Laboratory psychrometers: how can relative humidity be determined?

The psychrometer is the reference instrument for determining relative humidity (RH) from the dry-bulb and wet-bulb temperatures. With controlled airflow around the sensors and a psychrometric calculation (chart or software), it delivers a robust, traceable measurement that is independent of humidity sensor drift. In the laboratory and in the field, it is used to check hygrometers, qualify environmental chambers, monitor technical rooms and document stability studies. The result: simplicity, reliability and accuracy, available as sling (whirling) or aspirated (Assmann) models.

What types of psychrometers are there, and how do you choose the right one?

Principle. A psychrometer pairs two thermometers: one reads the air temperature (dry bulb), the other is wrapped in a water-soaked wick (wet bulb). Through evaporation, the wet bulb cools; the temperature difference between them is directly linked to the relative humidity. The method requires a stable air speed around the bulbs (typically 2–5 m/s), distilled water, a clean wick and, for metrological accuracy, a barometric pressure correction.

Main types. The sling psychrometer ventilates the bulbs by manual motion: robust, portable, ideal for field use. The aspirated psychrometer (Assmann) incorporates a fan to impose a constant flow: better repeatability, the laboratory and qualification benchmark. Fixed assemblies are available for climate chambers or cleanrooms. The sensing elements can be mercury-free liquid-in-glass thermometers (without mercury) or Pt100/thermistor probes connected to a meter/psychrometric calculator.

Selection criteria. Sensor accuracy (e.g. ±0.1–0.2 °C), stability of airflow, wick quality, ease of reading/calculation (charts, app, software), splash and dust resistance, cleanroom compatibility, ease of calibration and maintenance (wick replacement, cleaning). Also consider the response time and how you will compute RH (psychrometric table, built-in RH function).

Type	Principle	Applications	Benefits
Sling (rotary)	Ventilation by manual motion	Field, HVAC, quick checks	Portable, simple, battery-free
Aspirated (Assmann)	Fan ensuring constant flow	Laboratory, qualification, metrology	Accurate, repeatable, traceable
Fixed / integrated	Permanent installation with wick	Climate chambers, cleanrooms	Continuous monitoring, system integration

Best practice. Use distilled water, saturate the wick, avoid direct radiation, ensure sufficient ventilation, verify thermometer calibration and record the atmospheric pressure.

In which applications is a psychrometer used?

The psychrometer is the go-to wherever relative humidity governs quality or safety. In life sciences and pharmaceutical settings, it supports the qualification of climatic chambers, checks stability chambers and validates hygrometers. In food and beverage, it monitors warehouses, drying and packaging. In environment and meteorology, it provides reference measurements in the field. In HVAC and cleanrooms, it documents system compliance and hygrothermal comfort. In chemistry and cosmetics, it safeguards processes that are sensitive to water (powders, polymers, coatings). In education, it is a clear teaching aid to illustrate evaporation and heat balance.

Available psychrometer types

Accessory for psychrometer

Frequently asked questions about psychrometers

How does a psychrometer work?
It compares dry-bulb and wet-bulb temperatures; the difference, together with pressure, allows relative humidity to be calculated using a psychrometric chart or software.

Sling or aspirated model: which one to choose?
The sling type is rugged and portable for field use. The aspirated model (Assmann) delivers a constant airflow and better repeatability in the laboratory.

What accuracy can I expect?
With accurate thermometers (±0.1–0.2 °C) and good ventilation (2–5 m/s), RH uncertainty can be reduced to a few percentage points depending on conditions.

Do I need to correct for atmospheric pressure?
Yes, pressure affects psychrometric calculations. Recording local pressure improves accuracy, especially at altitude.

What are good operating practices?
Use a clean wick fully saturated with distilled water, avoid radiation, ensure adequate airflow, allow readings to stabilise and read promptly.

Maintenance and consumables?
Replace the wick regularly, rinse with distilled water, check the calibration of thermometers/probes and protect the instrument after use.

Do psychrometers contain mercury?
Modern models favour mercury-free liquids or Pt100 probes. Always check regulatory compliance at your site.

Can I use it in a cleanroom?
Yes, choose an aspirated model with compatible materials and low particle emission; avoid splashes and monitor the condition of the wick.