How to Size the Weekly Liquid Chlorine Dose to Last the Week
Why this matters
A weekly route pool gets chlorine once and has to hold it for seven days. Dose to a fixed number on every pool, and the shady screened pool sits at a needlessly high level all week while the sunny one with three kids runs dry by Thursday and turns on Friday. The green-pool callbacks on a route are mostly this: a dose that was never sized to the pool's own weekly loss. The method below uses two numbers you already have, last week's dose and today's reading, to set a target that still sits above the pool's minimum when you come back. Shock is a different job with different targets, owned by 'Shock Dosing Standard'.
Start state
- This week's panel, run per 'How to Run the Weekly Water Test Panel at the Pool', with free chlorine by FAS-DPD and a CYA reading no more than a month old.
- Last week's log line: the level the pool was dosed to and the date.
- The pool's volume from 'How to Estimate Pool and Spa Gallons'.
- The jug in front of you. This card is written for 12.5 percent trade-strength liquid chlorine. Read the label; a 10 percent product delivers proportionally less, and liquid chlorine loses strength in storage, faster in heat.
The one factor: 12.5 percent liquid delivers about 12.5 ppm of free chlorine per gallon in 10,000 gallons ('Shock Dosing Standard'), so 1 ppm in 10,000 gallons takes 128 over 12.5, or about 10.24 fl oz. The general dose formula belongs to 'The Dose Per 10,000 Gallons Factor Card'.
The route log behind the dose
ROUTE LOG Pool A 20,000 gal CYA 30 ppm
min FC for CYA 30 = 7.5% x 30 = 2.25 ppm
date FC found dosed to 12.5% liquid
Jun 03 2.4 8.5 0.98 gal
Jun 10 2.0 9.0 1.12 gal
loss Jun 03 to Jun 10:
(8.5 - 2.0) / 7 days = 0.93 ppm per day
target = 2.25 + 6.5 = 8.75 -> 9.0 ppm
dose = (9.0 - 2.0) x 2.0 x 10.24 = 143 fl oz
= 143 / 128 = 1.12 gal
The steps
Find the pool's floor: the minimum free chlorine for its stabilizer level. Use the routine minimum of about 7.5 percent of the CYA reading, the field ratio stated in 'Shock Dosing Standard'. Pool A at 30 ppm CYA has a floor of 2.25 ppm. Reason: stabilizer holds chlorine in reserve, so a stabilized pool at 2 ppm is not the same as an unstabilized one at 2 ppm. Skip it and you aim the week's dose at a number that sounds safe and is not, for that pool. Your health department's minimum still governs a regulated pool.
Figure last week's daily loss from the log. Daily loss is the level the pool was dosed to last visit, minus today's reading, divided by the days between. Pool A: 8.5 minus 2.0 is 6.5 ppm, over 7 days, 0.93 ppm a day. Reason: this is the pool's own demand (sun, temperature, bathers, debris) measured on the pool, which is better than any rule of thumb. The "dosed to" figure is the computed target unless someone measured after circulation; that assumption is fine as long as the gallons and the jug strength are right.
Check whether last week held. Today's 2.0 against a 2.25 floor: last week's dose fell short by about a quarter of a ppm. Small, and still the reason this step exists. Reason: a pool that arrives under its floor spent part of the week under-protected, and repeating last week's target repeats the shortfall.
Set this week's target: floor plus a week of loss, rounded up. 2.25 plus a week of loss (7 times 0.93, which is the measured 6.5 ppm) is 8.75; round up to 9.0 ppm. Reason for rounding up rather than to nearest: the loss is an average and next week can be hotter. A shop default of rounding up to the next whole ppm buys a small margin without overdosing.
Check the target against the stabilizer's shock level. Keep the weekly target under the oxidation target of about 40 percent of CYA in 'Shock Dosing Standard'. Pool A: 40 percent of 30 is 12 ppm, and 9 sits under it. Reason: a weekly target that reaches shock level means the weekly plan cannot carry this pool (see "What changes the plan").
Compute the dose and round only at the end. (9.0 minus 2.0) times (20,000 over 10,000) times 10.24 fl oz is 7 times 2.0 times 10.24, 143.4 fl oz, which is 143.4 over 128, or 1.12 gal. Call it 1.1 gal. Check it back: 1.12 gal times 12.5 ppm per gallon per 10,000 is 14.0 ppm per 10,000 gallons, halved for 20,000 gallons is 7.0 ppm. Seven plus 2.0 is 9.0. It closes.
Pour after any acid, at a return, pump running. Liquid chlorine goes in last of the day's products, at a different return from any acid, per 'The Dosing Order on the Deck: One Product at a Time'. Wear chemical splash goggles and gloves; liquid chlorine on skin or in an eye gets 15 minutes of clean water and follows the product's safety data sheet. Pour low to avoid splash on the customer's deck furniture and your clothes.
Log the target, not just the gallons. The next visit's step 2 needs "dosed to 9.0". A log that only says "1.1 gal" forces the next tech to recompute last week's target from the volume, and most will not.
Finish test: next visit's reading at or above the floor, ideally within about 1 ppm of it. Arriving well above the floor every week says the target can come down; arriving below it says the loss changed or something else did.
Worked twice: Pool B and the one bad week
A 15,000 gallon pool, CYA 30 ppm, so its floor is also 2.25 ppm. Its log for the last four weeks shows the pool dosed to 8 ppm each visit and arriving at 2.3, 2.5 and 2.4. That is a steady loss of about 0.8 ppm a day (8 minus 2.4 is 5.6, over 7 days).
Today it reads 1.0 ppm. On step 2 alone, loss this week is 8 minus 1.0, 7.0 ppm, over 7 days, 1.0 ppm a day, and step 4 would set a target of 2.25 plus 7.0, 9.25, rounded up to 10. But the customer mentions a party of about twenty people on Saturday. One heavy week inside a run of steady ones is an event, not a new normal.
- Target: use the typical loss. 2.25 plus 7 times 0.8 is 7.85, rounded up to 8.0 ppm.
- Dose: (8.0 minus 1.0) times 1.5 times 10.24 is 7 times 1.5 times 10.24, 107.5 fl oz, over 128 is 0.84 gal.
- Check: 0.84 gal times 12.5 is 10.5 ppm per 10,000 gallons; over 1.5 is 7.0 ppm. One plus 7.0 is 8.0.
- The note: "party Sat, FC 1.0 found, typical loss used." If next week also arrives low with no event, the loss has really changed and step 4 gets the new figure.
Pool B also arrived below its floor today, by 1.25 ppm. If combined chlorine is up as well after a heavy bather load, that is a shock decision under 'Shock Dosing Standard', not a bigger weekly dose.
What changes the plan
- The target reaches the shock level. A sunny unscreened pool with 1.8 ppm a day of loss at 30 ppm CYA needs 2.25 plus 12.6, about 14.9 ppm, above the 12 ppm shock level. One weekly dose cannot carry it. The choices are a feeder or floater carrying part of the load, a mid-week visit, raising CYA within the limits of 'Cyanuric Acid Management Standard' so less chlorine is lost to sunlight, or finding the demand (phosphates, algae starting, a broken cover). Pick one and tell the customer which, because the cost and the water both change.
- The jug is weak. If every pool on a route lands short by the same fraction, the problem is the product, not the pools. Old or hot-stored liquid chlorine has lost strength. Rotate stock and keep it shaded on the truck per 'Chemical Storage and Transport on the Truck'.
- The pool has a cell or a feeder too. A salt cell or tablet feeder supplies part of the demand. The measured loss is then net of that supply, and a change to the cell's output or the feeder dial changes next week's arrival reading without any change in the pool.
How to verify the method is working
Over four weeks, the arrival readings for each pool should sit close to its floor and not drift. Plot nothing: just read down the "FC found" column. A column that steps down week after week means the target is lagging a rising demand. A column well above the floor every week means the shop is pouring chlorine the pool does not need, and the CYA- and salt-side drift from it (see 'What Each Chlorine Product Does to the Other Readings') is accumulating for nothing.
References
- See related: Shock Dosing Standard (liquid chlorine strength, the 7.5 percent minimum and 40 percent shock ratios)
- See related: The Dose Per 10,000 Gallons Factor Card; How to Run the Weekly Water Test Panel at the Pool
- See related: The Dosing Order on the Deck: One Product at a Time; Cyanuric Acid Management Standard
- Product label and safety data sheet for the liquid chlorine in use