The Overnight Chlorine Loss Test (OCLT) is one of the most powerful diagnostic tools a pool technician has — and one of the most underused. It is the definitive way to know whether a SLAM treatment is truly complete, and it is equally useful as a standalone diagnostic for pools with chronic chlorine demand problems. This guide covers how to run it correctly, how to interpret results, and what to do when a pool fails.
The OCLT measures how much free chlorine is consumed in an 8-hour dark period. By testing after sunset and before sunrise, all UV-driven chlorine consumption is eliminated. Only biological and chemical chlorine demand remains.
In a healthy, balanced pool with no algae or bacteria load, chlorine loss overnight should be minimal — 1 ppm or less over 8 hours is the passing threshold. More than 1 ppm indicates active biological demand: something in the pool (algae, bacteria, or organic material) is consuming chlorine at a rate that exceeds natural background loss.
Take the evening FC measurement after sunset — specifically, after any direct sunlight has left the water. Testing at 7 PM in summer when the sun is still up will show UV-driven consumption that inflates your apparent overnight loss. Test at true dusk (after full sunset) or later.
If you are running the OCLT during a SLAM, bring FC to at least 2–3 ppm above the SLAM minimum threshold before the evening test. You need to start the test at a measurable level and still be above the minimum in the morning. If you start at exactly minimum and the pool fails, you'll have nothing left to measure accurately.
If the pool has an automatic chlorinator, inline chemical feeder, SWG, or tablet floater — turn them off for the duration of the OCLT. Any chlorine addition during the night will inflate your morning reading and make the test invalid. This is critical and is the most common OCLT error.
Take the morning FC measurement before any direct sunlight hits the pool — before dawn or immediately at first light is ideal. By 8 AM in summer, UV consumption may already be removing 0.5–1.0 ppm/hour in an unstabilized or low-CYA pool. Testing late in the morning inflates apparent overnight loss.
Subtract morning FC from evening FC. The result is the overnight chlorine loss.
| Overnight FC Loss | Result | Interpretation |
|---|---|---|
| 0.0–1.0 ppm | PASS | No significant biological demand; pool is clean |
| 1.1–2.0 ppm | BORDERLINE | Some demand remains; consider another 24h of SLAM before retesting |
| 2.1–4.0 ppm | FAIL | Active biological demand; continue SLAM |
| 4.0+ ppm | SIGNIFICANT FAIL | High organic or biological load; SLAM is not close to complete |
The OCLT is valuable as a standalone diagnostic tool for pools that chronically consume chlorine faster than expected. Common causes of chronic overnight chlorine loss:
Even in a perfectly maintained pool with zero biological demand, some FC is consumed overnight through natural chemical reactions. Typical normal loss ranges:
Loss above 1.0 ppm is always a signal worth investigating. Loss above 2.0 ppm in what appears to be a clean, balanced pool is a red flag that something in the system is consuming chlorine — biofilm, debris, or biology that isn't visible.
Customers rarely understand why you need to "wait and check again in the morning" before declaring a pool clean. Frame it this way: "The overnight test lets me confirm the algae is completely gone — not just cleared from the surface. Without it, I can't guarantee it won't come back in 3 days, and I don't make that kind of guarantee on a job I can't verify."
PoolLens can document OCLT results with timestamps and readings, which provides professional documentation of your treatment and protects you if a customer later claims the pool went green again within days of service.
PoolLens tracks evening FC readings, morning FC readings, and OCLT results for every account — with timestamps. Build a documented record of every treatment and protect yourself professionally. Works completely offline.
Open PoolLens Free →The OCLT measures how much free chlorine a pool loses overnight in darkness. Test FC at dusk after sunset, then test again in the morning before any sunlight hits the pool. A passing result is a loss of 1 ppm or less over 8 hours. Loss greater than 1 ppm indicates active biological demand.
A failing OCLT means active biological demand remains in the pool. Causes include: live algae surviving in the water or on surfaces, bacteria in the water column or biofilm in plumbing, dead algae decomposing in the filter, or organic contamination actively consuming chlorine.
Run OCLT correctly by: (1) Taking the first FC measurement after sunset to eliminate UV consumption, (2) Bringing FC above SLAM target before the test, (3) Turning off any automated chlorine additions (SWG, tablets, feeders), (4) Testing in the morning before sunrise, (5) Using an FAS-DPD test kit for accuracy.
Yes — the OCLT is valuable for diagnosing chronic chlorine demand problems in any pool. A pool that consumes more than 1–2 ppm FC overnight without any obvious algae may have biofilm in plumbing, a clogged sand filter harboring bacteria, or a significant organic load.