Learning to test and interpret results

[PhillipH]

New pool owner. I've been reading a ton about pool chem lately, and I'm trying to become competent and confident enough to manage my pool chemistry. You should know that I'm an engineer, and I'm overly technical and I like things to be precise. As you can probably imagine, I've been thrown for a loop by both the test strips and the DPD testing. I was hoping for a nice digital reading with 1/100th precision for every pool chem component. I've given up on that dream, but I was hoping you guys could help me gain confidence in testing my pool water. So . . .

My pool is gunite/plaster and 25k gals. Saltwater system with Aqua Logic and T-CELL-15. I just aquired the property, and the pool was previously maintained by a pool svc company on a weekly basis. The pool water is crystal clear and not alot of junk gets in the pool. The pool guy tells me that the pH is around 8 or a little over, and that's ok (I know you guys are gasping -- in all of my reading, I've never come across one other person who said that). He said that SWG pools naturally have a higher pH and that's ok. That's my background (over the last 3 weeks of learning and talking to the pool guy). So, getting to the point . . .

I want to correct any issues I have and manage things correctly going forward. Unfortunately, I'm really struggling with the test strips (AquaChek) and even with my DPD test kit (Pentair Pro-II -- Rainbow reagents #2, #3, #3b, #4, #5, #6, and #7). The pool stores uses the computerized method of color-matching. I can't get the test strips to even get close. The FC is at the extreme upper end of 10ppm, so it might even be higher than that. I can't even really tell what color the pH matches best to, if any. The stabilizer seems to match best to 150-ish, and that can't be right (I hope). TA looks like 180-240, but even that is hard to tell. See pool store results below.

I guess I feel a bit better about the DPD test kit, but I still don't have much confidence in it yet. FC appears to be up in the 10ppm range, which is the upper limit. This 10ppm reading (or whatever it is) has happened every time I tested (even when the store said 3.5). pH is still showing around 8 or higher. CH and TA seem to be a tad more dependable since you count the number of drops, and the solution changes color abruptly. I get TA=100 and CH=200 on the latest results.

The pool guy had the pump running 24/7 and the chlorinator at 50%. I actually cut it off for a few days (which apparently got the pool store FC reading down to 3.5), but I'm now running 9am-9pm and at 30%. Still learning there.

I just want some consistency and confidence in my readings!

First trip to the pool store:
FC: out of range
pH: 8.0
CH: 152
TA: 126
CYA: 56
Recommendation: add 4 lbs of pH Down (sodium bisulfate I think), add some calcium. I did this.

Second trip to the pool store:
FC: 3.5
pH: 7.9
CH: 200
TA: 130
CYA: 61

Sorry so long! Thanks for any help and/or sympathy.

Phillip

 

[PaulR]

Welcome! As the son of a chemical engineer and a math-major software dude, I can sympathize with wanting precision. Well... There's just fuzziness in pool chemistry; fortunately a little fuzz in the numbers will usually not lead to fuzz in the pool.

Most people here think using strips for basic pool parameters is a total waste of money. The drop-based tests are the way to go. You can check out Test Kits Compared in Pool School for what we think are the best kits around. It looks like your Pentair does DPD chlorine, pH, TA, and CH, so maybe you don't want a whole new test kit. But you will want to augment what you have with a CYA test, and a FAS-DPD chlorine test (not the same as DPD). I believe you can get these tests separately from Taylor, and also from tftestkits.net, if your Pentair supplier doesn't have them. Shop around.

Lacking a FAS-DPD chlorine test, you can dilute your pool water with chlorine-free water (e.g. distilled; tap water often does have chlorine in it) and use the DPD test on the diluted sample in order to extend its range.

I hope you noticed that your CH result from the pool store varied a lot. That shouldn't happen if the test is done correctly; CH varies very slowly if you don't purposely do something about it. As you become comfortable with your own testing you'll find you are doing a better job than the pool store.

Because you have a SWG, you should also read through Water Balance for SWGs to get an idea of what your target ranges ought to be. You already know your pH is too high; if you're comfortable handling it, muriatic acid works as well as bisulfate and doesn't add sulfates to the water.

Well, this has gotten a bit long but it seemed like you wouldn't have a problem with that. Don't hesitate to ask if something is confusing or you see conflicting recommendations. Lots of helpful people here!
--paulr

 

[polyvue]

Comments here to supplement PaulR's excellent advice.

This 10ppm reading (or whatever it is) has happened every time I tested (even when the store said 3.5).

Chlorine can lose potency fairly quickly and I wonder if that might explain the discrepancy in your strip/DPD test results that showed FC 10ppm, presumably conducted at poolside or nearby, and the testing of your water sample performed by the pool store, some distance away. Whatever the reason, you won't be disappointed in the precision offered by a FAS-DPD chlorine test.

The stabilizer seems to match best to 150-ish, and that can't be right

Since CYA isn't likely to have fallen out of solution on the way to the pool store, their turbidity test results may be closer to reality than your color-match w/test strips. Both tests (IMHO) are fuzzy. Test strips force you to choose between remarkably similar color blotches; the turbidity test demands an even more subjective determination: adjudge whether a little black dot has "disappeared" in solution. I've found both tests to be maddeningly imprecise, but I recommend purchase of the CYA turbidity test so you can (in)validate tests done at pool store and with AquaChek strips. If your stabilizer (CYA) level is over 90 ppm, you're going to have to drain pool water to reduce it.

I can't get the test strips to even get close.

Throw 'em out! Or just use them as a backup for a while. If you're not too happy with your current Pentair kit, you can save some money by investing in a new FAS-DPD complete test kit, which contains both FAS-DPD chlorine and CYA turbidity in addition to the other standard titrants and reagents.

Good luck to you. You'll get lots of help here, so you won't have to rely too much on sympathy!

Greg

 

[frustratedpoolmom]

Are you sure your kit tests chlorine up to 10? That's unusual for a DPD. Perhaps its 5 for chlorine and 10 for Br (Bromine)?

IMHO I would replace your kit with one of the recommended test kits - Paul gave you the link.

 

[PhillipH]

That's what I was hoping to hear about the strips. I didn't know how I could be messing that up, yet the results were just crappy to be honest. They weren't logical, and they were hard to interpret.

My Pentair DPD kit does test for CYA, and it's the turbidity test. Again, very hard for me to figure out what the precise number is. Is the sun/brightness affecting it, am I holding the stick at the right angle, etc, etc. I imagine I'll get better as I gain experience.

I hadn't heard of the FAS-DPD chlorine test. I like the sound of using titration vs eyeball color-matching. I will definitely look into this.

Sorry if I wasn't clear, but I did add some calcium per the pool store's advice. That's presumably what brought up the CA level (note: I have a gunite/plaster pool and wanted it higher than 152). Also, my DPD kit says "water hardness", but the pool store says "calcium hardness". I've read that magnesium is also measured in the "water hardness" test, so if true, I guess I need to always be aware of an expected discrepancy?

My DPD kit has the following on the chlorine side, and it claims to be able to measure FC and TC -- although there is no possible way you could measure a difference that is much less than about 2ppm. The chlorine scale shows the following I believe: 0.5, 1.0, 1.5, 2.0, 3.0, 5.0, and 10.0.

I'll plan on getting one of the 2 test kits mentioned. Once I drop $70 for the nice kit, I assume I can get reagent refills somewhere?

Any opinions on the AquaChek TruTest (or any other competitors)? I guess it's a poor man's version of digitizing the color-matching. They claim good accuracy, but I have a hard time believing it since they are using test strips to begin with. Maybe there are dependable competitors in this area? Do most people just use the DPD kits?

Thanks alot!

Phillip

 

[polyvue]

my DPD kit says "water hardness", but the pool store says "calcium hardness". I've read that magnesium is also measured in the "water hardness" test, so if true, I guess I need to always be aware of an expected discrepancy?

Yes. Substituting Water Hardness [aka Total Hardness or TH] for Calcium Hardness in effect overreports CH and could impact the calcuations in CSI/LSI. Not the end of the world. Earlier this year I tried to correlate Calcium Hardness and Total Hardness via coincident tests but got results proving every possible conclusion (CH>TH at times, CH=TH once, CH<TH other times.) Anyway, here's what the chemistry section of the Pool Calculator, found useful by many in this forum, says about this:

Total hardness is the sum of calcium hardness and magnesium hardness. Most test strips report TH instead of CH. The ratio of calcium to magnesium varies. As an approximation you can multiply TH by two thirds to get a rough estimate of CH. http://www.poolcalculator.com/chemistry.html#CH

My DPD kit has the following on the chlorine side, and it claims to be able to measure FC and TC -- although there is no possible way you could measure a difference that is much less than about 2ppm.

Using FAS-DPD, test a 25mL sample to measure TC, FC and CC within .2 ppm (testing with 10mL sample provides .5 ppm) Pretty good, huh?

Both Taylor FAS-DPD products highlighted in my previous post identify chlorine levels in the 0.2 - 25.0+ ppm range. Ditto for Leslie's FAS-DPD Service Kit, a re-branded Taylor K-2006. The TF100 appears to provide the same precision and sells refill kits on its website. Refills for Taylor and Leslie's kits are widely available. Hope this information helps.

 

[PaulR]

Both Taylor FAS-DPD products highlighted in my previous post identify chlorine levels in the 0.2 - 25.0+ ppm range. Ditto for Leslie's FAS-DPD Service Kit, a re-branded Taylor K-2006. The TF100 appears to provide the same precision and sells refill kits on its website. Refills for Taylor and Leslie's kits are widely available. Hope this information helps.

Given that the TF100 uses all Taylor reagents, it should produce results identical to the K-2006 or its Leslie's equivalent.

My local Leslie's carries refills for at least R-0001 thru R-0012, plus pH and OTO; haven't asked for R-0013 or the FAS-DPD ingredients.
--paulr