Two Humidity Sensors Disagree by 10%: Calibrating with Salt Solutions and Reference Checks

The greenhouse manager stared at the two displays mounted on the same bay, three metres apart, and did the arithmetic twice. One read 62 percent relative humidity. The other read 72 percent. Ten points apart, in the same air, at the same moment. The ventilation controller took its orders from the first sensor, so the vents stayed shut; the disease model ran off the second, so it was already flashing a warning about grey mould. One of the two numbers was wrong, but both sensors were new and both were the same model. The question had no obvious answer: when two instruments disagree, which one do you trust — and how do you find out?

This is one of the most common arguments in greenhouse and HVAC work, and it is almost never solved by buying a third sensor. Two humidity sensors disagree for understandable reasons, and most of them have nothing to do with a fault. The sensors may be measuring different air, they may be at different temperatures, one may have drifted, or the disagreement may be the honest result of two instruments with different accuracy. This article explains how a capacitive humidity sensor actually works, why two of them can read 10 percent apart without either being broken, and how to settle the argument with a salt-solution calibration and a reference check.

How a Capacitive Humidity Sensor Measures — and Why Temperature Is Half the Answer

Most modern humidity sensors are capacitive. Inside the probe, a thin polymer film sits between two electrodes. The film absorbs water vapour from the air, and as it does, its electrical properties change — it becomes more able to store charge. The sensor measures that capacitance and converts it into a relative humidity reading. The consequence that surprises many users is that the sensor measures the humidity of the air touching the film, at the film's temperature, not the temperature of the room.

Relative humidity is relative to temperature. Warm air can hold far more water vapour than cold air, so the same absolute amount of moisture reads as a much lower relative humidity in warm air than in cold air. A rule of thumb that holds well in greenhouse conditions is that a temperature difference of about 1 degree Celsius changes the relative humidity reading by roughly 3 percentage points — warmer reads drier. That means two sensors in the same room, one sitting in a sun patch or near a heater and the other in shade, can disagree by 10 percent purely because one is a few degrees warmer than the other. Neither is wrong. They are measuring different air at different temperatures, and each reading is correct for the spot it occupies. The sensor also lags behind fast changes, because the polymer film takes minutes to absorb or release moisture, so compare two sensors in stable conditions, not during a ventilation change.

Why Two Sensors Disagree: The Usual Suspects

Placement is the most common cause, and it is the first thing to check. A sensor mounted near a vent, a door, a heater, or a misting line is sampling a local microclimate, not the room. A sensor above a wet crop canopy reads higher than one over a dry walkway. A sensor in direct sun, even for part of the day, heats up and reads dry — a greenhouse probe without a radiation shield can sit several degrees above air temperature in sunshine and read 10 or more points low. Before suspecting the sensors, swap their positions: if the disagreement follows the location rather than the sensor, the sensors are fine and the mounting is the problem.

Calibration drift is the second cause, and it grows with time and exposure. The polymer film is sensitive to contamination: dust, chemical vapours, fertiliser aerosols, and the volatile compounds released by fresh paint or cleaning products can all coat the film and shift its response. A sensor that has spent a season above a fertigation tank or next to a chemical store will often read differently from a clean one, and the gap grows over weeks and months rather than appearing overnight. Ageing adds to the same effect: the film and its electronics drift slowly with heat cycling, so two sensors bought at different times can drift at different rates, even if the model number matches.

Finally, accuracy itself is part of the answer. A typical industrial humidity sensor is specified at around plus or minus 2 to 3 percent relative humidity, and that figure is a tolerance, not a guarantee of agreement. Two sensors, each within its own specification, can legitimately differ by 4 to 6 points — and in the field, with placement and temperature effects added, a 10-point gap between two ordinary sensors is not unusual.

Choosing a Sensor You Can Trust

When buying a temperature and humidity sensor, the accuracy figure on the datasheet is the starting point, not the whole story. Look for a sensor with a stated accuracy that includes the temperature range you will use it in, because a sensor specified at room temperature may be far less accurate at the cold or hot end of its range. A replaceable probe is worth having: it lets you send one element away for calibration while the sensor keeps working, and it turns a drifted sensor into a ten-minute fix instead of a replacement.

Check whether the sensor measures temperature at the same point as humidity. A sensor that reports both from the same probe can compensate its own humidity reading for its own temperature, which is what makes the reading meaningful. And be honest about the environment: a sensor in a greenhouse with misting, fertiliser aerosols and condensation needs a filter cap and a housing rated for that exposure, or it will drift faster than a clean-room sensor ever would.

Installation Mistakes That Create Disagreement

The most common mistake is mounting the sensor in direct sun without a radiation shield. A bare probe in a greenhouse or on an outdoor mast will heat up in sunshine and read dry, and the error follows the sun through the day — largest at midday, gone at night. The fix is a ventilated shield that blocks sunlight while letting air flow freely, and the shield itself needs to stay clean and white; a dirty or darkened shield absorbs heat and defeats its own purpose.

The second mistake is mounting too close to a heat source, a vent, or a wet surface. A sensor above a heater, beside a door that opens onto cold air, or over a wet floor is measuring a local condition, and no amount of calibration will make it agree with a sensor in the middle of the room. Mount at the height where the decision is made — canopy height in a greenhouse, breathing height in a room — away from walls, heaters and vents, with free air circulation around the probe.

Verification: Settling the Argument with Salt Solutions

When two sensors disagree and placement has been ruled out, the only honest way to settle it is to calibrate. The most practical field method is the saturated salt solution check, which uses a basic fact of chemistry: air in a sealed container above a saturated salt solution settles at a fixed, known relative humidity that depends on the salt. A saturated solution of common table salt — sodium chloride — holds the air at around 75 percent relative humidity at room temperature. A saturated solution of magnesium chloride holds it at around 33 percent. These are not sensor readings; they are physical constants, which is what makes the method trustworthy.

The procedure is simple but needs patience. Put a small dish of the salt with just enough distilled water to make a wet slurry — damp crystals with a little free water — into a sealed jar or bag with the sensor probe, and leave it undisturbed for 12 to 24 hours so the air inside reaches equilibrium. Then read the sensor. A healthy sensor should read within a few points of the expected value; a sensor reading 10 points off at 75 percent has drifted and needs adjustment or replacement. Test at two points if you can — the 33 percent and 75 percent salts together — because a sensor can be right at one humidity and wrong at another. Keep the jar at a stable room temperature and do not open it during the wait, or the equilibrium breaks and the reading means nothing.

The second check is the reference comparison. Put both sensors side by side in the same stable spot, away from sun and draughts, and log them for a day. If they agree within a few points in the same air at the same temperature, the original disagreement was placement, not the sensors. If the gap follows one sensor wherever it goes, that sensor has drifted. A handheld reference instrument, borrowed or hired for a day, settles the question faster: compare each sensor against the reference in turn, and the one that matches it is the one to trust.

How often to repeat this depends on the environment. In a clean indoor setting, an annual check is reasonable. In a greenhouse with misting, fertiliser and condensation, check every six months, and check immediately after any event that could contaminate the sensor. The signal that a sensor needs calibration is a gap that grows over time against a reference, or a reading that stops responding to real changes in the air.

Troubleshooting: Two Sensors, Two Readings

Symptom	Likely cause	What to check
Gap appears only in sunshine, disappears at night	One sensor is heating in the sun	Fit a ventilated radiation shield; check the shield is clean and white
Gap follows the location when sensors are swapped	Placement, not the sensors	Remount both at the same height, away from vents, heaters and wet surfaces
Gap follows one sensor wherever it goes	Calibration drift or contamination	Run a salt-solution check at 75 percent; recalibrate or replace the drifted unit
Gap grows slowly over weeks or months	Ageing or contamination of the polymer film	Compare against a reference; clean or replace the filter cap and recalibrate
One sensor reads near 100 percent and stays there	Condensation on the probe	Check for dripping water, remount so water runs off, let the probe dry fully

Checklist: Making Two Sensors Agree

• Mount both sensors at the same height, away from sun, heaters, vents and wet surfaces, with a ventilated shield outdoors or in a greenhouse.
• Swap the sensors' positions once: if the gap follows the location, fix the mounting, not the sensor.
• Compare readings only in stable conditions, not during ventilation changes or misting.
• Run a saturated salt check — table salt at around 75 percent, magnesium chloride at around 33 percent — and leave the jar sealed for 12 to 24 hours.
• Check against a reference instrument once a year, or every six months in a greenhouse.
• When one sensor is off by more than a few points at the salt check, recalibrate it or replace the probe; do not adjust the controller to split the difference.

Two humidity sensors that disagree by 10 percent are usually not both wrong — they are measuring different air, at different temperatures, or one of them has drifted. The way to find out is not to average them and hope, but to swap their positions, shield them from the sun, and run a salt-solution check that gives you a number no sensor can argue with. A sensor worth trusting is one whose accuracy is stated, whose probe can be replaced, and whose readings can be checked against a known reference — and the RS485 temperature and humidity sensor gives you both measurements from the same probe over RS485, so the temperature-compensated humidity reading can be logged and compared whenever the two displays on the wall start to disagree.