A psychrometer is used to determine relative humidity, the amount of moisture in the air expressed as a percentage. It does this indirectly by measuring two temperatures and using the difference between them to calculate how much water vapor the surrounding air contains. The device is one of the oldest and most straightforward tools for humidity measurement, and it’s still used in weather forecasting, agriculture, HVAC work, and museum conservation.
How a Psychrometer Works
A psychrometer holds two thermometers side by side. One measures the temperature of the surrounding air normally. This is called the dry bulb temperature. The other has its bulb wrapped in a small piece of wet cloth or muslin, and its reading is called the wet bulb temperature.
When air moves over the wet cloth, water evaporates from it. Evaporation pulls heat away from the thermometer, so the wet bulb always reads lower than the dry bulb. How much lower depends entirely on humidity. If the air is already saturated with moisture, very little water can evaporate, and the two thermometers read almost the same temperature. If the air is dry, evaporation happens quickly and the wet bulb drops well below the dry bulb. That gap between the two readings is the key measurement.
Once you have both temperatures, you look up the pair on a reference called a psychrometric table or psychrometric chart. These tables account for factors like elevation and atmospheric pressure, and they convert the temperature difference into a relative humidity percentage. The National Wildfire Coordinating Group, for example, publishes tables organized by elevation bands from sea level up to 8,500 feet, each calibrated to a different atmospheric pressure. At higher elevations, the same temperature gap corresponds to a slightly different humidity value because air pressure affects evaporation rates.
What Else You Can Find From It
Relative humidity is the primary value people use a psychrometer to determine, but it’s not the only one. A psychrometric chart plots several air properties at once. From just the dry bulb and wet bulb readings, you can also find the dew point temperature (the temperature at which moisture would start condensing out of the air), the amount of moisture per pound of dry air, and the total heat energy content of the air. These properties are especially useful in agricultural building design and HVAC system balancing, where engineers need a complete picture of air conditions to size equipment and manage ventilation.
Types of Psychrometers
There are two main types: sling psychrometers and aspirating psychrometers.
A sling psychrometer is a simple, hand-operated device. Two thermometers are mounted on a frame that pivots on a handle or swings on a short chain. You wet the cloth wick, then whirl the instrument through the air for one to three minutes. The spinning creates airflow over the wet bulb, driving evaporation. Its main advantages are low cost, portability, and a design with essentially nothing to break.
An aspirating psychrometer is a battery-powered (or sometimes wind-up) unit with a small built-in fan. The fan draws air steadily over the wet bulb, so instead of whirling the device you simply turn it on and wait about two minutes for the wet bulb reading to stabilize. The aspirating type is considered the more accurate of the two because the airflow is consistent and controlled. It also keeps the user’s body heat and breath farther from the sensors. In tight spaces like museum storage rooms or mechanical closets where there’s no room to swing a sling psychrometer, the aspirating version is the practical choice.
Accuracy Compared to Modern Sensors
Psychrometers are reliable when used correctly, but they’re sensitive to technique. Several factors affect accuracy: the speed of airflow over the wet bulb, the temperature of the water on the wick, how clean the wick is, and how well the two thermometers are calibrated to each other. The National Bureau of Standards historically recommended a minimum airflow of 900 feet per minute over the wick for dependable readings.
When conditions are less than ideal, errors add up. Testing by the U.S. Environmental Protection Agency found that a psychrometer under typical lab conditions (lower than recommended air velocity, room-temperature water on the wick, partially filled reservoir) produced a positive humidity bias of about 5 grains of moisture per pound of air compared to a precision dew point hygrometer. If the wick was dirty or had slipped off the bulb, the error climbed to 7 grains. Factoring in thermometer calibration drift, total error could exceed 10 grains per pound. For rough field measurements this is perfectly acceptable, but high-precision applications like emissions testing labs have largely replaced psychrometers with electronic dew point sensors.
Where Psychrometers Are Used
Weather stations and field meteorologists use psychrometers to report humidity, which feeds into forecasts for precipitation, fog, and heat index calculations. Wildfire crews rely on them heavily because relative humidity is one of the critical variables in predicting fire behavior. A reading below about 25% signals elevated fire risk.
In agriculture, psychrometers help manage greenhouse and livestock building environments. Knowing both relative humidity and dew point lets growers anticipate condensation on plant surfaces (a trigger for fungal disease) and lets livestock managers assess heat stress in animals, which depends on the combination of temperature and humidity rather than temperature alone.
Museums and archives use psychrometers as a calibration tool for the hygrothermographs and electronic sensors that continuously monitor storage areas. A quick spot check with a sling or aspirating psychrometer can reveal whether a wall-mounted sensor has drifted out of calibration, something that happens gradually and can go unnoticed for months.
Why Relative Humidity Matters
The reason so many fields care about the measurement a psychrometer provides comes down to health, comfort, and material preservation. Research on indoor environments consistently points to 40% to 60% relative humidity as the optimal range for human health. Within that band, airborne viruses survive less effectively, the body’s respiratory defenses function well, and conditions discourage dust mites and mold growth. Above 60% to 75%, mold becomes a real concern, triggering allergic and inflammatory reactions and worsening asthma. Below 40%, mucous membranes dry out, skin cracks, and static electricity becomes a nuisance.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends controlling indoor humidity so the dew point stays at or below about 59°F. At a typical office temperature of around 73°F, that translates to a maximum relative humidity of roughly 59%. Studies of office buildings worldwide have found that indoor humidity frequently drifts outside the 40% to 60% zone, meaning the simple act of measuring humidity with a psychrometer or any other instrument, and then acting on the reading, has a direct link to occupant comfort and respiratory health.