A test strip is only useful if you know what the colors mean and what to do about them.
This article explains the four readings on a typical cold plunge strip. For the rest of the upkeep kit, see the cold plunge accessories buying guide.
Short answer: watch pH first. Keep it between 7.2 and 7.8, fix the one reading that’s off, and test again before you get in.
What one strip reads
The Sirona Water Test Strips from Renu Therapy read four things in 10 seconds. A pack of 25 costs $15. AquaChek’s 7-way strips read seven, including chlorine and bromine, in 15 seconds, 100 to a bottle.
| Reading | Target | What it tells you |
|---|---|---|
| pH | 7.2 to 7.8 (Sirona) | How acidic or basic the water is |
| Total alkalinity | 80 to 120 ppm (Sirona) | How well the water resists pH swings |
| Sanitizer | Per your product’s label | Whether your last dose is still working |
| Calcium hardness | Not stated on the listing | How much dissolved calcium is in the water |
pH comes first
CDC recommends a pH of 7.0 to 7.8 for pools and hot tubs. Its reasoning applies to any treated water: as pH goes up, chlorine kills germs less well, especially above 8.0. As pH goes down, pipes are more likely to corrode.
So a sanitizer reading means little until pH is in range.
Alkalinity keeps pH steady
Alkalinity is the buffer. When it’s in range, pH holds. When it’s low, pH jumps around between tests. If your pH won’t stay put, look at this number.
Sanitizer
This shows whether the sanitizer you added is still present. The Renu treatment kit’s directions say to add sanitizer as needed, based on the strip. That’s the reading this line is for.
How much to trust a strip
Here’s what the spec sheet won’t tell you. CDC says test strips are less accurate than a DPD test kit, the liquid kit pool operators use.
You get speed and a $15 price with strips. You give up precision. That’s a fair trade for a home tub, as long as you treat a borderline color as borderline.
What to do with a bad reading
- Adjust the one thing that’s off, using the product made for it.
- Follow that product’s label for the dose.
- Wait, then test again before your next session.
You don’t need to drain the tub for a single off reading.
Do you need strips at all?
Maybe not. Renu Therapy says its tubs’ ozone and filter system removes the need for chemicals. If you add no sanitizer, there’s little to test for.
My rule of thumb: if you dose chemicals, test weekly, at the same point in your routine each time. If you don’t dose, keep a pack for when the water looks off.
If you’re still choosing a tub, start with the cold plunge buying guide.
Bottom line: pH between 7.2 and 7.8, one fix at a time, and a retest. A strip is a quick check, not a lab result.
Sources
- Renu Therapy, Sirona Water Test Strips listing (readings, read time, count, target ranges, price), checked September 2026
- CDC, “Home Pool and Hot Tub Water Treatment and Testing,” May 10, 2024: cdc.gov
- Renu Therapy FAQ (ozone system), accessed October 8, 2026: renutherapy.com