Why am I getting differences in Chlorine levels with 3 different samples taken at the same time.

[rbwamsley]

Good morning.

I'm currently SLAMing my pool and this morning I was testing for OCLT and I got 3 different results.

Notes on samples:

• Filter pump and cleaner pump had been running all night and were still running.
• All samples were pulled with in 10 min of each other.
• All samples were pulled at the same depth (waterline up to mid bicep)
• 2 month old TFPro kit (so reagents are good)
• Using auto stirrer
• FAS-DPD test with 10ml of water

Sample 1 = 24ppm
Sample 2 = 27ppm
Sample 3 = 25.5ppm

I don't understand how I'm getting such a large range of differences. Is this normal?

 

[Texas Splash]

Besides perhaps a very slight testing variance, the only things I can think of are:
- Water sample not retrieved from same location (sound like you do though)
- Collection vile or container dirty (but your TFP Pro vile is new so that "shouldn't" be an issue); might clean any items though to be sure
- Inconsistent drop rate/sizes of R-0871

 

[Newdude]

Inconsistent drop rate/sizes of R-0871

^^^^^^^ My vote.

Did you really make 48-54 perfect drops ? You can do your absolute best to slow it down and be as consistant as possible, but you are human. Personally I blame the morning cup of coffee because many people have problems with AM consistency. We get asked once a week why someone can't pass their OCLT a week after the SLAM should be over. If they don't drink coffee/tea then maybe those folks should.

When the time comes that you believe you are done, and have scoured every inch of the pool, every nook and cranny, every last inch..... and have STUPID clear water, you can let the FC fall into target range and try the OCLT when you only have to make 10-15 drops.

 

[rbwamsley]

Inconsistent drop rate/sizes of R-0871

Out of all those I would say this is highest possibility. I am going faster than normal as I'm dropping ~50 drops, but I'm doing that on all the samples.

It's quite annoying that there is so much deviation in my results (especially during a SLAM). I ended up taking the average of all 3 numbers as my result which still put me as passing the OCLT. However, I am worried that my test the previous night was inaccurate.

 

[Texas Splash]

It's quite annoying that there is so much deviation in my results (especially during a SLAM). I ended up taking the average of all 3 numbers as my result which still put me as passing the OCLT. However, I am worried that my test the previous night was inaccurate.

High SLAM levels can be a little tricky with more potential for variance. If you believe that you have completed the SLAM's 3 criteria, you do have the option of performing another OCLT in a day or two when the FC falls to a lower level. It might give you peace of mind and reassurance that your water is indeed all clear.

 

[rbwamsley]

Did you really make 48-54 perfect drops ? You can do your absolute best to slow it down and be as consistant as possible, but you are human. Personally I blame the morning cup of coffee because many people have problems with AM consistency. We get asked once a week why someone can't pass their OCLT a week after the SLAM should be over. If they don't drink coffee/tea then maybe those folks should.

I do blame lack of coffee for a lot of things.

When the time comes that you believe you are done, and have scoured every inch of the pool, every nook and cranny, every last inch..... and have STUPID clear water, you can let the FC fall into target range and try the OCLT when you only have to make 10-15 drops.

I started my SLAM last Thursday night already having "stupid clear water". I have algae that only reappears every 7-10 days. My only sign will be the OCLT. That is the problem...

I increased my FC to 35 this morning before I left for work to hit mustard algae levels. I won't be able to add more chlorine until ~5PM, so hopefully it stays quite elevated until then. I don't know how everyone with commuting jobs maintains their levels during the day. I'm doing my best...

 

[Newdude]

I started my SLAM last Thursday night already having "stupid clear water". I have algae that only reappears every 7-10 days. My only sign will be the OCLT. That is the problem...

Yes. And the OCLT at normal levels is much easier to be consistant.

The timing of it all always makes me wonder during pollen and farm planting season.

It could also be dead zones in those spots with little or no circulation thay just need more frequent brushing than the rest of the pool.

Going forward you would be well served to run 'hot' targeting 1 or 2 PPM over target level to ensure you never fall below it. With ongoing concerns, you need to pay extra close attention to the FC level compared to the folks who have it easier.

Time will tell. Keep us posted.

 

[rbwamsley]

The timing of it all always makes me wonder during pollen and farm planting season.

I actually posted a video in another thread of my algae problem, to make sure it was actually algae. Also, I'm only getting it on walls that get the most shade (none on the floor).

 

[rbwamsley]

Going forward you would be well served to run 'hot' targeting 1 or 2 PPM over target level to ensure you never fall below it. With ongoing concerns, you need to pay extra close attention to the FC level compared to the folks who have it easier.

Yep. That is the plan! Time permitting....

 

[Mdragger88]

It’s definitely alot of drops to keep track of 1st thing in the morning! Gotta hold your tongue the same both times lol
As @Newdude said - If you are suspecting mustard algae after the 24hrs is up be sure to keep fc above mustard algae minimum for your cya (9ppm) for a while - some need to maintain this longer than others. Heres the chart for easy reference

Chlorine/CYA Chart

EDIT: You will find a simplified and standardized version of this chart at Pool School, Chlorine / CYA Chart. Chlorine/CYA Chart by Chemgeek FC = Free Chlorine in parts-per-million (ppm) CYA = Cyanuric Acid in parts-per-million (ppm) CYA ........... Min FC3 ..... Target FC ...... Yel/Mstrd...

www.troublefreepool.com

 

[Bperry]

The test itself has a specific error tolerance but I don’t know what it is. I suspect the 10ml test has a larger error tolerance than the 25ml test so that’s possibly a reason.

One way to check is to test the same sample of water 3 times and see what you get. If you get different results and use a consistent drop method, any differences should be accounted for by testing tolerance.

 

[Jimrahbe]

rb,

Sorry, but I just don't see the issue. When doing a SLAM it is not all that important to keep you FC exactly at a fixed number. As long as you are keeping your FC above the SLAM level it is just fine. 24 to 27 is not that big of a deal.

Thanks,

Jim R.

 

[rbwamsley]

rb,

Sorry, but I just don't see the issue. When doing a SLAM it is not all that important to keep you FC exactly at a fixed number. As long as you are keeping your FC above the SLAM level it is just fine. 24 to 27 is not that big of a deal.

Thanks,

Jim R.

I guess I never thought of it this way. My assumption was that high chlorine levels could cause problems with equipment and the pool and based on you CYA, X was the amount you should keep it around. I was always trying to keep it at X and not go above that level.

 

[Mdragger88]

A slight scootch above (1-2ppm)is not an issue. Anymore just wastes chlorine & risks damage to equipment, surfaces & swimmers.