Meter displays psychrometric chart This item DOES NOT contain mercury! 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. Fan is powered by three size D batteries (order separately). What's included: one 2-oz water bottle, psychrometric slide rule, tables, chart, extra lamp, extra wicking material, and carrying case. SpecificationsRH range 10 to 100% RH resolution 1% RH accuracy ±3% full-scale Temp range -15 to 45°C Temp resolution 0.5°C Temp accuracy ±0.2°C Power three 1.5 V D batteries (sold separately) Dimensions 10"W x 4 1/2"H x 2"D