Am I Using the FAS/DPD Correctly?

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zollinmj

Member
May 6, 2024
22
Virginia, USA
👋 I finally found my way here after owning a pool for a couple years now! I'm trying to correct a few challenges I've been having and mistakes I've made, mostly keeping my pH in check, but plenty more I'm learning.

Context For Today: I've been using the Taylor K-2005 kit all along, but just bought the FAS/DPD supplement but I'm not sure I'm doing it right! I'm getting a reading of 17-18 PPM with it whereas I get a reading of 5ish PPM with the Taylor kit I've been using.

Just to confirm the FAS/DPD steps:
1. Fill to 10mL
2. Add 1 scoop R-0870 (that thing is small :) )
3. Mix by swirling (this state is the picture below)
4. Add R-0871 drop-wise while swirling until the liquid returns to clear
- This has been reliably 34-36 drops. I've tested four times over 3 days.

Relevant Info:
- pH: 7.4
- TA: 70
- FC: 17? 5? I don't know!!! 😆
- CYA: 70
- SWG: running at 30%
- Salt: 2800
- Pumping running almost 24hours
- CH: 225
- Size: 17,000 gallons

Questions:
1. Am I missing something obvious?
2. Any suggestions about what to do?
- I know the understanding here at TFP is to keep chlorine higher than industry recommends, but this seems a bit much! I'm probably going to dial the SWG to 10% to see what happens, but I'd like to understand the VAST discrepancy between the measurements I have before doing too much tinkering.

FAS_DPD-testing.png taylor-results.png
 
I'd trust the FAS DPD test over the color match test. Your description of the test steps for FAS DPD appears correct. The precision of the color match test is far less than the FAS DPD.

Sounds like your on the right path with reducing SWG output for this time of year to get back closer to 10FC on the FAS DPD test.

Also, just to prove to yourself that the scoop, though small, is more than sufficient..... after you complete the FAS DPD test (clear) add more pool water. Color returns which means you had more than enough colorizing reagent in the test vial.
 
The PH side of the testing block is on the right side. The left side is for chlorine/ bromine. The only part of the picture that puzzles me is I haven't ever see the left side with the pink color as it's always been shades of yellow.
 
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Any chance you're using the phenol red / R0014 instead of the R0002 ?

Vague accuracy aside, It's easy to grab the wrong bottle. Ask me how I know. 😁
 
PoolGate said:
Yeah this is quite odd. I would get a new test kit to make sure it is not your reagents.
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Yeah, I might, but I just bought these! 😀 . Any opinion about using the local pool store as a third opinion to check my results?

Rancho Cost-a-Lotta said:
Try testing your tap water and see what happens. Sounds like you're doing everything right. Just make sure you're getting your water sample away from the returns and at an elbow's length below water.
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Yeah, good thoughts and reminder about sample location. I was actually getting the sample near the return (elbow deep). I tried again on the far side of the pool and got the same result. I'll have to try the tap water to see.

bobloblaw42 said:
I'd trust the FAS DPD test over the color match test. Your description of the test steps for FAS DPD appears correct. The precision of the color match test is far less than the FAS DPD.

Sounds like your on the right path with reducing SWG output for this time of year to get back closer to 10FC on the FAS DPD test.

Also, just to prove to yourself that the scoop, though small, is more than sufficient..... after you complete the FAS DPD test (clear) add more pool water. Color returns which means you had more than enough colorizing reagent in the test vial.
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Yeah, I went ahead and lowered the SWG to 10%, will check it out again over the next few days to see if it's dropping. Good thought on re-adding the pool water, I'll try that next time.
 
Approaching this from another angle :

What is your SWG make/model, what is it set to power-wise - and is that the only source of chlorination currently being used ?

getting up to 17ppm+ if you're only running the SWG at 30% seems unlikely - unless the SWG is seriously over-sized or the pool is in the basement with the drapes drawn....
 
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Newdude said:
Any chance you're using the phenol red / R0014 instead of the R0002 ?

Vague accuracy aside, It's easy to grab the wrong bottle. Ask me how I know. 😁
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Hehe, I'm sure I've grabbed the wrong bottle a number of times, but in this case I think I'm good. I've done it a number of times with the same result and all my caps are color coded (also I can't even find R0014, I have R0004 -- for pH, but I don't think I've done that wrong 5 times 😆)
 
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Is the volume for your pH test 44ml or thereabouts? If so, R0004 is the correct reagent. If its something much lower, then R0014 would be the correct reagent.
 
SJPoe said:
Approaching this from another angle :

What is your SWG make/model, what is it set to power-wise - and is that the only source of chlorination currently being used ?

getting up to 17ppm+ if you're only running the SWG at 30% seems unlikely - unless the SWG is seriously over-sized or the pool is in the basement with the drapes drawn....
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Yeah, embarrassingly I don't actually know to be honest. Hayward but unsure of the model. Here's pictures. I just looked up the contract (and I didn't know anything at the time) and it doesn't list a specific model, just SWG. Any chance you can spot the model from these pictures?

There is no other source of chlorine and the generator has been set to 30% until this morning, when I turned it down to 10%.

IMG_2613.jpeg

IMG_2608.jpegIMG_2623.jpeg
 
Newdude said:
Yes.

Very. Very. Strong opinions.

Don't. 😁
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Oh interesting. I know in general pool stores don't fall in line with the approach around here for lots of reasons, but I was hoping just for measurement sake they could provide some useful information.

If it matters, my local store is Swim Gem and it doesn't LOOK like anyone is doing anything manually. It seems like they just drop the sample in a machine and it spits out numbers (and of course recommendations), but I'll pay closer attention when (if?) I go back.
 
Hayward Aquarite, with (I'm no expert, but that looks like) a T-Cell-15 equivalent, which would push about 1.5lb/chlorine a day at 100%

You can punch that into Pool Math to get an idea of how much chlorine is being generated in your 17,000 gallon pool, which comes out to about 3ppm/day at 30%.

Given the average pool would be losing 2ppm or more a day right now....hard to believe you're up to 17/18ppm
 
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zollinmj said:
but I was hoping just for measurement sake they could provide some useful information.
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All it will do is make more questions.

Their inaccuracies are WELL documented here, thousands and thousands of times. Don't take my word for it, read up from members just like yourself, until you believe it too. Then read more, some more, and some more after that to cement the thought.

*if they agree with you, you'll question why they weren't more off.

*If they don't agree with you, you'll question your more accurate results.

It's a no win. Plus the whole too many cooks thing. Don't double check with test strips, your neighbors way, or the local garbage man. It'll only add more confusion.

Use the fas/dpd powder test and believe it.
 
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Newdude said:
All it will do is make more questions.

Their inaccuracies are WELL documented here, thousands and thousands of times. Don't take my word for it, read up from members just like yourself, until you believe it too. Then read more, some more, and some more after that to cement the thought.

*if they agree with you, you'll question why they weren't more off.

*If they don't agree with you, you'll question your more accurate results.

It's a no win. Plus the whole too many cooks thing. Don't double check with test strips, your neighbors way, or the local garbage man. It'll only add more confusion.

Use the fas/dpd powder test and believe it.
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haha, well said. I imagine I'll get there soon.
 
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