TDS Meter and Phenol Red Disagreeing

[s0cr4t1c]

Second Edit: Now this is really weird. TDS meter reads 8.4 in neighbor’s pool but phenol red test reads 7.4. In my pool, the TDS meter is reading .3+ LOWER, but in his pool it’s reading a full 1.0 HIGHER than the chemical test. I can’t make any sense of this.

Edit: I just tested with a second phenol red test I have lying around and it’s closer to 7.8. I’m inclined to believe my TDS meter.

As the title says, I’m getting very different results from my tests. I’ve recalibrated the TDS meter a couple of times even swapping out for new calibration solutions, but it still consistently reads lower than the phenol red would suggest. Today, TDS says 7.7, but chemical test reads 8+. So which should I trust?

FC - 5
CC -0
CYA - 80
ALK -80
CH - 350
Borate - 40
pH - ?

 

[TinFoilHat]

I would believe the reagent drop test. Why do you have a TDS meter when CH doesn't need to be tested often enough to warrant such a meter?

 

[ajw22]

TDS meter is different then a pH meter.

TDS is Total Dissolved Solids which measures TA and other chemicals together - Read TDS - Further Reading

What pH meter are you trying to use?

I would trust the chemical drop test.

 

[mas985]

A TDS meter measures Total Dissolved Solids, not PH. I think you mean you have a PH meter or perhaps a combo TDS/PH meter.

PH meters need frequent calibration to measure properly and drop kit reagents require proper storage and periodic replacement. How old is the PH drop kit reagents and do you store it outside or inside?

 

[s0cr4t1c]

TDS meter is different then a pH meter.

TDS is Total Dissolved Solids which measures TA and other chemicals together - Read TDS - Further Reading

What pH meter are you trying to use?

I would trust the chemical drop test.

I use a TDS meter for salt and pH.

A TDS meter measures Total Dissolved Solids, not PH. I think you mean you have a PH meter or perhaps a combo TDS/PH meter.

PH meters need frequent calibration to measure properly and drop kit reagents require proper storage and periodic replacement. How old is the PH drop kit reagents and do you store it outside or inside?

I have a TDS meter with salt and pH functions. There was little disagreement between the meter and phenol red for months. Now they’ve diverged. As I edited up top, another test kit I had available reads closer to the TDS meter.

 

[s0cr4t1c]

A TDS meter measures Total Dissolved Solids, not PH. I think you mean you have a PH meter or perhaps a combo TDS/PH meter.

PH meters need frequent calibration to measure properly and drop kit reagents require proper storage and periodic replacement. How old is the PH drop kit reagents and do you store it outside or inside?

Drop kit is about 8 months old. Has been stored inside in a cool dark place.

 

[mas985]

I use a TDS meter for salt and pH.

I have a TDS meter with salt and pH functions.

I think what you have is a combo TDS/PH meter. Technically, it is incorrect to call it a TDS meter as it measures more that just TDS and a stand alone TDS meter does not measure PH.

There was little disagreement between the meter and phenol red for months. Now they’ve diverged. As I edited up top, another test kit I had available reads closer to the TDS meter.

Then I would say it is probably the drop test kit is the issue.

 

[s0cr4t1c]

I think what you have is a combo TDS/PH meter. Technically, it is incorrect to call it a TDS meter as it measures more that just TDS and a stand alone TDS meter does not measure PH.


Then I would say it is probably the drop test kit is the

I think what you have is a combo TDS/PH meter. Technically, it is incorrect to call it a TDS meter as it measures more that just TDS and a stand alone TDS meter does not measure PH.


Then I would say it is probably the drop test kit is the issue.

I feel like this divergence in results happened after I added boric acid to the pool. Can borates interfere with the drop test?

 

[JamesW]

Standard Solutions

tftestkits.net

 

[JamesW]

R-7062 pH Standard 7.2 (2 oz)
used in place of pool water in pH test
2 ounces per bottle
calibrated to 7.2 pH
confirms test method reliability

 

[JamesW]

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[JamesW]

I have a TDS meter with salt and pH functions.

What exact meter do you have?

Do you have calibration solution?

 

[s0cr4t1c]

What exact meter do you have?

Do you have calibration solution?

Braxlee Poolmeter from Amazon. I do have calibration solution. Recent replaced the solution and readings haven’t changed.

 

[s0cr4t1c]

Now I’ve dosed about 6 cups of MA into the pool (~20k gallons). According to Pool Math, that should’ve dropped the pH from 7.9 to 7.5 which is exactly the readings my meter was giving. Now the drop test looks like 7.6, so close enough. These results are diverging at higher pH for some reason.

 

[JamesW]

Which calibration solution do you have?

 

[s0cr4t1c]

Which calibration solution do you have?

Which calibration solution do you have?

View attachment 617800

Packets like those in this image, dissolved in freshly opened distilled water. Old solutions and new solutions gave similar readings.

 

[Bperry]

Now I’ve dosed about 6 cups of MA into the pool (~20k gallons). According to Pool Math, that should’ve dropped the pH from 7.9 to 7.5 which is exactly the readings my meter was giving. Now the drop test looks like 7.6, so close enough. These results are diverging at higher pH for some reason.

Some people read the higher pH levels incorrectly on the phenol test so that would be something to check you’re reading it correctly.