Sper Scientific 736740 Mason's Hygrometer

Measure Humidity - Mason's Hygrometer - Red Liquid.One thermometer bulb is kept constantly wet by a siphon reservoir while the other gives a dry temperature reading.Humidity is obtained by comparing the two readings on the enclosed chart.The red liquid masons hygrometer uses a safe and easy to read red liquid in place of the usual mercury.Dual scales of -10/50°C and 10/120°F