Psychrometers measure relative humidity without agitation. To use, turn on the fan and let it run for 1-1/2 to 2 minutes. Then, subtract wet bulb reading from dry bulb reading and use the included slide rule and tables to calculate the RH. The fan is powered by three size D batteries (order separately). This operation, when compared to the conventional sling psychrometers, affords a safer, convenient, and more accurate method of RH determination.