Which instrument uses dry bulb and wet bulb temperatures to calculate relative humidity?

The instrument that uses dry bulb and wet bulb temperatures to calculate relative humidity is the sling psychrometer. A sling psychrometer consists of two thermometers, one measuring the ambient air temperature (dry bulb) and the other measuring the temperature of air after it has passed over a moistened wick or cloth (wet bulb).

To determine relative humidity, the readings from both thermometers are compared. The difference between the dry bulb temperature and the wet bulb temperature is used in conjunction with psychrometric charts or formulas to calculate the humidity level in the air. This method is effective because as water evaporates from the wet bulb, it cools the thermometer, and the greater the evaporation (which occurs when humidity is lower), the larger the temperature difference will be.

Other instruments listed do not measure humidity in this way. A barometer measures atmospheric pressure, a hydrometer measures the specific gravity of liquids, and a thermometer measures temperature only. Therefore, the sling psychrometer is specifically designed for measuring relative humidity by utilizing both dry and wet bulb temperatures.