Unexpected CYA test result (I think). Can someone help me figure this out?

[DangerBoy]

Okay, I've just done a startup following the TFP method on my '95 Grandee (500 US gallons). I got the TA dialed in at 50, the pH at 7.5 and I've been adding dichlor over the last few days to get the FC up into acceptable ranges and steadily build up the CYA level towards the 30 - 40 ppm range.

I should also mention that for the first few days after I filled and heated up the tub (for the third time - but that's another story ) I was working on getting the heating system working properly and didn't do anything with chemicals. Over that time, I did go in for a few soaks so there was a bit of a bather load on the tub before the chemistry was started.

Following the TFP startup directions, I shocked the tub to 10 ppm FC. According to the pool calculator, I needed 34 g of dichlor to do that. I weighed out 34 g on my food scale and fed it to the tub. An hour or so later I tested for FC and got 4.0 ppm. I had a soak that night.

The next day, I tested FC and the water didn't turn pink even after I added several tiny scoops of the powder so I assumed FC had gone to 0 ppm. I then added 34 g of dichlor expecting my FC would go to 4.0 again. After an hour or two I tested FC again and got a result of 6.5 ppm. A little higher than I wanted but okay. I had two dips in the tub last night.

This afternoon, I tested FC again and got a result of 1.5 ppm. The CC was 1.0 ppm. The pool calculator told me I needed to add 15 g of dichlor to get FC to 6.0 ppm so that's what I added. After an hour or so I tested for FC and got a result of 6.5 ppm. I then tested for CYA and don't know what I got because I filled the comparator tube full and could still see the black dot. The water was only slightly cloudy even though I followed the directions to the letter and shook the mixture for 30 seconds or just a bit more. The R-0013 reagent is brand new and doesn't expire until Feb/20.

If I use the pool calculator and start with a CYA of 0 and set the target level at 44 ppm, it tells me I need to add 83 g of dry stablizer which I assume is dichlor. I have added 34g + 34g + 15g = 83 g in total so I was expecting the CYA level to test out to somewhere in the 30 - 44 ppm range but it's way lower than that although I really can't guess as to what it is since the number scale on the comparator only goes as low as 30 ppm.

What's going on here? Why did my CYA number come up so much lower than what I was expecting? Is there something I'm not understanding about the relationship between CYA and chlorine in dichlor?

 

[mknauss]

Dichlor is about 50% CYA and 50% chlorine.

 

[DangerBoy]

Dichlor is about 50% CYA and 50% chlorine.

I know that. It's actually 9 parts CYA to 10 parts chlorine. The Pool Calculator says that if I added 83 g of dichlor to my tub starting from 0 ppm, I should have a CYA concentration of 44 ppm. I did that in three stages and I'm guessing I only have half that amount of CYA at best. Why?

 

[jseyfert3]

I know that. It's actually 9 parts CYA to 10 parts chlorine. The Pool Calculator says that if I added 83 g of dichlor to my tub starting from 0 ppm, I should have a CYA concentration of 44 ppm. I did that in three stages and I'm guessing I only have half that amount of CYA at best. Why?

You have your termonology wrong. Stabilizer is pure CYA. Dichlor is chlorine stabilized with CYA.

I checked PoolMath effects of adding chemicals for both stabilizer and dichlor. Adding 83 grams of stabilizer will indeed add 44 ppm CYA. But adding 83 grams of dichlor will add 22 ppm CYA. So you should have around that much, not enough to test with the Taylor kit.

Recently I purchased the TFT CYA test kit standalone to go with my Taylor K-2006. That one comes with way more test regent and measures down to 20 ppm, much more suitable for a spa kept close to 30 ppm CYA.

Also you didn't ask but seemed a bit confused why you didn't reach the target FC with the specified addition of dichlor. That was because there was a huge chlorine load, and it was eaten up in the time between adding and the time you tested. A couple days of no FC and then a soak or two before 1st addition of FC? I'm surprised you had any FC left when you tested!

 

[Arobbert]

I personally don't bother to test for CYA. You can calculate how much Dichlor is needed to get to 40 and add that amount before switching to liquid chlorine. This can take a while depending on how much Dichlor you use. I prefer to use pure stabilizer so I can add it in one shot and avoid the PH drops caused by repeated use of Dichlor. I now only use stabilizer and liquid chlorine and very rarely dichlor.

Note that "CYA will decay at a rate of around 5ppm per month in a hot tub". I add stabilizer once a month to make up for this loss. I've tested a couple times before draining and the CYA level is in acceptable range.

Remember you need to add oxidizer (chlorine) after every use. The amount needed can vary depending on the number of people, duration of the soak and how clean you are when you get in. I figure about 3.5 PPM per person per bathing hour assuming I start with the correct base level as recommended for my CYA level. You will also need to add extra chlorine to maintain the required free chlorine level until your next soak. You should test before you get in until you get a feel for things. You may need to add more or less depending on your individual situation. Once you get a feel for it you can just repeat the process you know works and reduce your testing frequency.