How to Run the Weekly Water Test Panel at the Pool
Why this matters
Every dose you pour that day is computed from this panel, so a sloppy sample or a reading taken in the wrong order is not a small error, it is the wrong chemical in someone's pool. The two mistakes that cost the most are invisible on the ticket: a sample scooped next to the return that just carried last week's tablet plume, and a pH read on a high-chlorine sample that turned the indicator a false purple. This is the residential panel in the order that keeps each reading from contaminating the next, with the reason for the order at each step. Commercial pools keep their own log, under 'Commercial Pool DPD Titration to ANSI/APSP/ICC-11 Standards'; strip versus titration accuracy is covered in 'Water Test Accuracy (Titration vs Strip)'.
Start state
- A titration kit with in-date reagents. Check the dates on the FAS-DPD titrant, the pH indicator and the TA and CH titrants before the season and monthly after. Expired reagents read wrong without looking wrong. Reagents live in a shaded, closed case, never on the dash: heat ages them fast.
- Clean sample tubes, rinsed in pool water before filling, and a white card behind the comparator.
- A thermometer. Water temperature is part of the record, because it feeds the saturation index ('How to Calculate the Saturation Index on the Pool Deck') and because the turbidity test for stabilizer behaves differently on cold samples.
- The pump running, for at least a few minutes before you sample, so you are testing mixed water and not a layer that sat still overnight.
- Eye protection and gloves for the reagents. Several titrants are corrosive in the bottle; reagent on a finger goes into an eye. Rinse skin or eyes with clean water and follow the reagent's safety data sheet.
Where the sample comes from
PLAN VIEW: WHERE TO SAMPLE
+----------------------------------------------+
| SKIMMER no: pulls the surface film |
| |
| R1 RETURN no: carries product just added |
| |
| X SAMPLE HERE: deep side, 3 ft+ |
| from returns, skimmer, floater |
| and the steps |
| FLOATER no: tablet plume around it |
+----------------------------------------------+
SECTION: bottle mouth DOWN to elbow depth,
about 18 in, then turn it UP to fill
Surface water is thinner in chlorine (sunlight burns it off the top first) and dirtier in everything else. Water by a return or a floater is richer in whatever was just added. Elbow depth in the open deep side, away from all of them, is the pool's average.
The panel, in order
Sample and fill the tubes at once. Push the bottle down mouth first to elbow depth, turn it up, let it fill, bring it out. Fill each tube to its line from that one sample. Reason: every reading then describes the same water. Skip it, and two readings taken from two scoops a few minutes apart can disagree by more than the dose you are about to compute.
Free chlorine by FAS-DPD, first. Add the DPD powder to the measured sample: it turns pink in proportion to free chlorine. Titrate with the FAS reagent drop by drop, swirling, until the pink is just gone. In the common kits each drop is 0.5 ppm on a 10 mL sample and 0.2 ppm on a 25 mL sample; use the small sample when the pool is likely to read high. Reason for going first: chlorine decays in a sample sitting in the sun, so the clock starts at the scoop. Read it within a minute or two.
Combined chlorine, straight on. Into the same cleared sample, add the kit's combined chlorine reagent. If pink returns, keep titrating; the extra drops at the same ppm per drop are combined chlorine. Reason: total and free from one sample, read back to back, give a combined figure you can trust. Read separately, sample drift invents combined chlorine that is not there, and a false 0.5 ppm combined moves a shock target by 5 ppm under the breakpoint rule in 'Shock Dosing Standard'.
pH, after you know the chlorine. Phenol red reads pH in the 6.8 to 8.2 range on most comparators. Above the free chlorine level the kit's instructions name, chlorine can shift phenol red to a false purple-red that reads as high pH, or bleach it toward a false low; the fix is the kit's chlorine neutralizer drop added to the pH sample first, per the instructions. Reason for the order: you cannot know whether the pH reading is trustworthy until you know the chlorine. A tech who doses acid off a falsely high pH on a heavily chlorinated pool takes a balanced pool acidic.
Total alkalinity. Neutralize the chlorine first if the kit calls for it, add the indicator, titrate with the acid titrant until the color changes. In the common kits each drop is 10 ppm on a 25 mL sample. Note that TA on a stabilized pool includes some alkalinity from the cyanuric acid itself, which matters for the saturation index and is corrected there, not here. Reason it follows pH: you need both to decide between acid, bicarbonate or nothing, and they are read on the same visit.
Calcium hardness. Buffer, indicator, then the hardness titrant until the color turns. Each drop is typically 10 ppm on a 25 mL sample. Reason it is read every visit on plaster pools: CH only moves slowly, but it moves one way on a cal-hypo pool or a pool with hard top-up water, and the trend is what you are watching.
Cyanuric acid by turbidity. Mix the reagent with the sample, pour into the view tube in shade with your back to the sun, and stop when the black dot at the bottom just disappears. Read the scale at the liquid line. Reason for caution: it is the least precise test on the panel, the readable range is limited (the kit states it), and an over-range sample has to be diluted with distilled or known-zero water and the result multiplied by the dilution. Run it in daylight in the shade, not in direct sun and not under a truck light.
Salt, where a cell is fitted. Use a salt titration or a calibrated meter. Reason: the cell's display reads conductivity at the cell, corrected for temperature by the cell's own model, and a scaled or failing cell often reads low. A dose sized off the display is a dose sized off the cell's fault.
Record every reading with the water temperature and the time. Finish test: the ticket holds FC, CC, pH, TA, CH, CYA, salt where fitted, water temperature and the time sampled, each written as read, before any product is opened.
Worked example: a routine stop and a trap
Routine. A 20,000 gallon plaster pool at 9:40, pump running since 8:00, water 82 F. FAS-DPD on 10 mL: 6 drops to clear, so 6 times 0.5 is 3.0 ppm free chlorine. Combined reagent: pink returns, 1 more drop to clear, so combined chlorine 0.5 ppm and total 3.5 ppm. pH 7.6, no neutralizer needed at 3 ppm. TA on 25 mL: 9 drops, 90 ppm. CH on 25 mL: 28 drops, 280 ppm. CYA: dot disappears at 50 ppm. No cell. FC, CC, pH, TA, CH and CYA logged at 9:46 with the 82 F water temperature; salt marked not applicable rather than left blank, so the next reader knows it was not skipped. Six minutes from scoop to last entry is about right for the full panel; a panel that routinely takes twenty is a panel where the chlorine sample sat in the sun.
The trap, next stop. A customer tells you they "added some chlorine" on Saturday. FAS-DPD on 10 mL: 24 drops, so 24 times 0.5 is 12.0 ppm free chlorine. The pH tube, run without a neutralizer, shows a deep purple-red past the top of the comparator, which reads as pH 8.2 or above. A tech who skips step 4's reasoning reaches for acid. Instead the sample is re-run with the neutralizer drop added first and reads 7.7, inside the range. No acid goes in. The pool needs nothing but time for the chlorine to come down, and the ticket says so, with the customer's addition noted so next week's reading is read in context.
What would have happened. Acid sized for a drop from 8.2 to 7.5 on a pool that was really at 7.7 would have pushed it to roughly 7.0 or lower, with the TA dropping alongside. On plaster that is etching water; on a heater, corrosive water through the exchanger.
When the panel gets shorter or longer
- A salt pool. Add salt every visit, and expect the pH to trend up week to week from the cell.
- A pool whose readings swing. If two consecutive visits disagree by more than the dose between them explains, run the panel twice from two samples at the same visit. If the two samples agree, the pool changed. If they disagree, the technique or the reagents did.
- A cloudy or green pool. The panel still runs, but a heavy chlorine demand can bleach the DPD itself and read near zero at high chlorine. Run free chlorine on the smallest sample size the kit allows and treat an unexpected zero as a reason to retest, not a reading.
How to verify your testing
Once a month, split one sample and run the full panel on both halves. FC and CC should agree within a drop, TA and CH within a drop, pH within one comparator step. Twice a season, run the same sample beside a second kit or a lab reading. Drift beyond that points at reagent age first, technique second.
References
- See related: Water Test Accuracy (Titration vs Strip); Commercial Pool DPD Titration to ANSI/APSP/ICC-11 Standards
- See related: Shock Dosing Standard (breakpoint use of combined chlorine); How to Calculate the Saturation Index on the Pool Deck
- Test kit manufacturer's instructions for sample sizes, drop equivalents, neutralizer use and reagent dates
- Reagent safety data sheets for handling and first aid