Most pool test kits incorporate one of the following four methods:
- Color comparison - a reagent is added to water, and the resultant color is compared with a test strip. Another variation is a test strip that is dipped in the water, then compared against a standard set of colors. This can be done by adding the water to a plastic container called a "color comparator," which has a set of standard colors printed on its sides; or by dipping a test strip in the water, then comparing it against a set of standard colors on the bottle. Chlorine and pH are commonly tested in this way.
- Drop-count - also known as a "titration" or "titrametric" test. A reagent is added to a water sample, and the color is checked against a standard set of colors. Then, more reagent is added drop-by-drop, until the liquid changes to another color or turns clear. As many as three different reagents may be involved in this process: for instance, one may adjust the pH, while the other two produce color reactions. These tests are common for measuring hardness and alkalinity.
- Turbidity - also known as a "turbidimetric" test, this requires the tech to add drops of reagent to a water sample, then judge the water's turbidity (cloudiness) as more drops are added. Currently, the industry's only turbidity test measures cyanuric acid levels.
- Electronic testing, sometimes known as "electrometric" testing, takes advantage of the varying electrical conductivity of different compositions of water. The most common electronic tests are TDS meters, pH meters, and ORP electrodes. These tests do not require a reagent, and but may take hours to develop an accurate reading.
Two other types of light-based testing are also available, but these are more often used in laboratory settings.
- Colorimeters measure the amount of light that passes through a water sample. After a reagent is added to a water sample, a light is shone through the sample at a certain wavelength (such as red, which has a wavelength of 697 nm). The light strikes a photodiode (light sensor), which passes electronic information about the light's strength to a central computer, where its intensity is compared with the intensity of light passing through a sample without reagents.
- Photometers differ from colorimeters in that they use a specific wavelength of light. A light beam is passed through a monochromator (a device containing a prism or grating, which causes a specific wavelength of light to split off from the main beam). Photometers measure radiant energy as a function of the light's wavelength.
Types of Test Kits
Test kits are available for measuring a variety of factors, and some kits include multiple types of tests. Some commonly used types of test kits are as follows:
- OTO (ortho-Tolidine) is an older test for chlorine, which has been in use at least since 1913. Five drops of the reagent orthotolidine are added to chlorinated water, which turns yellow on contact; the darker the shade, the more chlorine the water contains. OTO tests cover a range of 0 to 10 ppm of chlorine, and measure FAC and chloramines. In addition, it will measure any other sanitizers in the water, such as hydrogen peroxide, and can suffer interference from iron, manganese, and nitrites. It should be noted that OTO is a poison and a membrane irritant, and has been linked to cancer.
- DPD (N,N-diethyl-p-phenylenediamine) is the most common modern chlorine test; it has been in use since 1957. The reagent DPD turns red in the presence of chlorine. One downside of this test is that DPD "bleaches out" in the presence of very high chlorine, which can make an excessively chlorinated sample appear to be chlorine-free. High calcium levels can create a cloudy sample; this can be prevented by adding the reagents to the sample cell before the water. Also a source of confusion is the fact that DPD reacts with all oxidizers present in the water, including bromine, ozone, and hydrogen peroxide.
- FAS-DPD (Ferrous Ammonium Sulfate-DPD) measures free and combined chlorine directly, eliminating the need for subtracting the combined chlorine from the total. The test uses two reagents: first, DPD is added to the sample, then FAS is added until the reaction's "end point" is reached. The two numbers of drops provide the free and combined chlorine readings, respectively.
- Syringaldazine (3,5-dimethoxyl-4-hydroxybenzaldazine) tests were developed in the 1970s, and are based on the reaction of free chlorine with syringaldazine. This test measures FAC in the range of 0.1 to more than 10.0 ppm. It is specifically a test for free chlorine, and is not subject to interference by other oxidizers. Syringaldazine is nearly insoluble in water, so it is impregnated onto test strip pads, which can be compared to a set of standard colors.
- TMB (3,3',5,5'-tetramethylbenzidine) tests represent the newest advances in test strip engineering. First available in 1996, TMB tests reliably measure chlorine concentrations from 0 to 20 ppm, using a reagent strip that is dipped into the pool's water, then compared against a standard color set.
- pH is most often tested with phenol red, a reagent that turns darker with higher pH. Phenol red gives a false reading (or sometimes no reading) for a pH below 6.8 or above 8.2. High chlorine or bromine levels may cause interference, so most manufacturers include a chlorine neutralizer with the test. Also, pH can be tested with an electronic pH meter, which is much more accurate than a reagent test.
- Testing the Waters — Pool & Spa News — 3.12.2010
- The Difference in Testing — Pool & Spa News — 2.12.2010