CYA at lower levels, trust the test or the amount of Dichlor added?

[SchralphMacchio]

I'm using Dichlor-then-bleach for a 430 gallon spa with EOS ozone generator.
Over the course of a week following water change, I added about 5.5 cumulative TBSP of Dichlor. This should have gotten me to just over 20 PPM of CYA.
However, using the 30 mL CYA test from TFTestKits I have the dot fairly visible with the test cylinder full (picture taken in shade outside on a bright day).



This causes me the following issues:

• Not totally confident about ideal target / shock / SLAM FC levels
• Unable to track CYA stability - which should theoretically be very stable, but my spa is placed right at my outdoor landscaping so it is subject to sprinkler spray onto the cover and lots of insects that can add to contamination

I'm was hoping to target 20 PPM of CYA, which seems like a good compromise of kill time and chlorine stabilization. I've read the Orenda blog post on the topic where CDC doesn't recommend going above 15 PPM as kill times are greatly increased while chlorine UV-stabilization barely increases. Our spa is often used during daylight hours and it's much easier for me to maintain comfortable chlorine levels with some CYA than none.

It seems like I should trust the stoichiometry and just go by the total mass of Dichlor added, and assume the CYA to be stable, and forget the test.
I'm wondering if there are good perspectives counter to that argument?

 

[Newdude]

20 is sketchy at best to read. One of the test tubes doesn't even have it, only the 30 mark and above.

Long term it's anyone's guess how long it remains, losing a couple parts per month. But when you first fill, you can trust you added 0- 30 (or any amount) if you are sure of your gallons.

 

[SchralphMacchio]

20 is sketchy at best to read. One of the test tubes doesn't even have it, only the 30 mark and above.

Long term it's anyone's guess how long it remains, losing a couple parts per month. But when you first fill, you can trust you added 0- 30 (or any amount) if you are sure of your gallons.

Thanks. I think I have high confidence of my gallons (spec sheet specifically says it's to the fill line), and also I can have high confidence in cumulative PPM FC added by using the FAS-DPD test 30 minutes after adding Dichlor. For example, my initial Dichlor addition of 2 TBSP got me a measured FC of 10 PPM, which more or less was expected based on the stoich inputs and 430 gallon assumption. This gave me confidence that I didn't need to carefully weigh my Dichlor granules to estimate accumulated CYA, I could just keep going by quick volume measurements.

5 TBSP accumulated should have gotten me to ~25 PPM FC and ~22 PPM CYA, but then the 30 mL CYA test tube wasn't fully clouded. I then added another 0.5 TBSP of Dichlor and re-did the CYA test later on (weeks later actually), but the visual CYA test results were mostly the same.

I guess I'll forget about the CYA test for now, and my CYA depletion sensor will be when 5 FC of chlorine starts feeling harsh on my skin again, like it did early into the Dichlor accumulation!

 

[5tan]

My only suggestion is to get a kitchen scale with 0.1g precision, and measure the mass of your dichlor, instead of going by volume (tablespoons). As many professional bakers know, each tablespoon/teaspoon/cup you scoop of granulated/powdered ingredients can be significantly different in mass, which is why many bakers measure all of their ingredients in grams to maintain quality standards relating to recipe reproducability.

For my 1643L spa, according to PoolMath, I need 65g of dichlor to mathematically reach 20ppm CYA. You could try test strips that measure stabilizer if you wanted a second opinion to your other test kit. If you want to be really certain of your stabilizer levels on a fresh fill, forget dichlor and buy pure stabilizer (usually for pools). Add the specified number of grams of pure stabilizer for your water volume and you're good to go.