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Photometer Water Testing

Have you ever wondered if digital pool photometers are accurate? It is hard to compare them to a reagent drop test or test strips and the only real way to test if that is accurate is to put them up against another photometer. I had just the opportunity to test their accuracy when I was able to use five PoolLab 1.0 photometers and my eXact iDip Photometer/Digital Tester.

Keep in mind this wasn’t done in a laboratory and I tried to get the exact same amount of sample water in each photometer, but there is a slight margin of error in this type of experiment. Even so, all of the test results from the testing were right in line with each other. This goes to show how accurate photometer testing really is.

Here is how a photometer works:

“Most photometers detect the light with photoresistors, photodiodes or photomultipliers. To analyze the light, the photometer may measure the light after it has passed through a filter or through a monochromator for determination at defined wavelengths or for analysis of the spectral distribution of the light.” -Wikipedia

The water sample in blanked or zeroed out so that the photometer tester gets an accurate reading on the state of the water sample before the reagent is introduced. Once the reagent is put in the water sample, the photometer tester will read the color change of the water sample. For example in the pH testing, the lighter the color the lower the pH, just like in a reagent test kit or the tabs on a test strip. The darker the color the higher the pH. The thing about the digital photometer is that it will read the color exactly and give you a digital readout of what the pH is based on the color spectrum. This eliminates user error and the need to guess at what color the reagent is.

The first test was for the pH. I used the same exact water sample in each PoolLab 1.0 and the eXact pH+ Digital Tester. The results were all within 00.10 of each other.

eXact pH +           = 7.73

PoolLab 1.0 Unit 1 = 7.73

PoolLab 1.0 Unit 2 = 7.67

PoolLab 1.0 Unit 3 = 7.63

PoolLab 1.0 Unit 4 = 7.63

PoolLab 1.0 Unit 5 = 7.59


The PoolLab 1.0 uses a pre-measured tablet that you simply break up in the photometer chamber. This releases the reagent. The eXact pH+ uses a probe that measures the pH in the sample and no regent is needed. Digital Testers are extremely accurate within +/- 0.1% so I was sure that the pH was 7.73 in this test.


Here are the results for the Alkalinity test with five PoolLab 1.0 and the eXact iDip photometer. The eXact iDip uses a pre-measured test strip vs the tablets of the PoolLab 1.0

eXact iDip Photometer = 146 ppm

PoolLab 1.0 Unit 1 = 156

PoolLab 1.0 Unit 2 = 153

PoolLab 1.0 Unit 3 = 146

PoolLab 1.0 Unit 4 = 147

PoolLab 1.0 Unit 5 = 149

The average between the 6 testers is 149 ppm and this is within the margin of error for the Alkalinity test. Basically 4 of the 6 results were right in line with each other with one being 10 ppm over the others. This can be accounted for if the water sample wasn’t exactly 10 ML in one of the units. The pre-measured tablets all came from the same package so the only real variable is the 10 ML water sample.

These results show that using a digital photometer for your pool and spa water testing is very accurate. If you used the correct water sample volume and the correct reagent amount, the test can be relied on accurate.

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