What Is the Overnight Chlorine Loss Test (OCLT)?
Quick answer: The Overnight Chlorine Loss Test (OCLT) measures Free Chlorine consumption without UV interference. A loss of 1 ppm or less between sunset and sunrise confirms the pool has no hidden chlorine demand. More than 1 ppm loss indicates residual algae or organic contamination that can’t be seen in the water. OCLT is one of the three criteria required to declare a SLAM process complete.
Understanding OCLT starts with a solid grasp of pool chemistry. The test only makes sense once you understand how chlorine works against CYA levels and what “chlorine demand” actually means.
Why the test exists
Chlorine disappears from a pool for two reasons: UV light from the sun breaks it down chemically, and biological or organic material consumes it. During the day, both are happening at the same time. You can’t separate them.
At night, there’s no UV. Any FC loss that occurs between sunset and sunrise must come from chemical demand, whether from algae, bacteria, or dissolved organic material. That’s what makes the OCLT meaningful: it isolates demand from photodegradation.
The CDC residential pool chlorine testing{:target=“_blank”} guidance notes that residual disinfectant monitoring is the foundation of safe pool operation. OCLT is the residential DIY version of that principle applied to diagnosing invisible demand.
OCLT is one of the three TroubleFreePool criteria for a successful SLAM process: CC under 0.5 ppm, OCLT under 1 ppm, and visually crystal-clear water. All three must pass before returning FC to normal levels.
How to run the OCLT
The procedure is straightforward, but timing matters:
At sunset, test the free chlorine (FC) level with a FAS-DPD kit and raise it to at least 10 ppm using liquid chlorine if needed; then record the exact FC reading. Next morning at sunrise, before any direct sunlight reaches the pool surface, test the FC again using the same method. Subtract the morning reading from the evening reading: that difference is your OCLT (oxidation-reduction complex time). Finally, compare the result to the interpretation table below.
The test window is sunset to sunrise. If you measure after sunrise, UV has already started destroying chlorine and any FC loss from sunlight is mixed into the result, which invalidates the reading.
OCLT can’t be replaced by a daytime FC test. UV destroys chlorine and masks algae demand. OCLT must be measured with FAS-DPD; test strips and DPD color comparators aren’t accurate at SLAM-level FC of 10 ppm or above. A Taylor K-2006 or TF-100 FAS-DPD kit costs $30-$55, and we consider it required equipment for any pool owner running OCLT or SLAM.
How to interpret OCLT results
| OCLT Result | Interpretation | Action |
|---|---|---|
| 0-1 ppm loss | No hidden demand; water is chemically clean | SLAM complete criterion met |
| 1-3 ppm loss | Residual algae or elevated organic load | Continue SLAM. Retest tomorrow |
| 3+ ppm loss | Active algae or severe organic contamination | Aggressive SLAM continuation required |
A result of exactly 1 ppm is a borderline pass. We recommend retesting the following night before declaring SLAM complete, since a single 1 ppm result could reflect normal variation. Two consecutive nights at 1 ppm or below confirms clean water.
The EPA chlorine demand guidance{:target=“_blank”} establishes that chlorine demand in treated water indicates the presence of oxidizable substances. In pool chemistry terms, that’s algae, combined chlorine precursors, and organic debris. A passing OCLT means none of those are consuming FC at a meaningful rate overnight.
Maintaining adequate cyanuric acid in pools directly affects how much chlorine you need to hold overnight. At 50 ppm CYA, minimum FC is 4 ppm; at 70 ppm CYA, it’s 6 ppm. Your starting FC for the OCLT should be well above those minimums (at least 10 ppm) to ensure any overnight loss is measurable.
Common OCLT mistakes
Measuring after sunrise. This is the most common error. Even 15-20 minutes of direct sunlight on the pool can consume 0.5-1.0 ppm FC. Take the sunrise reading before the sun hits the water surface.
Running a pool cover overnight. A pool cover doesn’t invalidate the test, but it changes conditions slightly. There’s less evaporation and different surface exposure. If you use a cover, note it when recording results. Consistently using or not using the cover gives you a better baseline over time.
Using test strips instead of FAS-DPD. Test strips have a maximum readable range of about 10 ppm and plus or minus 0.5-1 ppm accuracy at best. At the 10-20 ppm FC levels used during SLAM, test strips are unreliable. Always use FAS-DPD for OCLT readings.
Not accounting for rain overnight. Heavy rain dilutes pool water and can reduce FC readings independently of chemical demand. If it rained overnight, the OCLT result isn’t reliable. Retest on the next dry night.
When OCLT passes but pool is still cloudy
A passing OCLT (1 ppm or less overnight loss) means the chemistry is clean. But the water may still look cloudy or hazy. This is normal and doesn’t mean the SLAM failed.
Dead algae cells are microscopic particles that are no longer consuming chlorine but haven’t yet been filtered out. The chemistry is correct; the water needs mechanical clearing.
Run the pump 24/7 and clean the filter as pressure rises (when PSI climbs 25% above the clean baseline, backwash or clean the cartridge). A clarifier or flocculant can help aggregate fine particles into larger clumps the filter can capture. Cloudy-blue water post-SLAM typically clears within 24-72 hours of continuous filtration. If it’s still cloudy after 72 hours, check filter function before assuming a chemistry problem.
The same chemistry principles apply to hot tub water. If you maintain a spa alongside your pool, see our hot tub chemicals and hot tub water chemistry guides for how demand testing translates to a smaller water volume.
Understanding calcium hardness in pools becomes relevant after a SLAM: the high chlorine levels used during SLAM are hard on plaster surfaces with low calcium hardness. Test CH once SLAM is complete and supplement if needed.
Often overlooked.
FAQ
Is OCLT the same as shocking?
No. Shocking raises FC to a high level to kill algae or oxidize contaminants. OCLT measures how much FC the pool consumes overnight to determine whether any active demand remains. You typically shock before running an OCLT (to raise FC to the required starting level), but they’re different procedures serving different purposes.
Can I run OCLT without doing a full SLAM?
Yes. OCLT is useful any time you want to know whether your pool has hidden chlorine demand. If your pool looks clear but FC keeps dropping faster than expected, an OCLT can confirm or rule out algae as the cause. You don’t need to be in the middle of a SLAM to use the test. Raise FC to at least 10 ppm, wait overnight, and measure.
How often should I run OCLT?
Most pool owners never need to run OCLT in a normal season if FC is maintained. Run it when you suspect a problem: FC dropping unexpectedly fast, water with slight haze that doesn’t resolve, or during a SLAM to verify completion. The TroubleFreePool community uses it as the definitive “SLAM done” confirmation, not as routine maintenance.