Quick answer: Put greenhouse humidity sensors in the crop canopy, in an aspirated radiation shield, away from heaters, vents, fans, spray lines and direct sun, with at least one per zone that is controlled separately. Measure temperature at the same point, because near saturation a 0.5 °C temperature error shifts RH by about 3 percentage points. Check each sensor against a reference every season, and replace or recalibrate any that drift.
Quick answer
- Location: canopy height, shaded and aspirated, in each control zone, including the coldest one.
- Accuracy: ±2 % RH is common; temperature accuracy matters just as much near 90 % RH.
- Calibration: check against a reference hygrometer or saturated-salt standards (about 33 % and 75 % RH).
Where to put the sensors
| Purpose | Where | Why |
|---|---|---|
| Control the climate the crop feels | In or just above the active canopy, mid-row, in an aspirated shield | Air at the leaves is wetter and cooler than air at head height |
| Catch the worst condensation risk | In the coldest, least-ventilated zone: near cold walls, gables, under screens | Disease starts where leaves first reach the dew point |
| Diagnose roof or glazing condensation | Near that surface, with a surface temperature probe | Room RH does not show surface conditions |
| Control a dehumidifier or vent stage | In the air that equipment is meant to change, not at its discharge | A sensor in dry discharge air turns the unit off too early |
Avoid: direct sun on the sensor, heating pipes and unit heaters, vent openings, the discharge of fans or dehumidifiers, fogging and spray lines, and doors. The University of Alaska Fairbanks Cooperative Extension gives the same advice: shield sensors from the sun and keep them away from heaters, vents and drafts (UAF Extension).
How many: at least one per independently controlled zone. In large houses, add temporary loggers for a few weeks to find where conditions differ, then decide where permanent sensors go.
Measure temperature at the same point
RH depends on temperature. At 20 °C and around 90 % RH, a 1 °C change alters RH by about 6 percentage points with no change in the water in the air. So:
| Error at the sensor | Effect on RH near 90 % |
|---|---|
| Temperature reads 0.5 °C high | RH reads about 3 points low |
| Sensor in sun, 2 °C too warm | RH reads about 11 points low |
| Sensor RH accuracy ±2 % | ±2 points |
A sensor warmed by the sun or a heater will tell the climate computer that the crop is drier than it is. For condensation decisions, use the dew point or humidity deficit calculated from paired temperature and RH at the same point, and compare it with the leaf or surface temperature. See greenhouse humidity and disease for the night setpoints.
Choosing a sensor
| Feature | What to look for | Why |
|---|---|---|
| RH accuracy | ±2 % RH or better over 10–95 % RH | Greenhouse decisions sit at 80–95 % RH |
| Temperature accuracy | ±0.2–0.3 °C | See the table above |
| High-humidity behavior | Rated for condensing conditions; heated or hydrophobic-filter probes | Many capacitive sensors read wrong or drift after condensation |
| Shield | Aspirated (fan-ventilated) radiation shield | Passive shields still warm up in sun and still air |
| Response time | Short enough for vent and heating control | A slow sensor makes the climate swing |
| Output | Matches the climate computer: 4–20 mA, 0–10 V or digital | Avoid conversions that add error |
| Replaceable element | Swappable sensing head | Lets you rotate in calibrated heads |
Checking and calibrating
Check every sensor at least once a season, and after any condensation event that may have damaged it.
- Spot check with a reference: place a recently calibrated reference hygrometer next to the sensor in the shield, wait until both are stable, and compare. Do it at night when conditions are steady.
- Salt reference check: saturated salt solutions in a sealed container hold a known RH: about 75 % RH for sodium chloride and 33 % RH for magnesium chloride at room temperature. Seal the sensing head with the salt (not touching the liquid), wait several hours at a stable temperature, and compare.
- Calibration certificate: for sensors that drive control, use heads with a calibration certificate traceable to a national standard, such as NIST humidity calibration services in the US (NIST).
- Record the offset with the date, and replace or recalibrate when it exceeds your limit.
How much error is acceptable?
Add the independent error sources by root sum of squares, as standard uncertainty guidance does (NIST Technical Note 1297):
combined ≈ √(sensor² + reference² + temperature effect²)
Example: sensor ±2 % RH, reference ±1 % RH, temperature effect ±3 % RH → √(4 + 1 + 9) ≈ ±3.7 % RH.
If your night setpoint is 89 % and the combined uncertainty is ±3.7 %, the true RH could be over 92 %. Either improve the temperature measurement and shielding, or set the control margin with that uncertainty in mind.
Use the data to improve control
- Log events with the data: irrigation, screen moves, vent and heating steps, door openings and dehumidifier starts. Most humidity spikes have a cause in that list.
- Compare zones: a zone that is always wetter needs more air movement, heat or dehumidification.
- Alarm on the dew point margin, not only on RH.
- Re-check placement after any change to crops, screens or equipment.
When the data show that the house cannot hold its night target with venting and heating, a greenhouse dehumidifier removes the water directly. See how to choose a greenhouse dehumidifier.
FAQ
Frequently asked questions
Where should a humidity sensor go in a greenhouse?
In the crop canopy, in an aspirated radiation shield, away from heaters, vents, fans, spray and direct sun, with at least one per control zone.
How accurate do greenhouse humidity sensors need to be?
±2 % RH and ±0.2–0.3 °C. Near saturation, temperature accuracy affects RH as much as the RH element does.
Why does my greenhouse humidity sensor read 100 %?
Condensation on the sensing element, or a sensor that has drifted after repeated wetting. Use condensation-resistant or heated probes, and check against a reference.
How do I calibrate a greenhouse humidity sensor?
Compare it with a calibrated reference in the same shield, or with saturated salt references (about 75 % and 33 % RH), and record or correct the offset.
How often should sensors be calibrated?
Check at least every season, and after condensation events. Recalibrate or replace when the offset exceeds the margin your control relies on.
Sources
- University of Alaska Fairbanks Cooperative Extension, Controlling the greenhouse environment.
- NIST, Humidity calibration services; NIST Technical Note 1297, Guidelines for evaluating and expressing the uncertainty of measurement results.
- Greenspan, L. (1977), Humidity fixed points of binary saturated aqueous solutions, Journal of Research of the NBS 81A.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
