readings. I see now how both readings on the test strip and taylor were both spot on. Which I knew about this before I pushed more TA into the pool. Could have stoped it at 120...live and learn.

As far as the CYA goes yes I'm using pucks and would like to use a different method. Possibly cheap and zero additions to other chemical areas. Maybe I'll do the Chlorine bleach route next year.
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Postby chem geek » Fri 22 Aug, 2008 20:27

You can't lower the TA just by adding acid. You have to add acid to lower the pH, then aerate the water to make the pH rise a bit, then add acid to lower it back down, etc. The procedure is described in this post.

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