Not getting a chlorine reading

[Kcb10]

Hi,

I have an in ground spa which we don’t heat and use as a pool. I estimate it is around 1700 gallons (this was sort of tricky to calculate with the wrap around bench and stairs).

Lately, I have noticed the need to add chlorine more often after starting to use stabilized powdered chlorine. Pool store keeps saying it’s the heat, but that didn’t make sense to me since this is stabilized and prior to that liquid chlorine was lasting longer. My CYA level last I checked was 89 and I was told that was within the limits, but I could pour in 5 or 6 times the recommended chlorine and still get a 0 reading after an hour. I drained water today and refilled and lo and behold without adding anything the chlorine read was up. My main question/ concern is how do I truly know how much chlorine is in there? I understand it was being “locked” or held up by the CYA, but I was adding a lot daily so isn’t it still “in” there? Don’t want to send the kids swimming if in reality the chlorine levels are high. I do plan to have the CYA checked again to see what it came down to.

 

[randytsuch]

You need to get a good test kit (test kits compared)
Then test yourself, don't trust pool store numbers. That is how you know exactly what's in your water.

Randy

 

[Kcb10]

Right, I have a Taylor kit. I guess I was just worried all that chlorine was in there even if I wasn’t getting a reading because of the stabilizer and could be harmful.

 

[randytsuch]

Right, I have a Taylor kit. I guess I was just worried all that chlorine was in there even if I wasn’t getting a reading because of the stabilizer and could be harmful.

Taylor with FAS/DPD test? That's the best way to measure CL.

CYA will not throw off your readings.
Since you just refilled, your CYA should be zero now. A high CYA will make your spa hard to maintain without algae, but it won't mess up the FAS test results.

Randy

 

[mgtfp]

A proportion of the chlorine in the pool forms a chemical bond with CYA, that's how chlorine gets protected by CYA from UV. This chlorine doesn't actively participate in killing germs and algae anymore, but is held in reserve. It still shows up as part of FC in the FAS-DPD test. As more "active" chlorine gets used up, more "reserve" chlorine gets released.

That's why it's important to adjust FC according to the CYA level, following the FC/CYA Levels. Same ratios of FC/CYA provide same amounts of "active" chlorine.

With no CYA at all, FC has a half life under UV light of something like half an hour, so you do need some.

Welcome to TFP! A good starting point to understanding the TFP way of pool care is by reading through the Pool Care Basics.