Sling psychrometers measure the atmospheric humidity. These systems are essential for calibrating hygrometers. Sling psychrometers are available in various sizes ranging from pocketsize models to larger units approximately a foot in length. Sling psychrometers are the standard for accurately measuring ambient temperature and humidity. These psychrometers are used to measure cave air. Sling psychrometers tend to lose accuracy at humidity levels below 20% RH and temperatures below 32oF.