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Cold plunge water testing: strips, targets, and how often
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.
Frost turns readings into doses
Type in your strip reading and Frost gives you the exact gram or milliliter dose for your tub volume and product — chlorine or peroxide, plus after-session top-ups sized to how many people plunged. No subscription, no ads, works offline.
Sources
- CDC — Healthy Swimming: recreational water pH and disinfection basics
- Plunge — water care guidance
- Icebound Essentials — Cold Plunge Water Maintenance Guide
- Morozko Forge — cold plunge guidance
This article is general guidance. Always confirm with your own water testing and your equipment manufacturer's instructions.