Biguanide (PHMB)
The sanitizer in a chlorine-free pool. Nothing in a standard test kit can see it.
- Safe range
- 30 – 50 mg/L (Manufacturer guidance. PHMB is a proprietary chemistry and is not covered by the Model Aquatic Health Code, so there is no public-health code figure to cite here.)
- Ideal range
- 40 – 50 mg/L
- Measured in
- mg/L
What it is
Polyhexamethylene biguanide — PHMB, sold as Baquacil and similar — is a sanitizer that works nothing like chlorine. It binds to the cell walls of bacteria and ruptures them, rather than oxidising them. That difference is why a biguanide pool has no chlorine smell, does not need stabiliser, and holds its sanitizer far more steadily in sunlight.
Why it matters
It is the only thing sanitising the water, so the reading is the same class of number as free chlorine in a chlorine pool: below the band, the pool is not protected. It also degrades slowly and predictably rather than burning off in an afternoon, which makes it forgiving to manage and easy to forget about until it has drifted well down.
How to measure it
You need biguanide-specific test strips. This is not a preference — an ordinary pool strip has no pad that responds to PHMB, and its chlorine pad will sit blank in your water and read as though the pool has no sanitizer at all.
Test strips
- How trustworthy
- Dependable
- Cost
- Low cost
- Effort
- Under a minute, wherever you drew the sample
A plastic strip with reagent pads on it. Dip it, hold it level, and match each pad against the chart on the bottle at the time the label tells you to. Most strips read several parameters at once.
The standard test for these systems, and usually sold by whoever sells the sanitizer. A biguanide strip reads the sanitizer and the peroxide oxidizer together, which are the two numbers this kind of pool is actually run on.
Watch out: Check the bottle says biguanide or PHMB before you buy. A three-way or seven-way pool strip looks almost identical on the shelf and can tell you nothing whatsoever about this water.
Where to find it: Pool shops, hardware stores, supermarkets and online, sold both per parameter and as multi-packs. Buy strips made for your kind of water: pool strips and drinking water strips read very different scales for the same chemistry.
Liquid reagent kit, colour match
- How trustworthy
- Dependable
- Cost
- Low cost
- Effort
- A couple of minutes, at a sink or poolside
A measured sample in a plastic comparator block, a few drops of reagent, and a colour you match against printed standards beside the tube. The two-tube kit sold everywhere for pools is this.
Some brands supply a drop-count test for the sanitizer. More precise than a strip and worth having if you are trying to work out how fast your pool is consuming it.
Where to find it: Pool shops and online. Reagents expire, so buy replacement bottles rather than a whole new kit each season.
Free test at a pool shop
- How trustworthy
- Dependable
- Cost
- Free
- Effort
- A trip, and a sample carried carefully
Take a sample in and they will run it, usually on a photometer, usually free, and print you a full panel. The catch is that the printout ends in a list of things to buy.
Only useful if the shop actually handles biguanide. Many do not stock it and their bench kit cannot read it, so call before making the trip.
Where to find it: Most pool and spa retailers. Ask which parameters they cover before driving over.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Sample below the surface, away from the returns, as with any pool test.
- • Biguanide moves slowly, so testing weekly is generally enough — there is no daily burn-off to keep ahead of.
If it is too low
Sanitizer is running out and the water is losing its protection. Biguanide falls slowly, so a low reading usually means several weeks without a top-up rather than one hot weekend.
- • Add the sanitizer supplied for your system, and only that one. Biguanide products are a closed family and are not interchangeable with chlorine equivalents.
- • Retest after a full turnover rather than immediately; it disperses more slowly than liquid chlorine.
If it is too high
No safety problem, but you are spending money on sanitizer the pool cannot use. Biguanide is not consumed quickly, so an overdose stays with you for a long time.
- • Stop dosing and wait. Biguanide degrades slowly, so this corrects itself over weeks rather than days.
- • Dilution works if you are impatient, but it is rarely worth draining water over.
Common questions
Can I switch between biguanide and chlorine?
Not by simply adding the other one. The two react with each other and the usual result is cloudy water and a gummy residue through the filter. Converting in either direction means draining, cleaning the surfaces and replacing filter media, so it is a deliberate job rather than a change of mind.
Why does my chlorine test read zero?
Because there is no chlorine in the pool, which is correct for this system. A chlorine or DPD test is measuring something your water does not contain; it is not telling you the pool is unsanitised. Use a biguanide strip to see the reading that matters.
Does biguanide need stabiliser?
No. Cyanuric acid protects chlorine from ultraviolet light, and biguanide is not vulnerable to sunlight in the same way. There is nothing for stabiliser to do here, and adding it achieves nothing.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.