Swimming pool and spa water testing can be done in a number of ways, and each method has its advantages and limitations. Understanding the differences is key to selecting the right tools for the job — especially when accuracy matters.
Let’s look at the three most common chlorine test methods used by pool professionals: test strips, DPD drop tests, and DPD-FAS titration.
Test Strips
Test strips are the quickest and easiest method. Dip the strip in the water, wait a few seconds, and compare the resulting colors to a chart. They’re inexpensive and convenient for spot checks or when time is tight.
However, test strips can lack precision. Colors may be difficult to match, especially in poor lighting, and results may vary based on how long the strip was submerged or how the user interprets the colors. Also, they usually provide broad ranges (e.g., 3–5 ppm), not precise values.
DPD Drop Tests
DPD stands for diethyl- pphenylenediamine, a reagent used to measure chlorine levels. With a DPD test kit, you fill a test cell with water and add specific drops that react to chlorine. Free chlorine produces a pink color, and the intensity of the pink is compared to a color chart. To get total chlorine, additional drops are added.
DPD is generally more accurate than test strips and allows a clearer distinction between free chlorine and combined chlorine (a sign of chloramine buildup). However, like test strips, color matching can be subjective and inconsistent in different lighting.
DPD tests are a good middle ground — easy to perform, with more reliable results than test strips, but still limited by visual comparison.
DPD-FAS Titration For the most accurate chlorine testing, especially in commercial or high-risk environments, DPD-FAS (Ferrous Ammonium Sulfate) titration is the gold standard. Instead of color matching, this test involves a color change reaction. You add a powder to the sample, which turns the water pink in the presence of free chlorine. Then, you titrate with a reagent drop by drop until the pink color disappears. Each drop corresponds to a specific concentration, allowing precise measurement — often down to 0.2 ppm.
The DPD-FAS method is especially useful for high chlorine levels (over 5 ppm), where color matching tests may max out or become unreliable. It also allows clear measurement of combined chlorine, which can be essential for diagnosing water quality issues.
Choosing the Right Method
• Use test strips for quick residential checks or preliminary readings.
• Use DPD for reliable routine service visits with acceptable accuracy.
• Use DPD-FAS when exact readings are needed — such as for health department compliance, troubleshooting cloudy water, or balancing public pools and spas.
Each method has a place in the service tech’s toolkit. The key is knowing when speed outweighs precision — or when precision can’t be compromised.
