What should I believe? PoolMath or Taylor CYA test?

[DangerBoy]

I've been doing the TFP chlorine spa startup method for the past week. Over the last 5 or 6 days I've been maintaing my FC levels at about 6.0 by testing and adding small amounts of dichlor each day and keeping track of the number of grams of dichlor that I've added. Today, I added the last 8 grams which brought me to an accumulated total of 133g. According to PoolMath Effect of Chemicals calculator, that should've brought the CYA level up to 35 ppm which is where I want to hold it at.

I let the tub circulate with both jut pumps running for about 10 minutes after adding the dichlor and then tested for CYA. I got a result of less than 30 ppm CYA. If I had to guess, I'd say it was about 27-ish ppm but my comparator only goes as low as 30 so it's kind of hard to guess exactly.

So what should I believe here? The math says I should be right around 35 ppm. The test says CYA is maybe only around 27 ppm plus or minus whatever the margin of error is on that test which I suspect is not insignificant. Should I add a little more dichlor over the next few days until I finally do get a test reading in the mid thirties or should I just go with what the math says and switch to using only bleach for chlorine now that I've hit my theoretical target of 35 ppm CYA. Or should I have circulated more/waited longer to test after adding the last bit of dichlor?

 

[Texas Splash]

DB, the PoolMath tool is great for evaluating what effects of adding chemicals should be. However something we cannot be certain of in all cases is the accuracy from the manufacturer of the product being used. While it "should" produce x-percentage amount added, it's always possible the batch was slightly off when produced. Also, remember that CYA tests don't always register immediately. The dissolved dichlor should show rather quickly, but it may still need another day or two. Let's not forget the CYA tests have between a 10-15% variance due to the complexity of the test and watching the black dot disappear. Also note the viewing tube isn't designed to try and split a CYA result in half. Generally if it looks like it's halfway between two "10th" levels, we round up. So if (for example) it looked like 35, I'd be inclined to call it 40. Sounds like you're very close. As you noted, it might be good to simply go the bleach route for a couple more days and retest the CYA to see what you get. Hope that helps.

CYA Testing:
Proper lighting is critical for the CYA test, so you want to test for CYA outside on a bright sunny day. Taylor recommends standing outside with your back to the sun and the view tube in the shade of your body. Use the mixing bottle to combine/gently mix the required amounts of pool water and R-0013 reagent, let sit for 30 seconds, then gently mix again. Then, while holding the skinny tube with the black dot at waist level, begin squirting the mixed solution into the skinny tube. Watch the black dot until it completely disappears. If it helps, pour a little, look away, then look back and pour some more. Once it disappears, record the CYA reading. After the first test, you can pour the mixed solution from the skinny view tube back to the mixing bottle, shake, and do the same test a second, third, or fourth time to instill consistency in your technique, become more comfortable with the testing, and validate the CYA reading. If you are still questioning your own results, have a friend or two do the same test 2-3 times. Share your results only after everyone is done to see if you came up with the same average results.

 

[ajw22]

So what should I believe here? The math says I should be right around 35 ppm. The test says CYA is maybe only around 27 ppm plus or minus whatever the margin of error is on that test which I suspect is not insignificant.

CYA scale is logarithmic, not linear. Halfway between is not 5. You cannot eyeball values between the 10's for that reason. So you always should round up to the next highest value.

The precision of the CYA test tube is to the 10's at most.