How to Test Hot Tub Water

Most water problems are not caused by bad testing. They are caused by testing at the wrong moment, or trusting a strip that has been sitting in a hot cupboard since last summer. A reading you cannot rely on is worse than no reading, because you act on it.

Testing properly takes about a minute. Here is how to get a number you can trust.

Not sure where your water is? Check your readings.

How often

• Two to three times a week if the tub is in regular use
• Before every soak if the tub is used occasionally, or has been sitting
• Two or three times in the first week after a fresh fill, since new water moves more than settled water
• After heavy use, rather than waiting for the next scheduled test
• After any top-up, particularly from a different source
• Weekly is enough on a cold plunge, since cold slows everything down

The most useful habit is testing before soaking rather than on a fixed day. Sanitizer can fall to zero between sessions with nothing visibly changing, so a reading from Tuesday tells you very little on Friday.

Taking the sample

Where and when you draw the water changes the number.

• Test mid-tub, away from any inlet or return. Water beside a return has just come through the system and does not represent the whole body
• Draw from elbow depth, not the surface. The top inch is where oils and debris sit
• Circulate first, or stir thoroughly with a paddle on a tub without a pump. Still water stratifies, and a sample from the top of an unstirred tub is not the water you are soaking in
• Wait fifteen to twenty minutes after adding anything before you retest. Testing straight after a treatment tells you where the treatment is, not where the water is
• Test at soaking temperature where you can, since readings shift with temperature

Using a test strip

Strips are accurate enough for routine use, and most of the ways they go wrong are avoidable.

1. Dip the strip for the time on the packaging. Longer is not better, and it will wash the reagents off
2. Remove it and hold it flat, pad side up
3. Do not shake the water off, and do not flick it. That runs the reagents into each other and skews neighbouring pads
4. Wait the stated time before reading. Reading early gives a low result, reading late gives a high one
5. Match against the chart in natural light, holding the strip close to the printed scale rather than at arm's length
6. Read all the pads, not just the one you are worried about

Store strips properly, which is where most inaccuracy actually comes from. Keep them cool, dry and out of sunlight, close the lid immediately, never touch the pads with wet fingers, and do not decant them into another container. A tub-side cupboard in summer is one of the worst places you could keep them.

Check the expiry date. Expired strips read low, which means you dose upward on a reading that was never true.

When strips are not enough

Strips are good for pH, alkalinity and sanitizer day to day. Two situations warrant more.

Calcium hardness on a new fill or a new source. Strips read hardness less precisely than the other parameters, and hardness is the reading you most want to be right, particularly if your water is soft. A liquid test kit or your municipal water report gives a better figure.

When something does not add up. If the readings contradict what the water looks like, or a correction produces no change at all, test with a second method before you act. Two strips from the same tube can agree with each other and both be wrong.

Taking a sample to be tested elsewhere works too, with one caveat: use a clean container, fill it from elbow depth, keep it out of the sun, and get it tested the same day. A sample that has sat in a warm car has already changed.

Keeping a record

Four numbers and a date, written down, will tell you more over a season than any single test.

Patterns are what you are looking for. pH that climbs steadily every week points at aeration. pH that swings in both directions points at low alkalinity. Sanitizer that falls off a cliff after weekends points at bather load rather than a fault. None of those are visible in a single reading, and all of them are obvious across six.

It also tells you when water is genuinely finished. When readings stop responding predictably to correction, that is the signal to change it rather than treat it again.

What the four numbers are

Test them, and correct them, in this order, because each one affects the ones after it.

1. Total alkalinity, typically 80 to 120 ppm. Buffers pH. More here
2. pH, generally 7.4 to 7.6 depending on your tub. Decides comfort and sanitizer effectiveness. More here
3. Calcium hardness, typically 150 to 250 ppm. Protects surfaces in both directions. More here
4. Sanitizer, at the working range for your tub. More here

If you have an AlumiTub

The published targets are alkalinity 80 to 120 ppm corrected first, pH 7.4 to 7.8, calcium hardness 150 to 250 ppm, and bromine 2 to 3 ppm and never above 3, which is lower than the 3 to 5 ppm typical of a conventional spa.

Calcium hardness is the one worth testing with a liquid kit at least once per fill, because water below roughly 100 ppm is chemically aggressive toward a marine-grade aluminum interior and strips are least precise on exactly that reading.

On a Wood Fired tub without a circulation pump, stir thoroughly with a paddle before sampling. Still water in a large tub stratifies more than people expect, and a surface sample will read differently from the water around you.

Test before soaking rather than on a fixed schedule, and record the numbers. The water diagnostic will keep the history for you and show the drift.

Not sure where your water is? Check your readings.