Overthinking the DPD powder dose

Brad_C

Well-known member
Nov 15, 2018
134
Perth, Western Australia
G'day all,

I find myself going through far more DPD powder than is probably reasonable when doing the FAS titration.

My FC level is always so high I use a 5ml sample for 1ppm/drop, but I've still been using a heaped scoop of DPD powder.

A couple of days ago I thought I'd run a couple of tests to see how little powder I can get away with and I want to make sure I have my understanding straight.
The DPD powder is a buffer and the DPD indicator. So, provided there is enough powder there to adequately buffer the ph and turn it pink then that'll be enough? Right?

I did a couple of tests, and using a 5ml sample I can get down to ~1/4 of a scoop and still get a reliable result (ie it's the same result as doing the same test with the same water qty and a heaped scoop).

Am I right in thinking the DPD is being used as an indicator in this reaction? It's not like the borate mannitol test where you need enough mannitol to completely react with the borate.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,889
Tucson, AZ
The R-0870 powder is a mixture of several different dry compounds - phosphate buffers, pH adjuster, metal ion chelator and the DPD indicator dye. The DPD only makes up about 2% of the dry weight of the mixture. The requirement to use 2 scoops with a 25mL sample is to ensure that there is both enough dye to react with the chlorine in the water as well as to buffer the pH of the water sample to the correct level (it lowers the pH of the resulting mixture). To little DPD with high FC risks a false low result because you may not be adding enough dye and too little powder for the sample volume could cause the pH to be off which risks creating false-low CC results and other problems with the test.

I realize reagents can be expensive and hard to get in some areas but trying to minimize usage like you are doing, as well as using such a small sample volume, risks making your test results meaningless.

Perhaps you should test less frequently and/or lower your FC level....
 

Brad_C

Well-known member
Nov 15, 2018
134
Perth, Western Australia
I realize reagents can be expensive and hard to get in some areas but trying to minimize usage like you are doing, as well as using such a small sample volume, risks making your test results meaningless.
It's not as bad as it seems. The Clear Choice Labs test uses a 10ml sample as standard, so I'm only using half of that.

Perhaps you should test less frequently and/or lower your FC level....
That's what I'm working towards. (un)fortunately following the TFP method over the last 6 months has dropped my daily chlorine consumption down so far I'm struggling to reliably get the SWG set to somewhere vaguely maintainable. Sure, it's a nice problem to have, but as of this morning the FC is at 16. The Chlorinator is set to 60% and I filter 4 hours in the evening.

We're coming off-season now, so the sunlight is eating less chlorine. Last year (and for the 20 before that) I just left the chlorinator up and the pool would eat everything I threw at it. Now I'm in that learning curve of trying to get it dialed in. Don't test it for 3 days? Find the FC has jumped 9PPM.

So it's about getting the best value from the reagents while I get this stuff figured out, and learning I don't have to tip anywhere near as much powder in as I have been.

Appreciate the input.
 

cfherrman

TFP Guide
May 10, 2017
2,385
Hays, Kansas
The 5 mil test can be +/- 5 ppm easy, I really don't like when people use it, Matt is correct your putting a band aid on a migraine.

Use the 10 mil test with a healing scoop, or you will be chasing your tail from testing error
 

Brad_C

Well-known member
Nov 15, 2018
134
Perth, Western Australia
The 5 mil test can be +/- 5 ppm easy, I really don't like when people use it, Matt is correct your putting a band aid on a migraine.

Use the 10 mil test with a healing scoop, or you will be chasing your tail from testing error
With the Clear Choice Labs test kit, the standard test is 10ml with 0.5ppm resolution therefore a 5ml test is 1ppm.
Oddly enough I haven't actually seen any testing error. Repeated tests varying the sample size and powder volumes always come out within the margin of error (1ppm 5ml - 0.5ppm 10ml), which is why I was wondering how little I could get away with. Turns out I can probably reliably use 1/2 the amount I was using, but lower than that I risk inadequate pH buffering.

Matts reply got me off my backside to check out the chemistry, so I now have a better understanding of exactly what is going on. Any day you learn something is a good day.
 

RoyR

Bronze Supporter
Jul 31, 2018
247
Escondido/CA
It would be interesting to see what people are running at...I have a 16500gal pool, with an XF pump running at 2050rpm, from 7-5pm, with The IC60 set at 30% in sunny San Diego.

FC 3-4 ppm
PH 7.8
TA 90
CH - 350
CYA - 60-70
Salt 3400 ppm
Temp - 71
 
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the_sphynx

Well-known member
Feb 19, 2019
60
North San Diego County
Roy,
I am filtering from 8a-5p mainly on low speeds but an hour in there at a higher rate. Have the IC40 set at 40% and my FC is about 6-8ppm. The pool builder left it at 50% but my FC was pretty high IMHO (about 10-12ppm) so I ratcheted it back a bit. Still playing with it to dial it in and suspect that as it starts getting sunnier I'll have to bump it. I am in Valley Center BTW.
 

the_sphynx

Well-known member
Feb 19, 2019
60
North San Diego County
Marty,
That's cool. Had no idea SWG was an option on the app until you said something. Looks like with my IC-40 at 40% running for 9 hours, it will generate 1ppm of FC per day. I have read the same FC for the past couple of tests so I guess I have found the stable mark for this time of year.

No swimmers yet other than the dog for about 5 minutes the other day :). Thanks for all of the help you have provided to me and others here.
 

Teald024

TFP Guide
The Chlorinator is set to 60% and I filter 4 hours in the evening.
@Brad_C . If you are able to maintain the FC at a constant level, then the SWG is set appropriately for your usage. You have two options: turn off the SWG for a couple of days until FC drops to where you want it, OR dial back the SWG to a much lower level for a week until it drops to where you want it. Once you get to the desired FC/ CYA ratio, then turn the SWG back to 60%.
 

Brad_C

Well-known member
Nov 15, 2018
134
Perth, Western Australia
@Brad_C . If you are able to maintain the FC at a constant level, then the SWG is set appropriately for your usage.
That's the problem. I can't maintain the FC at a constant level, so I'm having to do frequent testing and adjustment to try and find that SWG setting which provides consistency. I planned ahead when I set the system up, and my SWG can contribute up to 30ppm/day to the pool, so that allows me to really tune the filter time and SWG output to cope with anything.

Having said that, I don't want to have to keep turning it off but I also need to know/trust that a good blow (which drops a large amount of organic matter into the pool) isn't going to tank the FC. So I want to keep it on the high side while I get it figured out, but 18 is a lot higher than I'd like.

It's just a matter of getting a handle on the pool as we transition to the off season, as leaving the chlorinator on a setting that worked for summer (and previously in winter) has seen the FC head towards shock level. I just need more frequent tests as I learn about how the pool responds with the current chemical balance.

Pre discovering TFP (so last October), the pool generally had a CYA of 30, TA of > 250 and no borates). It was *easy* to look after if I remembered to test and add acid every 2 days, and left the chlorinator wound up. It just used everything I put in it. Since then I'm down to a TA of 90 (was 60 but our fill water is really high), CYA of 80 and 55ppm borates. It's a completely different pool, and now if I forget to look at it the pH stays stable and the FC goes through the roof. So it's just adjusting to the new behaviour and trying to learn where the chlorinator needs to be to maintain a sane FC level. Unfortunately at high FC levels the test just eats reagent (at 18 one bottle of titrant would last me less than a month for daily tests). So I'm also learning where I can compromise accuracy in the interest of conservation (for example I don't really care if it's 16 or 18, I just need to know it's > 10).
 

Teald024

TFP Guide
A bit off topic, but I think that you're overthinking it. With a CYA of 80, you only need target FC at 6ppm with a min of 4ppm. Drop the SWG to 30%. It's not going from 18ppm to 4ppm in a day in a clear pool. Let it ride for a couple of days and check again. With that monster SWG you can get FC pack up quick if you need to.
If you can really make 30ppm a day (1.25 lbs in 5000 gallons) then you only need to run at most ~2.5 hours at 100% (or bit over 4 hours @ 60%) in the summer. Much less now that end of season is approaching. I'd estimate you only need to run at 30% for 4 hours right now to break even.
 

Brad_C

Well-known member
Nov 15, 2018
134
Perth, Western Australia
A bit off topic, but I think that you're overthinking it.
....
I'd estimate you only need to run at 30% for 4 hours right now to break even.
Not off topic at all. I *am* most certainly overthinking it and I think your sums sound about right. I'll let it fall to 8 or so then wind the chlorinator up to 30% and see how it rides.

Ta.