22 Testing, Maintenance & Troubleshooting Practice Questions & Answers

1. 1. An operator is collecting a water sample for free chlorine and pH testing at a public pool. Which sampling practice best represents the water the bathers are actually in?

   • A.Dip the sample out of the skimmer basket, since all the water passes through there
   • B.Push the inverted tube to about elbow depth away from return inlets and skimmers, then let it fill, holding the tube near the top✓ Answer
   • C.Skim the sample off the surface directly in front of a return inlet
   • D.Reach over from the deck and fill the tube at the waterline beside the shallow-end steps

   Water leaving a return inlet is freshly treated and reads artificially high, while a skimmer throat or the steps holds surface film and stagnant water. A sample taken at mid-body depth away from inlets and skimmers reflects the bulk of the pool, and holding the tube near the top keeps hand oils out of the sample.

   Source: PHTA Pool & Spa Operator Handbook, water sampling and testing chapterReport a problem with this question

2. 2. An operator needs to know how much of the chlorine residual is still available to disinfect and how much has already combined with contaminants. Which statement about the two common field methods is correct?

   • A.DPD reads chlorine and pH at the same time, so a separate pH test is unnecessary
   • B.OTO separates free from combined chlorine; DPD reports total chlorine only
   • C.Both methods report free chlorine only, so combined chlorine cannot be measured in the field
   • D.DPD can be run for free and then for total chlorine, so combined chlorine is found by subtraction; OTO indicates total chlorine only✓ Answer

   DPD is the common field chemistry that distinguishes the fractions: the first reading gives free available chlorine, the completed reaction gives total, and total minus free is the combined chlorine. OTO reacts with all chlorine present and therefore cannot tell the operator whether a residual is doing useful disinfection or is already tied up as chloramine.

   Source: PHTA Pool & Spa Operator Handbook, chemical testing chapter (DPD and OTO methods)Report a problem with this question

3. 3. A pool has just been heavily superchlorinated. A color-matching DPD chlorine test on it flashes faint pink and then fades to clear. What should the operator conclude and do?

   • A.The chlorine has been consumed and the residual is gone, so more chlorine should be added at once
   • B.The very high chlorine bleached the reagent, so the reading is falsely low; dilute the sample with chlorine-free water, retest and multiply by the dilution factor✓ Answer
   • C.The reagent is reacting with combined chlorine only, so the free chlorine reading should be taken as zero
   • D.The result is valid and simply low, so the pool can reopen once pH is confirmed

   At high chlorine levels the DPD dye is oxidized past its colored form and bleaches out, producing a colorless or near-colorless result that looks like no chlorine at all. Recognizing the flash of color as the tell, the operator dilutes a measured sample with chlorine-free water, retests and multiplies the result; dosing more chlorine on a bleached-out reading would be a serious error.

   Source: PHTA Pool & Spa Operator Handbook, testing chapter — DPD bleach-out interferenceReport a problem with this question

4. 4. Immediately after superchlorination, the phenol red pH test on the same water turns an odd purplish shade that does not match any color on the comparator. What is happening and what is the correct step?

   • A.The pH is genuinely far above the top of the scale, so acid should be added until the purple disappears
   • B.The sample was taken too deep, so it should be retaken at the surface
   • C.High chlorine converted the phenol red indicator, so the color is meaningless; add a chlorine-neutralizing reagent to a fresh sample before the indicator, or retest after the residual falls✓ Answer
   • D.The comparator has faded, so the test should be read against a white deck instead

   A strong chlorine residual oxidizes phenol red into chlorophenol red, which develops colors the pH comparator was never made to read, so the result is an artifact rather than a pH. Neutralizing the chlorine in the sample first (or waiting for the residual to drop) restores a readable pH; chasing the false color with acid can drive the water badly out of balance.

   Source: PHTA Pool & Spa Operator Handbook, pH testing with phenol redReport a problem with this question

5. 5. Why do many operators run a FAS-DPD titration instead of a color-matching DPD comparator when the chlorine residual is high or when precision matters?

   • A.The titration reports chlorine and total alkalinity in a single test
   • B.The titration needs no reagents and therefore cannot expire
   • C.The titration reads oxidizer strength in millivolts and replaces the need for a residual test
   • D.The titration counts drops to a clear endpoint, giving finer resolution, and it does not bleach out the way a color-matching DPD reading does at high chlorine✓ Answer

   In a FAS-DPD titration the operator adds titrant drop by drop until the pink color disappears, and the drop count is the measurement, so resolution is set by the drop rather than by the eye matching a color block. Because the reading is taken at an endpoint, a strong residual does not bleach the result into a falsely low number the way a straight color comparison can.

   Source: PHTA Pool & Spa Operator Handbook, FAS-DPD titrationReport a problem with this question

6. 6. Why does a test kit carry dated reagents with storage instructions, and why should bottles from different kits not be substituted for one another?

   • A.The dates are only for inventory control and have no effect on the readings
   • B.Reagents lose strength with age, heat and light, and each is formulated to work with its own comparator and sample volume, so old, hot-stored or mismatched reagents give wrong answers that look perfectly normal✓ Answer
   • C.Reagents are inert, but the labels are required so inspectors can confirm a kit was purchased
   • D.Any reagent of the same chemical name may be swapped in freely, since the chemistry is identical between manufacturers

   Test reagents are consumable chemistry that degrades over time and faster in heat and sunlight, and each reagent set is matched to a specific comparator, sample volume and drop size. A degraded or mismatched reagent still produces a color and a number, so the operator has no visible warning that the whole day's log is wrong.

   Source: PHTA Pool & Spa Operator Handbook, test kit care and reagent storageReport a problem with this question

7. 7. The water in a pool is dull and faintly green, yet the operator's chlorine test reports a strong free chlorine residual. What is the best next step?

   • A.Trust the number, since an instrument reading always outranks an impression of the water
   • B.Treat the disagreement as a reason to verify the test — retest with a fresh sample, a clean tube and fresh reagents, and run a diluted sample if interference is possible — before dosing on that number✓ Answer
   • C.Ignore the test and keep adding oxidizer until the color clears, checking nothing
   • D.Assume the pool is fine and record the reading, since color alone is not a water quality concern

   A reading that contradicts what the water plainly looks like usually means an interference or a procedural fault — bleached reagent, expired or mismatched reagents, a contaminated tube, or a sample drawn at a return inlet. Verifying the test first costs minutes, whereas dosing chemical on a wrong number can push the water further out of balance and hide the real problem.

   Source: PHTA Pool & Spa Operator Handbook, testing practice and troubleshootingReport a problem with this question

8. 8. Weeks after a patron reports becoming ill following a swim, an investigator asks the facility for proof of how the water was operated that day. Why is the daily chemical log the operator's most important document?

   • A.It is contemporaneous evidence that the water was tested and operated as required at the time, and missing or blank entries are read as evidence that the checks were not done✓ Answer
   • B.It is required only by equipment manufacturers to keep a warranty in force
   • C.It substitutes for an incident report, so no separate account of the event is needed
   • D.It can be reconstructed afterward from memory, which is why entries are made in pencil

   A log written at the time of each test, with the reading, the time and the tester's initials, is the only record that shows the facility was under control when the exposure occurred. Because it cannot honestly be recreated later, gaps in the log are treated as gaps in operation, which is why entries are made immediately and kept for the period the health authority requires.

   Source: PHTA Pool & Spa Operator Handbook, record keeping chapter; CDC Model Aquatic Health Code guidance on operator recordsReport a problem with this question

9. 9. An outdoor pool turns cloudy green over a hot weekend. Beyond adding an oxidizer, which approach best describes correct treatment of a free-floating green algae bloom?

   • A.Brush all surfaces, run circulation and filtration continuously, hold the oxidizer residual until the color clears, then clean the filter and correct what allowed the bloom✓ Answer
   • B.Shut the pump off during treatment so the chemical is not filtered out of the water
   • C.Drain and refill, because algae cannot be killed once the water has turned
   • D.Stop feeding sanitizer and add a clarifier so the filter can remove the algae mechanically

   Brushing lifts algae off surfaces and into the water where the oxidizer can reach it, continuous circulation carries treated water everywhere and lets the filter remove the dead cells, and the filter is cleaned afterward because it loads heavily. Treatment only holds if the underlying cause is fixed — a residual that was allowed to fall, poor circulation, or dead spots where treated water never reaches.

   Source: PHTA Pool & Spa Operator Handbook, algae control chapterReport a problem with this question

10. 10. A yellowish, powdery growth keeps reappearing on the shaded wall of a pool that otherwise holds a normal sanitizer residual. It brushes away in a cloud and settles back overnight. Which statement is correct?

    • A.It is pollen and dirt, so vacuuming it out is all that is needed
    • B.It is a harmless alga that always dies off on its own at any normal residual
    • C.It indicates excess calcium and is removed by adding acid
    • D.Mustard algae tolerates an ordinary residual, so it takes vigorous brushing plus a stronger oxidizer treatment, and every item that touched the water must be disinfected or it will be reintroduced✓ Answer

    Mustard or yellow algae is notably more resistant than green algae and survives residuals that look adequate on the log, which is why it returns to the same shaded wall again and again. Treatment combines mechanical removal by brushing with a stronger oxidizer dose, and poles, brushes, vacuum heads, floats and swimwear must be disinfected because they carry the organism back into the pool.

    Source: PHTA Pool & Spa Operator Handbook, algae control chapterReport a problem with this question

11. 11. Why are the dark, firmly attached spots of black algae harder to eliminate from a plaster pool than a free-floating green algae bloom?

    • A.It lives only in the water column, so it is removed by filtration alone
    • B.It is not an alga but a mineral stain, so only acid will remove it
    • C.It is unaffected by any oxidizer, so the only remedy is replastering the pool
    • D.It grows in layered colonies under a protective sheath and anchors into pores and cracks in the surface, so the oxidizer cannot reach the living layer until the spot is broken open by aggressive brushing✓ Answer

    Black algae protect themselves with a tough outer layer and root into the porous surface, so a residual that easily kills suspended green algae never contacts the living cells underneath. Effective treatment is mechanical first — break the spots open with a stiff brush suited to the surface — and only then does the oxidizer, held at strength with good circulation, reach and kill the colony.

    Source: PHTA Pool & Spa Operator Handbook, algae control chapterReport a problem with this question

12. 12. A pool stays persistently cloudy even though the free chlorine test reads normally and there is no visible algae. How should the operator narrow down the cause?

    • A.Drain and refill immediately, since cloudiness cannot be diagnosed in place
    • B.Assume the cause is always low chlorine and repeat shock treatments until it clears
    • C.Work through the candidates in turn — filtration and flow, water balance tipping toward scale formation, very high dissolved solids or stabilizer, and the earliest stage of algae growth — checking each rather than dosing blindly✓ Answer
    • D.Add a clarifier first and use the response as the diagnosis

    Cloudiness is a symptom with several possible causes, so the operator eliminates them systematically: filter condition, pressure and flow, turnover actually achieved, the direction of the saturation index, the level of dissolved solids and stabilizer, and any sign of an early bloom. Dosing chemicals before the cause is identified adds more dissolved material to water that may already be overloaded, making the haze worse.

    Source: PHTA Pool & Spa Operator Handbook, troubleshooting chapter — cloudy waterReport a problem with this question

13. 13. A brownish discoloration is spreading on the plaster floor of a pool. What is a sound way for the operator to tell an organic stain from a metal stain?

    • A.Spot-test a small area: an organic stain from leaves or debris lightens where an oxidizer is applied, while a metal stain does not respond to the oxidizer but fades under an ascorbic-acid type reducing treatment✓ Answer
    • B.Only a laboratory analysis of the water can distinguish them, since surface tests are meaningless
    • C.Any stain that will not brush off is by definition a metal stain
    • D.Color alone identifies the source, so no test is needed

    Organic material is destroyed by oxidation, while metal staining is a deposited metal that responds instead to a reducing or sequestering treatment, so the two react in opposite ways to a small spot test. Identifying the type also points to the source — iron reads rusty brown, copper blue-green, manganese dark — and metals usually enter through the fill water or are dissolved out of equipment by aggressive water, which must be corrected or the stain returns.

    Source: PHTA Pool & Spa Operator Handbook, staining and discolorationReport a problem with this question

14. 14. A pool shows a hard white-grey crust at the waterline and the heat exchanger is found to be coated with rough deposits. What does this indicate and what is the correct response?

    • A.The water is aggressive and corrosive, so calcium should be added
    • B.The water is oversaturated and scale-forming, so the operator rebalances the water to bring the saturation index back toward balance, removes the deposits, and remembers that only replacing water lowers calcium hardness✓ Answer
    • C.It is an algae colony and will disappear after a shock treatment
    • D.There is too much chlorine, so the residual should be cut back until the crust dissolves

    Scale is mineral coming out of solution because the combination of pH, alkalinity, calcium hardness and temperature has driven the saturation index positive, and the hottest surface in the system — the heat exchanger — scales first. The fix is to move the index back toward balance and physically remove what has already deposited; a sequestering agent can help hold minerals in solution, but calcium hardness itself only comes down through dilution with water that contains less of it.

    Source: PHTA Pool & Spa Operator Handbook, saturation index and scale controlReport a problem with this question

15. 15. Which practice correctly matches cleaning method to pool surface?

    • A.Use a soft nylon brush on vinyl liners and fiberglass, reserve stiff stainless bristles for unpainted plaster or concrete, and never let undissolved granular chemical sit on a liner or fiberglass surface✓ Answer
    • B.A stainless-steel bristle brush is safe on every surface as long as it is used gently
    • C.Vinyl liners need a monthly abrasive scrub to keep them from becoming porous
    • D.Granular chemical should be broadcast onto a vinyl floor so it dissolves slowly where it lies

    Stainless bristles are meant for hard, unpainted plaster and concrete; on vinyl, fiberglass or painted surfaces they cut and scratch, and the damage is permanent. Undissolved granular chemical resting on a soft surface concentrates in one spot and can bleach or degrade it, which is why granular products are pre-dissolved or broadcast over a deep area with the circulation running and then brushed.

    Source: PHTA Pool & Spa Operator Handbook, pool surfaces and cleaningReport a problem with this question

16. 16. How often should free and combined chlorine and pH be tested at a public facility?

    • A.Only when the water looks or smells wrong, since testing clear water wastes reagent
    • B.At least as often as the health authority having jurisdiction requires, and more often when bather load, weather or chemical feed performance make the water change faster, with every test logged✓ Answer
    • C.Exactly three times a day at every facility in the country, regardless of use
    • D.Once a week is sufficient as long as an automated controller is installed

    Minimum testing frequency is set by the health code in force at the facility, so the operator's job is to look that requirement up and meet it rather than rely on a remembered number, since it varies between jurisdictions. The required minimum is a floor, not a target: heavy bather load, rain, high temperature or a feeder that is drifting all change the water faster than the schedule assumes, and the log is what proves the tests were done.

    Source: PHTA Pool & Spa Operator Handbook, testing frequency; local health code having jurisdictionReport a problem with this question

17. 17. When is vacuuming to waste the right choice, and what must the operator manage while doing it?

    • A.It is only for large leaves and other coarse debris that would clog the vacuum hose
    • B.It sends the debris through the filter twice for a more thorough cleaning
    • C.It should be the routine method for every vacuuming because it saves backwashing the filter
    • D.It is used for very fine or biological debris such as dead algae and silt that would pass through or blind the filter; because that water leaves the system, the operator must add make-up water, keep the pump from losing prime, and retest and rebalance afterward✓ Answer

    Vacuuming to waste routes the dirty water past the filter and out of the system, which is the right call when the material is too fine to be captured or would quickly blind the media, as dead algae does. The trade-off is that pool water is being discarded, so the level must be maintained to keep the skimmers and pump supplied, and the fresh make-up water changes the chemistry enough that testing and rebalancing follow.

    Source: PHTA Pool & Spa Operator Handbook, cleaning and vacuumingReport a problem with this question

18. 18. A 30,000-gallon pool measures cyanuric acid at 80 ppm and the operator wants to bring it to 40 ppm. No chemical destroys stabilizer. Assuming dilution is proportional and the replacement water contains none, about how much water must be replaced?

    • A.15,000 gallons✓ Answer
    • B.7,500 gallons
    • C.3,000 gallons
    • D.The full 30,000 gallons

    Dilution is proportional, so cutting the concentration in half means replacing half the volume: 30,000 divided by 2 equals 15,000 gallons of pool water drained and replaced with water that contains no stabilizer. This is the general principle behind everything that cannot be treated chemically — stabilizer, calcium hardness and dissolved solids come down only by replacing water.

    Source: PHTA Pool & Spa Operator Handbook, cyanuric acid and dilutionReport a problem with this question

19. 19. Using the common operating rule of thumb that days between drain-and-refill equals spa volume in gallons divided by (3 times the average number of bathers per day), how often should a 750-gallon spa averaging 25 bathers per day be drained?

    • A.About every 90 days
    • B.About every 10 days✓ Answer
    • C.About every 30 days
    • D.About every 3 days

    Applying the given rule: 3 times 25 bathers equals 75, and 750 gallons divided by 75 equals 10 days. The formula exists because a spa holds very little water per bather, so dissolved solids, body oils and treatment by-products build up quickly and dilution — not more chemical — is what resets the water; the health authority having jurisdiction may also set its own requirement.

    Source: PHTA Pool & Spa Operator Handbook, spa operation — drain and refill rule of thumbReport a problem with this question

20. 20. A heavily used spa foams over the jets, its readings swing between tests, and a scum line forms at the waterline. What explains this and what is the durable fix?

    • A.Adding a defoaming product is the complete remedy, since the foam is the problem
    • B.Small volume, high temperature, aeration and a very high bather load per gallon concentrate soaps, oils, body products and dissolved solids fast; the lasting fix is drain-and-refill dilution plus pre-soak showering and more frequent testing✓ Answer
    • C.It is caused by high calcium hardness and is corrected by adding acid
    • D.Foam means excess sanitizer, so chemical feed should be stopped until it subsides

    A spa concentrates contamination far faster than a pool because each bather brings the same load into a tiny, hot, aerated volume, and the surfactants that result are what foam when the jets and blower agitate the water. Defoamer only hides the symptom and adds more dissolved material; dilution removes what cannot be oxidized away, and the warm plumbing and air lines must also be purged periodically because biofilm shelters organisms from the sanitizer.

    Source: PHTA Pool & Spa Operator Handbook, spa and hot tub operationReport a problem with this question

21. 21. How does out-of-balance water damage a pool heater?

    • A.Scale-forming water deposits mineral on the hot heat-exchanger surfaces, insulating them and restricting flow so the heater overheats and loses efficiency, while aggressive water and low pH corrode and can perforate the exchanger — which is also why chemical is fed downstream of the heater✓ Answer
    • B.A thin scale layer is desirable because it improves heat transfer into the water
    • C.Only calcium hardness matters to a heater; pH and alkalinity have no effect
    • D.Heaters are made of inert materials and are unaffected by water chemistry

    The heat exchanger is the hottest wetted surface in the system, so water that is oversaturated drops mineral there first; the deposit insulates the metal and chokes flow, which raises internal temperature and can trip limits or crack the exchanger. Water on the aggressive side does the opposite kind of harm, dissolving the metal itself, and injecting chemical ahead of the heater exposes it to undiluted product — which is why feed points are located after the heater.

    Source: PHTA Pool & Spa Operator Handbook, pool heating chapterReport a problem with this question

22. 22. What is the operating principle behind winterizing an outdoor pool in a climate with hard freezes?

    • A.Pouring antifreeze into the pool water itself is what protects the structure
    • B.The pool should be drained completely, since an empty shell cannot be damaged by cold
    • C.Everything can simply be switched off and left full, because moving water is what causes freeze damage
    • D.Water expands as it freezes and will split pipes, pumps, filters and heaters, so the water level is lowered as needed, lines and equipment are drained or blown clear and plugged, and the water is balanced and the pool covered before closing✓ Answer

    Freeze damage happens because trapped water expands and bursts whatever contains it, so winterizing is fundamentally about removing water from every place it can be trapped and giving the remaining water room to expand. Draining the pool itself is the wrong answer: an empty shell can float or heave from groundwater and the surface can crack and dry out, so the pool is left with water at the appropriate level, balanced, covered and checked through the off season.

    Source: PHTA Pool & Spa Operator Handbook, seasonal opening, closing and winterizingReport a problem with this question

Practice questions based on the Pool & Hot Tub Alliance (PHTA) Pool & Spa Operator Handbook and public CDC healthy-swimming guidance. CPO and Certified Pool/Spa Operator are marks of the Pool & Hot Tub Alliance; this site is not affiliated with or endorsed by PHTA. Water-quality limits, turnover requirements and bather loads are set by the health code where your facility operates — always follow that code and your local health department, and confirm current course requirements before testing. About the CPO certification →