Though you think you’re using the most accurate water-test
kit available, it still may read incorrectly or differently than
expected. To save yourself confusion, time and money, you need to
understand the test equipment you’re using and what causes it
to sometimes report false readings.
Here’s a look at the top causes of inaccurate readings for
liquid kits, test strips and electronic devices.
Most liquid test kits use a chemical indicator called DPD to
measure free chlorine. However, high levels of combined chlorine
(chloramines) can cause false positives with these kits.
Low to moderate levels of combined chlorine are neutralized in most
DPD kits. But combined chlorine occasionally will build up to such
a point that it seems to indicate a level of free chlorine is
present when, in fact, there may be none.
A strong chloramine odor may confirm that there’s a high
level of combined chlorine present. Otherwise, pay close attention
to the chlorine reaction when adding the DPD indicator (usually DPD
No. 2). Free chlorine reacts immediately with DPD to turn pink,
while combined chlorine generally takes longer to produce a result.
Therefore, if color develops within a few seconds or minutes, you
are likely seeing combined chlorine reacting with the DPD
Elevated sanitizer levels can cause problems for liquid test kits,
too. First, high levels of chlorine (greater than 15 ppm) or
bromine (greater than 20 ppm) can cause the pH indicator to turn
purple, which does not match the pH color scale. This is similar to
the high pH colors, 8.0 to 8.4 ppm, so be careful not to mistake it
for a high reading and then add acid when it’s not
If chlorine or bromine levels appear to be on the high side and the
pH is reading purple, the best thing to do is wait for the
sanitizer level to drop and test again. If you’re pressed for
time, you could add a drop of sodium thiosulfate to neutralize the
chlorine. This is a less than ideal practice because thiosulfate
solutions have a high pH. Thus, you are increasing the pH of the
sample and again getting potentially high results.
Another problem caused by high sanitizer levels is when the DPD
indicator is “bleached out.” A flash of color occurs
when the indicator is added and then quickly becomes colorless. If
you see this happening, dilute the sample and retest. For example,
you may use half sample water from the pool and half fresh water
— ideally, distilled water because it contains no chlorine,
and little or no chlorine demand.
You can then run the test again using the same procedure. This will
provide you with a result that is half the actual value, so you
will need to multiply it by 2 to get the correct reading. Higher
sanitizer levels may require greater dilution to get an accurate
Tablet test kits
Tablet test kits are another method of testing pool and spa water.
These kits are similar to liquid kits, and many of the chemical
reactions are the same. Therefore, you can expect to get
inaccuracies similar to what is reported in the liquid
Additionally, you can usually determine if a tablet is unusable by
inspecting the tablet before using it. If a tablet appears to be
more powder than tablet, or if it is discolored, it usually
indicates that the reagent within the tablet has been contaminated
and is no longer usable.
Similar to DPD kits, elevated sanitizer levels also can cause
problems for test strips. Because test strips and liquids use the
same pH indicator — phenol red — high levels of
chlorine (greater than 15 ppm) or bromine (greater than 20 ppm) can
cause the pH indicator to turn purple, which does not match the pH
color scale. This problem tends to be more obvious with test
strips, forming a shade of purple that does not closely resemble
the 8.4 or other high-end color blocks.
Total alkalinity may be affected as well. Like the pH reading,
elevated sanitizer levels can cause a color that does not match
anything on the scale, in this case, a royal blue. Generally, the
royal blue alkalinity accompanies the purple pH, making it easier
to determine when this problem is occurring. The best thing to do
is wait for the sanitizer to drop to an acceptable level and test
Another common issue for test strips is faded or “washed
out” results. You can identify this situation when test
strips react to form colors that appear to be in the same family as
the ones on the chart, but are significantly lighter or less
intense. This happens most commonly when test strips have been
contaminated, generally by heat or humidity.
Test strips should be stored at room temperature, and use dry
fingers when handling them to prevent moisture from entering the
bottle. Seal the bottle immediately after use.
Electronic devices don’t provide the same kind of indicators
as liquid kits and test strips to let you know when you’re
getting inaccurate readings. When using them, you need to be aware
when false readings occur.
There are two things to consider with most electronic devices:
First, are they calibrated correctly? Most units require a
single-point calibration. This means you have a single solution of
a known concentration by which to set your meter, thus ensuring it
is reading accurately. Some devices require a multiple-point
calibration with two or three solutions. If you’re getting
questionable readings with your meter, calibrate it to be sure.
Some units are factory calibrated and don’t require periodic
Second, determine if the device and its accessories are clean. For
example, test tubes or vials used with a handheld colorimeter might
become discolored or faded over time. Recalibrating can help, but
it may not prevent this from causing false high and/or low
Faded or discolored vials or tubes should be replaced, especially
if the coloration is not uniform across the entire surface.
Cleaning the device can help, but be sure to follow
manufacturer’s recommendations. If not done properly, it can
damage the device. Regardless of which type of water-test kit
you’re using, you can still rely on a proven method to verify
the accuracy of a questionable result. Double-check the readings
you’re getting against another type of test to confirm that
they are in the same general ballpark. If one method tells you the
chlorine is low while the other shows that it’s acceptable,
one of them is not reading accurately.