How to use and read pool test strips: mistakes you can avoid
A test strip is quick and convenient, but only reliable if you store it properly, use it exactly as the manufacturer says and read it at the right moment. Here's how to avoid the most common mistakes.
By AQELIO-redactie5 min read
Short answer: a test strip gives a reliable indication if you store it dry, closed and within its shelf life, take the water sample properly, stick exactly to your manufacturer's dip and reading times, and compare the colours in good light with the chart from the same container. If you doubt the result, test again with a new strip or with a more accurate method such as a DPD test kit.
Why the "how" matters so much
A test strip works with reagents on small pads that change colour. That reaction is sensitive to moisture, time and light. The CDC calls test strips usable for home pools but less accurate than a DPD test kit, and stresses that you should always follow the manufacturer's instructions. Those instructions differ by brand: with some strips you dip for just one second, with others a little longer. So don't use a general rule of thumb — use the text on your container.
1. Store strips properly
Always close the container tightly. Moisture from the air can affect the pads. Close the lid right after taking a strip.
Dry and not too warm. Store the container as the manufacturer specifies, usually cool, dry and out of direct sunlight. So not in a damp pump house or next to the chemicals.
Take a strip with dry fingers and don't touch the test pads.
2. Mind the shelf life
Check the expiry date on the container, preferably when you buy it. Expired strips can give unreliable colours. If the pads of an unused strip already look discoloured compared with the "0" colour or their original colour, stop using that container.
3. Take the sample the right way
Don't test at the surface right next to a return jet, a floating chlorine dispenser or the spot where you just added a product: the water there isn't representative. Follow your strips' instructions if they specify a depth or method. Don't test straight after adding a product, unless the label asks you to.
4. Use the strip exactly as the manufacturer says
Dip the strip for as long as the container states and take it out straight away.
Shake off excess water — or not — exactly as the instructions say; manufacturers differ on this.
Hold the strip horizontally if asked to, so colours don't run into each other.
Read within the time stated on the container. Reading too early or too late can give a different colour.
Use each strip once.
5. Compare colours in good light
Read in bright daylight or good, neutral light. Coloured light or dusk distorts colours.
Hold the strip right next to the colour chart from the same container; charts from different brands aren't interchangeable.
If a colour falls between two blocks, note both values and judge your water on the uncertainty, not on the most favourable result.
If you find it hard to tell colours apart, ask someone to check with you or use a method with a digital readout.
Common mistakes and what to do instead
| Mistake | Consequence | Better |
|---|---|---|
| Leaving the container open | Pads absorb moisture and show different colours | Close straight away, store dry |
| Using a general reading time | Colour not yet developed, or developed too far | Use the time on your container |
| Reading in shade or under lamp light | Colours are misjudged | Read in bright, neutral light |
| Using another brand's chart | Colour scale doesn't match the strip | Always use the chart from the same container |
| Testing right after dosing | Result describes one spot, not the whole pool | Wait and test elsewhere |
When should you retest or use another method?
Unexpected or contradictory result: test again with a new strip.
Chlorine looks like zero right after dosing: according to the CDC, above 10 ppm (mg/l) a test can bleach out and wrongly show a low or zero reading. Use a method that handles high values, or have the water tested.
Before a larger correction or with persistent problems: check with a more accurate method such as a DPD test kit, or have a water sample tested by a pool professional.
Strips expired or discoloured: use a new container.
A photo of a strip helps you record and compare results, but follows the same rules: good light, the right reading time and the chart from the same container next to it. What to do with the values is covered in pool pH and chlorine: why you should measure both and 7 common pool maintenance mistakes.
Frequently asked questions
Can I use a test strip twice?
No. The reagents on the pads reacted during the first test; a second test with the same strip isn't reliable.
How long does an opened container last?
That differs per product. Follow the expiry date and storage instructions on the container, and replace strips that look abnormal.
This article is general information, not personal advice. Always follow the label of your products and the instructions for your equipment; when in doubt, ask a pool professional.
Sources
- Home Pool and Hot Tub Water Treatment and Testing — U.S. Centers for Disease Control and Prevention (CDC)
- AquaChek Yellow 4-in-1 test strips — product instructions (example of manufacturer instructions) — Trevi / AquaChek
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