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An overnight chlorine loss test compares free chlorine after sunset with free chlorine before sunrise. It helps you investigate demand without daytime sunlight confusing the result. For the comparison to mean anything, chlorine additions and salt generation must not replace what the water consumes.
The test measures a change under recorded conditions. It does not identify a particular organism, certify safe swimming, or replace a full water assessment. Use it when unexplained chlorine loss needs a clearer measurement.
Choose a test that can show a small change
Use a free chlorine method with adequate range and resolution at the concentration you expect. A FAS-DPD drop test is useful because it measures an endpoint rather than a subtle color match. Follow its instructions for sample size and drop counting.
For example, Taylor’s method uses 0.5 ppm per drop with a 10 mL sample or 0.2 ppm with 25 mL. Keep the same sample size for both readings. Smaller increments do not remove sampling or technique errors.
A broad color band cannot establish a precise 0.5 ppm overnight change. An ordinary yellow chlorine comparator may measure total chlorine rather than free chlorine. Check what the kit actually measures using the testing method guide.
Prepare the pool and the record
Pick a night when you can test after sunset and again before sunrise. Avoid swimming, water replacement, or other deliberate changes during the observation. Note unexpected rain or equipment events afterward.
Establish an appropriate starting sanitizer level using the pool’s treatment requirements. There must be enough measurable chlorine for the comparison, within the test’s range. The test itself does not require a universal 10 ppm starting dose.
If you add chlorine before the test, allow it to mix according to the treatment instructions before recording the starting value. A reading beside a recently dosed return is not representative. Use the same sampling location and method in the morning.
Keep circulation operating normally for the chosen procedure. Disable salt generation or automatic dosing using the controller’s documented setting while preserving circulation. Do not bypass a flow switch, open a chemical feeder, or improvise around pressurized equipment.
If you cannot suspend chlorine input safely, record that limitation. The difference will show net change with input, not untreated overnight demand. Have the system operator arrange a suitable test rather than guessing how much chlorine was generated.
Take the two readings
- After sunset and adequate mixing, measure free chlorine promptly from a fresh sample.
- Record the value, sample size, time, and dosing or generation state.
- Leave the pool unused during the test and note any changes in conditions.
- Before sunrise, collect a fresh sample using the same method and measure free chlorine again.
- Subtract the morning reading from the evening reading.
- Restore the normal sanitizer controls and correct any inadequate residual using the proper treatment instructions.
Do not keep an evening sample overnight and compare it with a fresh morning sample. That would partly test what happened in the container. Both readings should represent the pool at the time recorded.
A recording sheet that exposes hidden inputs
| Record | Evening | Morning or overnight notes |
|---|---|---|
| Time and free chlorine | Start time; FC in ppm | End time; FC in ppm |
| Test method | Kit, sample size, drop factor | Same method confirmed |
| Chlorine supply | Last addition; generator or feeder state | Any automatic or manual addition |
| Circulation | Operating state | Outage or changed schedule |
| Pool conditions | Cover state; water appearance | Rain, refill, swimming, unusual events |
| Other chemistry | Recent products and relevant readings | Combined chlorine if required by the chosen protocol |
In a hypothetical valid test, 6.0 ppm in the evening and 4.5 ppm in the morning gives 1.5 ppm loss. If a salt cell operated overnight, that same subtraction would understate actual consumption. A higher morning reading is a reason to investigate input or measurement, not report negative demand as a pass.
Interpret within the protocol you are using
A commonly used residential benchmark is no more than 1 ppm overnight loss. Pool Logger’s published recovery instructions use that benchmark alongside water clarity and combined chlorine checks. It is a practical recovery criterion, not a public health certification or a universal permission to swim.
If your treatment protocol specifies additional completion criteria, evaluate all of them. A result close to a threshold deserves attention to test resolution and repeatability. Avoid making a large treatment change from an uncertain single drop.
Large verified loss means there is demand to investigate, but the test does not prove algae is the only cause. Recent chemical treatments and other oxidizable material can affect chlorine. Return to the chlorine loss troubleshooting guide with the record.
A morning zero cannot reveal when chlorine ran out. It also prevents the test from measuring further demand after that point. Restore appropriate treatment and repeat under suitable conditions rather than treating zero as a complete demand measurement.
Small loss does not explain every cloudy pool or rule out every hazard. Keep visibility, sanitizer requirements, and other water readings in the decision. The free and total chlorine guide helps interpret the related measurements.
The Taylor K-2006 on Amazon includes the FAS-DPD method needed for this comparison. If your existing kit already provides suitable free chlorine measurements, you do not need another complete kit.
