Quick answer: A relative humidity gauge (hygrometer) shows how much water vapour the air holds as a percentage of the maximum it could hold at that temperature. Dial (mechanical) gauges are typically accurate to about ±3–5 % RH and drift; good digital capacitive sensors reach about ±2 % RH; a sling psychrometer or a calibrated reference is used to check them. Two gauges in the same room often differ by 5 % RH or more, usually because of accuracy, drift or a small temperature difference. Check any gauge with a salt test or a reference before trusting it for control.
Quick answer
- Dial gauge: about ±3–5 % RH; fine for a rough indication.
- Digital capacitive: about ±2–3 % RH; the usual choice for monitoring and control.
- Check: saturated salt (about 75 % RH with table salt) or a calibrated reference.
Types of RH gauge
| Type | How it works | Typical accuracy | Good for | Limits |
|---|---|---|---|---|
| Mechanical dial (hair or synthetic element) | Element stretches with humidity and moves a pointer | About ±3–5 % RH | Rooms, storage, display | Drifts; slow; needs regular adjustment |
| Digital capacitive (most electronic gauges) | Polymer film between electrodes changes capacitance with humidity | About ±2–3 % RH (better on premium sensors) | Monitoring, loggers, control | Drift after condensation or contamination |
| Resistive | Resistance of a hygroscopic film changes | About ±3–5 % RH | Low-cost controls | Less accurate at extremes |
| Psychrometer (wet and dry bulb) | Temperature drop of a wet thermometer | About ±2–5 % RH depending on use | Field checks | Needs airflow, clean wick, pure water |
| Chilled-mirror dew point hygrometer | Cools a mirror to the dew point | Dew point about ±0.2 °C | Laboratory reference | Expensive; needs care |
The National Weather Service explains RH, dew point and how they relate (NWS).
Gauge, meter, sensor, probe, transmitter: what the terms mean
| Term | What it usually is | Typical use |
|---|---|---|
| RH gauge / hygrometer | Wall or desk display, dial or digital | Room indication |
| RH meter / tester | Handheld instrument, often with a probe and data hold | Spot checks, surveys, restoration, HVAC service |
| RH sensor | The sensing element itself (capacitive chip, film) | Built into gauges, controllers, dehumidifiers |
| RH probe | Sensor in a protective tube with a filter cap, on a cable | Ducts, cold rooms, process air, enclosures |
| RH transmitter | Probe plus electronics giving 4–20 mA, 0–10 V or Modbus output | BMS, dehumidifier control, data logging |
| RH data logger | Records readings over time | Storage, transport, compliance |
IP rating: probes for wash-down, outdoor or condensing environments are often IP65–IP67. IP67 means dust-tight and protected against temporary immersion; it protects the electronics, not the sensing element from condensation, so use a heated or condensation-tolerant probe where the air reaches saturation.
How to measure relative humidity in a room
- Place the meter at the height you care about (breathing height, product level or canopy), away from windows, heaters, vents, people and direct sun.
- Let it settle: digital meters 5–10 minutes; dial gauges 30 minutes or more after moving.
- Read RH and temperature together. RH without temperature is incomplete.
- Check a few spots in larger rooms: corners and near cold walls can be much more humid.
- Log over a day if the reading matters: humidity changes with occupancy, cooking, weather and HVAC cycles.
- Convert to dew point if condensation is the concern (see below).
Dew point vs relative humidity
Relative humidity is how close air is to saturation at its current temperature; it changes when temperature changes. Dew point is the temperature at which that air would start to condense; it stays the same when air warms or cools without gaining or losing water. Example: 20 °C and 60 % RH has a dew point of about 12 °C; warm that air to 25 °C and RH drops to about 44 %, but the dew point is still 12 °C. Use RH for comfort and materials, dew point for condensation risk. The US National Weather Service explains both.
Why two gauges show different readings
| Cause | Typical effect |
|---|---|
| Accuracy of each gauge (±3 % each) | Up to 6 % RH apart even when both are "in spec" |
| Drift with age or after condensation | Several % RH, sometimes much more |
| Temperature difference between the gauges | Near 50 % RH at 20 °C, 1 °C difference ≈ 3 % RH; near 90 % RH ≈ 6 % RH |
| One gauge in sun, near a heater, a person or a window | Reads drier or wetter than the room |
| Slow response (dial gauges) | Lags behind changes by tens of minutes |
Put both gauges side by side, away from heat sources, wait at least 30 minutes and compare again before deciding which one is wrong.
How to check a hygrometer
Salt test (about 75 % RH):
- Put a few spoonfuls of table salt in a bottle cap or small dish and add a few drops of water to make a damp paste (not dissolved).
- Seal it in a zip bag or airtight box with the gauge, not touching.
- Keep it at a steady room temperature for 8–12 hours.
- The air inside should be about 75 % RH. The difference is your gauge's error at that point.
Second point (about 33 % RH): magnesium chloride gives about 33 % RH; testing at two points shows whether the error changes across the range. These fixed points come from published saturated-salt data (Greenspan, 1977).
Reference comparison: for control and compliance, compare against a calibrated reference instrument and keep the calibration record. NIST publications describe humidity measurement methods and their limits (NIST BSS 157).
Choosing a gauge
| Use | Choose |
|---|---|
| Home, office, checking a room | Digital gauge, ±3 % RH, with temperature |
| Mold prevention | Digital gauge; aim to stay below 60 % RH, ideally 30–50 % (US EPA) |
| Warehouse or production monitoring | Digital sensors ±2 % RH with logging and alarms |
| Controlling a dehumidifier | Sensor ±2 % RH at the controlled zone, not at the unit outlet |
| Greenhouses and high humidity | Sensors rated for condensing conditions, in aspirated shields; see greenhouse humidity sensor placement |
| Laboratories, calibration | Reference instrument with a traceable certificate |
Related: US EPA, greenhouse humidity sensor placement.
Using the reading
- Always read temperature too. RH alone does not tell you how much water is in the air.
- For condensation, use the dew point and compare it with the coldest surface temperature.
- Place gauges where the decision applies: at product level in a warehouse, at canopy height in a greenhouse, in the coldest part of a basement.
If your gauge shows humidity staying above your target, see best dehumidifier settings and our industrial dehumidifiers.
FAQ
Frequently asked questions
What is the difference between dew point and humidity?
RH is the percentage of saturation at the current temperature; dew point is the temperature at which the air would condense. Dew point doesn't change when air simply warms or cools.
What instrument measures relative humidity?
A hygrometer: digital capacitive meters, dial gauges, psychrometers or, for reference work, chilled-mirror dew point hygrometers.
What is an RH probe?
A humidity sensor in a protective housing on a cable, used in ducts, rooms and equipment; with an output it becomes a transmitter.
What device measures relative humidity?
A hygrometer: in practice, a digital capacitive sensor (handheld meter, logger or transmitter), a dial gauge, or a psychrometer, which measures it from wet-bulb and dry-bulb temperatures.
What humidity measuring devices are used in medical and pharmaceutical rooms?
Calibrated digital sensors or transmitters with traceable calibration certificates, connected to a monitoring system that records and alarms; see temperature and humidity monitoring.
How accurate are relative humidity gauges?
Dial gauges typically ±3–5 % RH; good digital gauges ±2–3 % RH. Accuracy is usually worse at very high and very low humidity.
Why do my two hygrometers read differently?
Each has its own error, and small temperature differences change RH. Side by side, they can differ by 5 % RH or more.
How do I calibrate a hygrometer at home?
Seal it with a damp table-salt paste for 8–12 hours at a steady temperature. It should read about 75 % RH; adjust or note the offset.
What is a good indoor humidity reading?
Between about 30 and 50 %, and below 60 % to limit mold growth (US EPA).
Sources
- National Weather Service, Humidity.
- NIST (NBS) Building Science Series 157.
- Greenspan, L. (1977), Humidity fixed points of binary saturated aqueous solutions.
- US EPA, A Brief Guide to Mold, Moisture and Your Home.
