Quick answer: To catch condensation before it happens, measure two things and compare them: the dew point of the air (from a temperature and RH sensor in the air that touches the risk surface) and the temperature of the coldest surface (a surface probe on the roof, wall, door, cooled plate or cable entry where water would form first). Alarm on the margin between them, for example when the surface is less than 3 K above the dew point, rather than on a fixed RH. Add an outdoor reference sensor, place sensors away from heaters, fans and dehumidifier outlets, and log everything on the same clock.
Quick answer
- Air sensor: in the air next to the risk surface, not in a dehumidifier or heater airflow.
- Surface sensor: on the coldest surface where water would form first.
- Alarm: surface temperature − dew point < about 2–3 K (set your own margin).
Why RH alone misses condensation
Condensation forms on a surface when its temperature falls below the dew point of the air touching it. An enclosure can read 60 % RH in the middle while a cold roof panel is already wet. Example:
- Air in a battery container: 25 °C, 60 % RH → dew point about 16.7 °C.
- Roof panel on a clear night: 15 °C.
- Result: water forms on the roof and drips, although RH looks moderate.
The US National Weather Service defines dew point as the temperature to which air must be cooled to reach saturation (NWS glossary).
Where to put sensors
| Sensor | Where | Avoid |
|---|---|---|
| Air temperature and RH (for dew point) | In the air near the expected risk surface; in stagnant zones; at more than one height in tall enclosures | Dehumidifier or heater discharge, fan jets, near the door seal |
| Surface temperature | Coldest surfaces: roof, north or shaded walls, doors, cooled plates, AC ducts, cable entry plates, busbars near openings | Surfaces warmed by equipment unless that is the risk point |
| Outdoor reference | Shaded, ventilated, near the enclosure | Sun, exhaust outlets |
| Equipment inlet and outlet | Before and after dehumidifiers, coolers | Using them as the control sensor for the space |
| Leak / water sensor | Floor or drip tray under the risk point | — |
How many: one air point and one surface point per enclosure is the minimum; containers and large rooms need several, because air is rarely well mixed. Use temporary loggers during commissioning to find the coldest and wettest spots, then fix permanent sensors there.
Alarms that work
| Alarm | Trigger example | Why |
|---|---|---|
| Dew-point margin warning | Surface temp − dew point < 3 K | Time to act before water forms |
| Dew-point margin alarm | Margin < 1 K | Condensation imminent |
| High RH | Above an agreed limit, for example 75 % | Corrosion risk even without condensation |
| Water detected | Leak sensor wet | Water already present |
| Sensor fault | Out of range, no data | Monitoring lost |
Set the margin from your sensors' accuracy: with ±2 % RH and ±0.3 K sensors, the dew-point uncertainty near 60 % RH is roughly ±0.5–0.6 K, so a 1 K "imminent" alarm is about the tightest sensible setting.
Time alignment
Condensation events often last minutes at sunrise, after a door opening or when cooling switches on. Log at 1–5 minute intervals, keep all sensors on the same clock, and record equipment states (cooling, heater, dehumidifier, door switch) alongside. Otherwise you cannot tell what caused an event.
Calibration and maintenance
- Check RH sensors against a reference every 6–12 months; replace sensors that have been wetted.
- Check surface probes are still bonded to the surface.
- NIST describes hygrometer calibration services and humidity standards (NIST, Hygrometers).
Commissioning sequence
- Map the enclosure: cold surfaces, openings, equipment, airflow.
- Install temporary loggers at candidate points for 1–2 weeks, including cold nights.
- Choose permanent points from the data.
- Check each sensor against a reference.
- Test alarms by simulating values.
- Record the baseline margins at typical conditions.
Acting on the data
If the margin is often small:
- seal leaks and cable entries;
- add insulation or heat to the cold surface;
- lower the air's dew point with a cabinet or container dehumidifier;
- check that cooling cycles do not pull surfaces below the dew point.
See condensation risk in battery enclosures, enclosure heater vs dehumidifier, our electrical cabinet dehumidifiers and energy storage dehumidifiers.
FAQ
Frequently asked questions
Where should a dew point sensor be placed?
In the air next to the surface most likely to condense, away from heaters, fans and dehumidifier outlets, with a surface temperature probe on that surface.
What dew point margin should trigger an alarm?
A warning at about 3 K and an alarm at about 1 K between surface temperature and dew point are common starting points; set them from your sensor accuracy.
Is RH enough to monitor condensation?
No. Condensation depends on surface temperature; a moderate RH can still condense on a cold surface.
Sources
- US National Weather Service, Glossary: dew point.
- NIST, Hygrometers; NIST SP 250-83, humidity calibration.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
