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. 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. Dim: 12" x 27/8" x 11/4" (304 x 73 x 32mm). Weight: 4oz (108g).