Swimming Pool Water Testing: The Complete Guide for UK Service Engineers

Your water testing results define your reputation as a pool service engineer. Get them wrong and you risk algae blooms, corroded surfaces, or — worse — a compliance failure that costs your client their public health clearance. Get them right, consistently, and you become the engineer clients trust with their most expensive asset.

This guide covers the specific parameters UK service engineers need to test, how often PWTAG expects you to test them, how to calculate the Langelier Saturation Index on-site, and what your compliance records need to include.

The Six Parameters Every UK Pool Engineer Must Test

PWTAG's Swimming Pool Water guidelines set out clear targets for pool water quality. These are the six chemical parameters you should be recording at every service visit.

Free chlorine

Free chlorine is the active sanitiser killing bacteria and pathogens. PWTAG's acceptable range is 0.5–3.0 mg/L, with most well-managed pools sitting between 1.0 and 2.0 mg/L. Below 0.5 mg/L the water isn't being disinfected effectively. Above 3.0 mg/L you risk skin and eye irritation for bathers.

Test free chlorine using a DPD No. 1 tablet or reagent in a photometer. Avoid test strips for professional work — photometers give you the precision PWTAG expects.

Combined chlorine

Combined chlorine (chloramines) forms when free chlorine reacts with nitrogen compounds from bathers — sweat, urine, cosmetics. PWTAG sets the limit at ≤1.0 mg/L, and ideally below 50% of the free chlorine reading. High combined chlorine causes the "swimming pool smell" that bathers complain about and indicates insufficient oxidation.

Calculate it by subtracting free chlorine from total chlorine: Combined = Total − Free. If your combined reading exceeds the limit, the pool likely needs superchlorination or a review of bather load and ventilation.

pH

pH controls how effectively chlorine works. At pH 7.2, roughly 65% of free chlorine exists as hypochlorous acid (the active form). At pH 8.0, that drops below 25% — meaning you need far more chlorine to achieve the same kill rate.

PWTAG recommends pH 7.0–7.4 for chlorine-disinfected pools, with 7.2–7.4 as the optimal range. Test pH at the same time as chlorine, because one reading is meaningless without the other.

Total alkalinity

Total alkalinity acts as a buffer, preventing pH from swinging after chemical additions or heavy bather loads. PWTAG's guideline range is 80–200 mg/L as CaCO3. Low alkalinity means pH instability — you'll find yourself constantly adjusting. High alkalinity makes pH stubborn and hard to bring down.

Alkalinity doesn't change as rapidly as chlorine or pH, so weekly testing is sufficient unless you've recently made large chemical adjustments.

Calcium hardness

Calcium hardness affects water balance and surface longevity. PWTAG's guideline range is 75–500 mg/L as CaCO3, though the ideal depends on pool construction. Soft water (low calcium) dissolves grout and etches plaster. Hard water deposits scale on heat exchangers, tiles, and pipework.

This parameter shifts slowly and is largely determined by source water. Monthly testing is adequate for most pools unless you're investigating balance issues.

Temperature

Water temperature affects disinfection rates, bather comfort, and LSI calculations. Record the exact temperature at each visit — you'll need it for the Saturation Index. PWTAG doesn't set a universal temperature limit (it varies by pool type), but most standard heated pools operate between 26°C and 30°C. Hot tubs typically run at 37–40°C, which demands stricter chemistry controls because chlorine degrades faster at higher temperatures.

PWTAG Testing Frequency: How Often and When

Different parameters need different testing intervals. Here's what PWTAG guidance expects for pools with regular public or commercial use. For help structuring your maintenance round to meet these frequencies, see our round planning guide.

Before every opening and at intervals throughout the day (public/commercial pools):

Free chlorine
pH
Temperature

At every service visit (your minimum for private client pools):

Free chlorine
Combined chlorine (total minus free)
pH
Temperature

Weekly:

Total alkalinity

Monthly:

Calcium hardness
Microbiological testing (sent to an accredited laboratory)

Microbiological testing

Chemical testing tells you what's in the water now. Microbiological testing tells you whether the treatment regime is actually working over time. PWTAG recommends monthly laboratory testing for:

Total viable count (TVC): <100 cfu/mL at 37°C
Coliforms: <10 per 100 mL
E. coli: none detected in 100 mL
Pseudomonas aeruginosa: <10 per 100 mL

You can't do microbiological testing on-site — samples must reach an accredited lab within a defined time window (typically within 24 hours, refrigerated). If you're not currently offering this as part of your service, it's worth adding. Clients operating shared or commercial pools have a duty under health and safety law to monitor microbiological quality, and most don't know how to arrange it themselves.

How to Take Accurate Water Samples During Your Round

Bad samples produce bad data. When you're testing five to fifteen pools in a day, it's tempting to rush — but a few simple habits keep your results reliable.

Sample from the right location. Take water from at least 30 cm below the surface and away from return jets, skimmers, and chemical dosing points. Near-surface samples can be skewed by undissolved chemicals or UV degradation. Samples near return jets will show artificially high chlorine readings.

Rinse the sample container. Rinse your test tube or cuvette three times with pool water before filling it. Residual chemicals from the previous test (or the previous pool on your round) will contaminate the reading.

Test immediately. Chlorine levels change within minutes of sampling, especially in warm water or direct sunlight. Run your DPD test within 60 seconds of taking the sample. If you're using a photometer, blank it before each test — don't carry a blank reading from three pools ago.

Record before you move. Write down every reading before walking to the plant room or picking up the dosing chemicals. Memory is unreliable when you're doing this fifteen times a day. A mobile app or structured log sheet prevents transcription errors later.

Calibrate regularly. Photometers drift over time. Follow the manufacturer's calibration schedule — most recommend monthly calibration against a known standard solution. If your readings suddenly don't match what the water looks like, check calibration before doubting the water.

Understanding the Langelier Saturation Index

The Langelier Saturation Index (LSI) tells you whether pool water is balanced, corrosive, or scale-forming. It's the single most useful calculation for predicting surface damage and heat exchanger scaling — two problems that cost your clients real money.

The formula

LSI = pH − pHs

Where pHs (the saturation pH) is the theoretical pH at which water is perfectly balanced with respect to calcium carbonate:

pHs = (9.3 + A + B) − (C + D)

Factor	Formula	What it represents
A	(log₁₀[TDS] − 1) / 10	Total dissolved solids factor
B	−13.12 × log₁₀(°C + 273) + 34.55	Temperature factor
C	log₁₀[Calcium hardness] − 0.4	Calcium factor
D	log₁₀[Total alkalinity]	Alkalinity factor

All concentrations are in mg/L as CaCO₃. If you don't test TDS, use 1,000 mg/L as a default for pools.

Interpreting the result

LSI Value	Meaning	Risk
−0.3 to +0.3	Balanced	Ideal range — water is neither corrosive nor scale-forming
Below −0.3	Corrosive	Dissolves calcium from grout, plaster, and metal fittings
Above +0.3	Scale-forming	Deposits calcium on heat exchangers, tiles, and pipework

A worked example

A residential pool you're servicing reads: pH 7.4, temperature 28°C, calcium hardness 250 mg/L, total alkalinity 120 mg/L, TDS 1,000 mg/L.

A = (log₁₀(1000) − 1) / 10 = (3 − 1) / 10 = 0.20
B = −13.12 × log₁₀(301) + 34.55 = −13.12 × 2.479 + 34.55 = 2.03
C = log₁₀(250) − 0.4 = 2.398 − 0.4 = 2.00
D = log₁₀(120) = 2.08

pHs = (9.3 + 0.20 + 2.03) − (2.00 + 2.08) = 11.53 − 4.08 = 7.45

LSI = 7.4 − 7.45 = −0.05

This water is balanced — the LSI of −0.05 sits within the ideal ±0.3 range, meaning the water is neither corrosive nor scale-forming. No corrective action needed.

Why LSI matters for your business

Most pool owners have never heard of the Saturation Index. When you can show a client their water balance on a simple scale and explain what it means for their surfaces and equipment, you've demonstrated expertise that a paper logbook can't match. It's also a natural conversation starter for upselling preventive maintenance.

Recording Results: What PWTAG Compliance Records Need

If your clients operate pools for public or commercial use, they have a duty of care under the Health and Safety at Work etc. Act 1974 to maintain safe water quality. Your test records are their evidence of compliance.

PWTAG's Code of Practice specifies that pool water quality records should include:

Date and time of every test
Who conducted the test (your name or engineer ID)
All parameter readings (free chlorine, combined chlorine, pH, temperature, alkalinity, hardness)
LSI calculation (where performed)
Any corrective action taken — what chemical was added, how much, and why
Equipment used (photometer model, calibration date)
Microbiological results (monthly lab reports, filed with routine records)

Records should be kept for at least five years — this is the standard PWTAG recommends, and many local authorities expect it during inspections.

What good records look like

Each site visit should produce a single, structured record that links the readings to the corrective action. A chlorine reading of 0.3 mg/L is only useful if the record also shows that you dosed 200g of sodium hypochlorite to bring it back to 1.5 mg/L, and a follow-up reading confirmed it.

Paper logbooks work, but they're hard to search, easy to lose, and impossible to share with clients in real time. If you're managing ten or more sites, a structured digital log saves you significant admin time and produces cleaner records for inspections. If you're evaluating digital options, our pool management software buyer's guide covers the features that matter most for UK service businesses. Our free PWTAG water testing log template gives you a structured starting point with PDF export.

Quick-Reference Parameter Table

Parameter	PWTAG Guideline	Test Frequency	Method
Free chlorine	0.5–3.0 mg/L	Every visit	DPD No. 1 (photometer)
Combined chlorine	≤1.0 mg/L	Every visit	DPD No. 1 + No. 3 (total − free)
pH	7.0–7.4	Every visit	Phenol red (photometer)
Total alkalinity	80–200 mg/L CaCO3	Weekly	Acid demand test
Calcium hardness	75–500 mg/L CaCO3	Monthly	EDTA titration
Temperature	Pool-type dependent	Every visit	Digital thermometer
TVC	<100 cfu/mL	Monthly	Lab (accredited)
E. coli	None in 100 mL	Monthly	Lab (accredited)
Pseudomonas	<10 per 100 mL	Monthly	Lab (accredited)
Coliforms	<10 per 100 mL	Monthly	Lab (accredited)

Put Your Testing Data to Work

Accurate water testing is the foundation of professional pool maintenance — but the value doesn't stop at the poolside. Every reading you log is data that helps you spot trends, predict problems, and demonstrate your expertise to clients. Accurate testing data also supports your pricing — see our UK pool maintenance cost guide for help building your rate card.

Use our free pool water chemistry calculator to run the full LSI calculation and get chlorine dosing recommendations from your readings. If you're still recording results on paper or in scattered spreadsheets, you're spending time on admin that could be automated.

PoolRound is building exactly this — a tool designed for UK pool and hot tub service engineers that connects your water chemistry data to your maintenance rounds and client reports. Join the waitlist to get early access.

Sources

PWTAG Standards — Pool Water Treatment Advisory Group standards and guidance hub
PWTAG Code of Practice — Industry code of practice for pool water treatment
PWTAG Technical Note TN53: Microbiological Testing — Guidance on microbiological sampling and limits
HSE: Spa-pool systems — HSE guidance on Legionella control in spa pools