Quick answer: A temperature and humidity measuring device (thermo-hygrometer) reports air temperature and relative humidity together, and many also calculate dew point. Pick the type by the job: a handheld meter for spot checks and surveys, a wall display to watch a room, a data logger to record conditions over days or weeks, a transmitter (4–20 mA, 0–10 V or Modbus) to feed a control system, or wireless sensors to monitor many points remotely. Typical good accuracy is ±2–3 % RH and ±0.3–0.5 °C. Read temperature and RH together: the same air shows a different RH if it warms or cools.
Quick answer
- Spot checks: handheld with dew point.
- Records and audits: data logger.
- Control and BMS: transmitter or networked sensor.
Types compared
| Type | Best for | Typical accuracy | Output | Watch out for |
|---|---|---|---|---|
| Handheld thermo-hygrometer | Surveys, troubleshooting, checking other sensors | ±2–3 % RH, ±0.3–0.5 °C | Display; some log or connect to an app | Allow time to settle; hand heat |
| Wall display | Watching a room, warehouse or office | ±3–5 % RH | Display | Often no record; placement on cold walls |
| Data logger | Recording conditions for audits, storage, transport, investigations | ±2–3 % RH | Download or cloud | Logging interval; battery life |
| Transmitter | Feeding a BMS, climate computer or dehumidifier controller | ±2 % RH or better | 4–20 mA, 0–10 V, Modbus | Wiring, power, calibration access |
| Wireless sensor network | Many points in large buildings, greenhouses, warehouses | ±2–3 % RH | Gateway and software | Signal coverage; battery replacement |
| Reference instrument (chilled mirror, calibrated probe) | Calibrating other sensors | Dew point ±0.2 °C or better | Lab | Cost; handling |
NIST describes humidity measurement instruments and calibration (NIST, Hygrometers). For more on RH sensor accuracy and checking, see relative humidity gauges.
What the readings tell you
| Reading | Use |
|---|---|
| Temperature | Comfort, process limits, product temperature |
| RH | Mold and corrosion risk, product equilibrium moisture |
| Dew point | Condensation risk: compare with the coldest surface temperature |
| Absolute humidity (g/kg or g/m³) | Comparing indoor and outdoor air; whether ventilation will dry a space |
Example: a room at 22 °C and 55 % RH has a dew point of about 12.5 °C. A cold water pipe at 10 °C in that room will sweat even though 55 % RH sounds fine.
Placement
- Where the decision applies: at product level in a warehouse, at canopy height in a greenhouse, near the coldest surface for condensation.
- Away from sun, heaters, lights, air outlets and people.
- Not on cold outside walls unless that is what you want to measure.
- One per zone with different conditions.
See dew point sensor placement and industrial temperature and humidity monitoring.
Choosing by application
| Application | Device |
|---|---|
| Home, office | Wall display or small logger |
| Warehouse, archive, storage | Wireless sensors or loggers with alarms |
| Greenhouse | Aspirated transmitters at canopy height; see greenhouse sensor placement |
| Water damage drying | Handheld with dew point and g/kg, plus moisture meter |
| Controlling dehumidifiers | Transmitter in the controlled zone |
| Pharmaceutical, food, lab | Calibrated loggers with traceable certificates |
Related: greenhouse sensor placement.
If readings show humidity staying high, see how to size a dehumidifier and our industrial dehumidifiers.
FAQ
Frequently asked questions
What device measures temperature and humidity?
A thermo-hygrometer: handheld, wall-mounted, logging or transmitter versions.
How accurate are temperature and humidity meters?
Good ones are about ±2–3 % RH and ±0.3–0.5 °C; cheap displays can be ±5 % RH or worse.
Should I measure dew point too?
Yes, whenever condensation matters. Dew point compared with surface temperature shows whether water will form.
How often should humidity sensors be calibrated?
Typically every 6–12 months, more often after exposure to condensation or contamination.
Sources
- NIST, Hygrometers.
- ASHRAE Handbook—Fundamentals, Measurement and instruments; Psychrometrics.
