Cold plunge water testing: strips, targets, and how often
The three readings that matter, the strips that measure them, and a testing cadence that actually sticks.
A cold plunge needs exactly three readings: sanitizer level (chlorine or peroxide, depending on which you run), and pH. That's it — you can ignore the other six pads on a pool strip. Here are the targets, then the strips, then the cadence.
Target readings
| Reading | Target range | Notes |
|---|---|---|
| Free chlorine | 1–3 ppm | Chlorine path only |
| Hydrogen peroxide | 50–100 ppm | Peroxide path only |
| pH | 7.2–7.6 | 7.3–7.5 is the sweet spot |
If you've kept a hot tub, the chlorine number looks low — spas run 3–5 ppm. A plunge gets away with 1–3 ppm for two reasons: bather load is far lower (minutes per session, not an hour of soaking), and near-freezing water dramatically slows microbial growth. Cold is doing part of the sanitizer's job. It is not doing all of it — unsanitized plunge water still fouls, just slower.
pH matters more than most plunge owners think: outside 7.2–7.6, chlorine loses effectiveness and water starts irritating skin and eyes. The same 7.2–7.8 band is the standard for all recreational water, per CDC guidance.
They neutralize each other — you pay for both and end up with neither. Pick one path. Switching means a full drain and refill, not a transition.
Which strips to buy
- Chlorine path: any standard pool/spa 4-in-1 strip (free chlorine + pH is all you'll read). They're cheap and everywhere.
- Peroxide path: regular pool strips cannot read H₂O₂. You need dedicated high-range hydrogen peroxide strips that cover 0–100+ ppm (often sold as "peroxide test strips" for hydroponics or sanitizing). A 0–25 ppm low-range strip will just read maxed-out and tell you nothing.
- Either path: the pH pad on any strip works.
How often to test
| When | What to do |
|---|---|
| 2–3× per week | Routine strip test; dose back to target if low |
| After a heavy-use day | Test same day — each person-session consumes roughly 0.3 ppm of free chlorine (or ~2 ppm of peroxide) |
| Water looks or smells off | Test before touching anything — never dose blind |
The after-session drain is the number people miss. Two people doing daily plunges consume the entire chlorine residual in roughly a week — which is exactly why "I dosed it last weekend" so often ends in cloudy water.
Gear this guide uses
The right strips for your path — and the catch most peroxide plungers hit.
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- On the chlorine path, any cheap 4-in-1 pool/spa strip reads free chlorine and pH — all you need.
- On the peroxide path, standard strips can't read H₂O₂; you need dedicated high-range hydrogen peroxide test strips (0–100+ ppm).
- Because plunge doses are tiny, a 0.01 g digital scale beats eyeballing fractions of a gram.
Sources
This article is general guidance. Always confirm with your own water testing and your equipment manufacturer's instructions.