slingpsychrometers

A sling psychrometer is a simple instrument used to measure relative humidity by comparing temperatures from two thermometers mounted on a sling or frame: a dry-bulb thermometer and a wet-bulb thermometer whose bulb is wrapped with a moistened wick. The device is swung through the air to expose the thermometers to ambient conditions, creating evaporative cooling on the wet bulb.

Operation involves moistening the wick, swinging the device until the readings stabilize, and then recording both the dry-bulb and wet-bulb temperatures. The wet-bulb temperature is typically lower than the dry-bulb temperature due to evaporation. The readings are then used to determine relative humidity through a psychrometric chart or corresponding calculations that relate the temperature pair to vapor pressures, often with reference to the ambient air pressure.

Calculation and interpretation rely on the relationship between saturation vapor pressure and actual vapor pressure. Using the dry-bulb and wet-bulb values, one can estimate the amount of moisture in the air and derive the relative humidity and dew point. In practice, standard charts at sea level are common, with minor corrections for nonstandard pressure.

Applications and limitations: Sling psychrometers are widely used in meteorology, agriculture, HVAC testing, and education because they are inexpensive and robust. Accuracy depends on proper airflow over the bulbs, a well-maintained damp wick, and stable ambient conditions; drafts or an excessively dry wick can introduce error. They provide a measure of relative humidity, not dew point, without additional data.

Variations and modern relevance: While digital hygrometers and chilled-mirror devices offer ease and precision, the sling psychrometer remains a low-cost, portable option for fieldwork and teaching.