Shock Dosing Standard

Purpose

This procedure guarantees that a shock dose is computed from the pool's own measured combined chlorine and stabilizer level, poured in a form that will not wreck the water balance it is meant to protect, and verified by a re-test before anybody is told the pool is open. Two failures live here, both routine. Under-dosing leaves combined chlorine in place, so the customer smells "too much chlorine" in a pool that is short of it and the shop returns twice. Over-dosing in the wrong form drives calcium hardness or cyanuric acid permanently upward, and the only correction for either is replacing water.

Scope

Covers oxidation of a residential or light-commercial pool or spa: selecting the product form, computing the dose, dosing, verifying, and setting the reopening condition.

It does not cover algae remediation, which the algae treatment SOP owns and which is a different target level over multiple visits. It does not cover a fecal, vomit or blood contamination event, which is a hyperchlorination response run against a published contact-time table rather than a breakpoint calculation. Acid additions on the same visit belong to the muriatic acid SOP, and commercial titration technique belongs to the DPD titration SOP.

Roles and responsibilities

What sets the dose

Two rules compete for the target, and you take the higher of the two.

Breakpoint. Combined chlorine is chlorine already bound to nitrogen compounds, and destroying it takes an excess of free chlorine. The working field rule is a free chlorine level about ten times the measured combined chlorine, so the dose in ppm is ten times CC minus the free chlorine present. CC is never measured directly: it is total chlorine minus free chlorine, which is why a test that reads only free chlorine cannot size a shock.

Stabilizer. Cyanuric acid holds chlorine in reserve and lowers the fraction available as active sanitizer, so a stabilized pool needs a higher number on the meter for the same effect. The widely used field ratio is a routine minimum free chlorine near 7.5 percent of the cyanuric acid reading and an elevated target near 40 percent of it for oxidation. That ratio comes from industry and lab work, not from a code; the code side is your health department's minimum free chlorine and stabilizer cap. The CDC's Model Aquatic Health Code recommends cyanuric acid at or below 90 ppm outdoors and binds you only where your state or county has adopted it.

Product form decides what else the dose adds to the water:

Trade percent on liquid is available chlorine by volume, so the numeral is close to the ppm one gallon delivers per 10,000 gallons: a 10 percent product gives about 10 ppm. The last two rows are why product choice is part of the calculation and not a stock question: on a pool already at 90 ppm cyanuric acid, a 20 ppm dose of dichlor adds another 20 ppm of stabilizer and makes the next shock harder than this one.

Procedure

1. Close the pool and post the reopening condition before you open a container. Nobody in the water, nobody on the deck downwind of the pour. Accept when the closure note is physically posted at the gate with the reading required to reopen written on it, not spoken to whoever answered the door. Wrong is a verbal "give it a few hours." Elevated free chlorine bleaches swimsuits and irritates eyes and airways at levels a swimmer cannot see, so an unposted pool is a trap for the next person home from school.

2. Test free chlorine, total chlorine, pH, cyanuric acid, calcium hardness and temperature, and write all six down. Sample at elbow depth away from returns with in-date reagent. Accept when free and total are read off the same sample within the same minute, because combined chlorine is their difference and a drifting sample invents it. Wrong is a strip reading, or total assumed equal to free. Re-test rather than estimate: a CC reading wrong by 0.5 ppm moves the target by 5 ppm.

3. Correct pH into the 7.2 to 7.8 band before oxidizing, not after. Hypochlorous acid, the active fraction, falls off sharply as pH rises, so a shock poured at pH 8.2 spends part of itself getting nowhere. Accept when pH sits inside the band on a fresh test. Wrong is dosing first and correcting after, which is also how an acid addition and an oxidizer addition end up minutes apart. If acid is required, add it, circulate, re-test, and only then oxidize; acid meeting hypochlorite in a skimmer or a bucket makes chlorine gas.

4. Compute both targets, take the higher, and check it against the stabilizer ceiling. Write the breakpoint figure and the stabilizer figure side by side on the ticket, then the dose in ppm and the product volume. Accept when the required target sits at or below 40 percent of a cyanuric acid reading that is itself at or under 100 ppm. Wrong is a computed target the pool cannot practically hold. Stop and quote a partial water replacement instead, escalating to the shop lead before the customer hears a number.

5. Verify strength on the label and pre-dissolve any granular product in a bucket of pool water, product into water, never the reverse. One bucket, oxidizer only, filled with water first. Accept when no undissolved grains remain at the bottom before anything is poured. Wrong is broadcasting cal-hypo dry across a vinyl liner or dark plaster, which bleaches where the grains land. Cal-hypo dust is a respiratory irritant, so open the pail downwind with your face away from the rim, and it will ignite on contact with trichlor or any organic in the bucket, so that bucket is single-purpose and rinsed before it goes back on the truck.

6. Dose with the pump running, distributed around the perimeter, and after sundown where you can choose. Ultraviolet light destroys free chlorine quickly, so a mid-afternoon shock on an unstabilized pool loses a meaningful share before it works. Accept when the pour is spread across at least three points with visible return flow. Wrong is a single-point pour, a skimmer pour, or a shock left under a closed solar cover, which traps gas against the cover and delaminates it. Take the cover off first and leave it off until the re-test.

7. Circulate at least one full turnover before any re-test, and brush the walls and floor during it. Accept when the pump has run continuously for the turnover time you calculated from flow and volume, not for "a few hours." Wrong is a re-test at 40 minutes, which reads the plume rather than the pool and reliably reports a false high. A salt cell's boost or superchlorinate mode is not a substitute for this dose: it raises output over days, not to breakpoint in one turnover.

8. Re-test at turnover and accept only on combined chlorine, not on free chlorine. Accept when combined chlorine reads 0.2 ppm or below with free chlorine still above the routine minimum for the pool's stabilizer level. Wrong is a free chlorine reading that looks impressive over a CC that has not moved, which means the dose never reached breakpoint and the pool needs the balance of the calculated amount, not a fresh calculation. Do not reopen on a partial result.

9. Reopen only on the posted condition, and record who made the call. Free chlorine back inside the operating band and CC at or below 0.2 ppm. Accept when that reading is taken by the technician, or read to the technician from a customer test with the value stated. Wrong is a customer who pulled the sign and swam at 15 ppm: record it as a customer reopening, because the shop's exposure changes when the instruction was posted and ignored.

The record this produces

One shock line per event on the customer ticket, carried into the pool's chemical history.

• Six pre-dose readings with the time, including total chlorine, since it is the field most often skipped.
• Both computed targets, the breakpoint figure and the stabilizer figure, and which one governed.
• Product form, strength, and amount added in label units.
• Closure and reopening: the condition posted, the time posted, the reading and time at reopening, and who read it.
• Exceptions: any dilution quoted, any dose split, any customer reopening.

The next technician reads the last two shock lines first, because a pool needing breakpoint every month has a bather-load or fill-water story rather than a chlorine story. The shop lead reads the governed-target column: if the stabilizer figure governs on most tickets, the route is being sold too much stabilized product.

One shock, filled in

A 20,000 gallon plaster pool, complaint of chlorine smell and stinging eyes, which is a combined chlorine complaint. Readings at 14:10: free chlorine 3.2 ppm, total chlorine 4.6 ppm, so combined chlorine is 1.4 ppm. pH 7.6, in band. Calcium hardness 300 ppm. Cyanuric acid 110 ppm. Water 86 F.

Breakpoint target: ten times 1.4, which is 14 ppm free chlorine, so the dose is 14 minus 3.2, or 10.8 ppm. In 20,000 gallons the 12.5 percent liquid delivers 6.25 ppm per gallon, so that is 1.7 gallons.

Step 4 fails. The stabilizer figure is 40 percent of 110, which is 44 ppm free chlorine, and the higher of the two governs. The dose would be 44 minus 3.2, or 40.8 ppm, which is 6.5 gallons of the 12.5 percent liquid into a pool that would then sit closed for days. The cyanuric acid reading is also above the 100 ppm ceiling in the acceptance condition. Stop rule taken: no shock today, dilution quoted, shop lead approves before the customer is told.

Dilution: bringing 110 ppm to 60 ppm means replacing one minus 60 over 110 of the volume, about 45 percent, so roughly 9,000 of the 20,000 gallons. Drained to the sanitary connection the local authority requires, refilled, circulated overnight.

Re-test the following afternoon: cyanuric acid 58 ppm against the predicted 60, free chlorine 1.8 ppm, total chlorine 2.6 ppm, so combined chlorine 0.8 ppm. Breakpoint target is now 8 ppm; the stabilizer target is 40 percent of 58, or 23 ppm, which still governs. Dose is 23 minus 1.8, or 21.2 ppm, which is 3.4 gallons of the 12.5 percent liquid. Liquid is chosen over cal-hypo because 21 ppm from cal-hypo would add about 15 ppm of calcium hardness to water already at 300 ppm.

Poured at 19:30 across four points, pump running, cover off, brushed. Re-tested after one turnover: free chlorine 19 ppm, combined chlorine 0.2 ppm. Step 8 accepts on the CC value; free chlorine below the 23 ppm target is decay, not failure, and the CC result proves breakpoint was passed. Pool stays closed. Next morning free chlorine 9 ppm, CC 0.0 ppm, and 9 ppm is well above the 4.4 ppm routine minimum for 58 ppm of stabilizer, so it reopens on a technician reading at 09:40 and the note comes down.

References

• CDC Model Aquatic Health Code - recommended cyanuric acid ceiling for outdoor pools; a model code binding only where your state or county has adopted it.
• Your health department's pool code for minimum free chlorine, stabilizer cap, and closure and reopening rules on regulated pools.
• 29 CFR 1910.1200 - labeling and employee access to safety data sheets for every oxidizer carried.
• Product label and safety data sheet for available chlorine percentage, incompatibilities and first aid.
• See related: the algae treatment SOP for remediation targets; the muriatic acid SOP for pH correction; the salt cell SOP for why boost mode is not a shock.