A sling psychrometer is a handheld instrument that measures humidity using two thermometers, one dry and one wrapped in a wet cloth wick. By swinging the device through the air and comparing the two temperature readings, you can determine the relative humidity, dew point, and wet-bulb temperature of the surrounding environment. It’s one of the oldest and simplest tools for measuring moisture in the air, and it’s still used today in HVAC work, meteorology, painting and coating applications, and classroom science.
How a Sling Psychrometer Is Built
The device consists of two identical thermometers mounted side by side on a flat backing, with a handle or chain attached at one end that allows the whole assembly to spin freely. One thermometer, called the dry bulb, is left exposed to the air. The other, called the wet bulb, has its sensing end wrapped in a small fabric sleeve or wick. Before taking a reading, you soak that wick in clean water at roughly the same temperature as the surrounding air.
The design is deliberately simple. There are no batteries, no sensors, no moving electronic parts. That simplicity is both its greatest strength and its main limitation: it’s cheap, rugged, and never needs firmware updates, but it relies entirely on the operator’s technique.
The Physics Behind the Reading
The sling psychrometer works on the principle of evaporative cooling. When you swing the device, air flows over both thermometers. The dry bulb reads the actual air temperature. The wet bulb, however, loses heat as water evaporates from its wick, and its temperature drops.
How far the wet-bulb temperature drops depends on how dry the air is. In very dry air, water evaporates quickly, pulling a lot of heat away from the thermometer and creating a large gap between the two readings. In humid air, evaporation slows because the air is already holding a lot of moisture, so the two readings stay close together. If the air is completely saturated (100% relative humidity), no evaporation occurs, and both thermometers read exactly the same.
The difference between the dry-bulb and wet-bulb temperatures is called the wet-bulb depression. Once you have that number, you look it up on a psychrometric chart or table to find relative humidity, dew point, and other properties of the air.
How to Take a Reading
Start by soaking the wick in clean water. Then hold the handle and spin the psychrometer in a circular motion, keeping it at arm’s length so your body heat doesn’t interfere. Most operators spin the device for about one to two minutes, though the key indicator is consistency: you’re looking for the wet-bulb reading to stabilize. After spinning, quickly read the wet bulb first (since it will begin warming the moment airflow stops), then the dry bulb.
Repeat the process two or three times to confirm the wet-bulb temperature has reached its lowest stable point. If consecutive readings match, you have a reliable measurement. The technique matters more than it might seem. Spinning too slowly reduces airflow over the wick and gives a falsely warm wet-bulb reading, which skews the humidity calculation upward.
Accuracy and Common Errors
A sling psychrometer can be reasonably accurate when used carefully, but operator technique introduces significant variability. A U.S. military field comparison found that with careful measurement, a sling psychrometer could achieve relative humidity accuracy within about 4%. In typical field use, though, the average error jumped to around 9%. By contrast, electronic meteorological instruments in the same study maintained errors of roughly 2%.
Most errors come from a few common sources:
- Dirty or old wicks that don’t absorb water evenly, leading to inconsistent wet-bulb readings
- Insufficient spinning time, so the wet bulb hasn’t fully stabilized
- Using warm water instead of ambient-temperature water to wet the wick
- Reading the thermometers too slowly after stopping, allowing the wet bulb to warm back up
- Body heat or exhaled breath influencing the thermometers at close range
When you control for these factors, the instrument performs well enough for many practical applications. But for high-precision work, electronic humidity sensors are more reliable and far less dependent on the person holding them.
Limitations in Cold Weather
Sling psychrometers become tricky to use when temperatures drop below freezing. The water on the wick can freeze, and a frozen wick behaves differently than a wet one. If the wick freezes, the wet bulb needs at least five minutes to stabilize rather than the usual one to two, and the psychrometric tables you use to interpret the reading must be ice-bulb tables rather than standard wet-bulb tables. Operators need to note whether the wick is wet or frozen before recording a measurement, since using the wrong lookup table will produce an incorrect humidity value.
At extremely low temperatures, keeping the wick wet at all becomes impractical, and most users switch to electronic instruments.
Where Sling Psychrometers Are Still Used
Despite being a centuries-old design, the sling psychrometer hasn’t disappeared. HVAC technicians use them as a quick field check for humidity in buildings and ductwork. In painting and industrial coating work, verifying that humidity is below a certain threshold is critical to preventing adhesion failures, and a sling psychrometer provides an independent, battery-free confirmation. Meteorology students still learn on them because the device makes the physics of humidity visible in a way that a digital readout doesn’t.
Weather stations have largely transitioned to electronic sensors for routine observations, but sling psychrometers remain a backup tool and a calibration reference. Their simplicity means they don’t drift out of calibration the way electronic sensors can, as long as the thermometers themselves are accurate. In remote field settings where electronics are impractical or unreliable, a sling psychrometer, a bottle of distilled water, and a printed psychrometric table can do the job with no power source required.