Abstract: Relative humidity is an environment factor that significantly affects biological experiments, but is difficult to precisely control. Amodified device for easy control of the humidity in regular thermal and light growth chamber was described in this paper. The device included three parts: 1) air pump; 2) three pipe collected containers of salt solution for producing expected humidity; and 3) a perspex chamber, in which, experimental materials can be placed. The modified device worked well for humidity control with an error of only 1%, while a digital data logger was used to monitor the relative humidity controlled by saturated solution of sodium tartrate every three minutes for 6 h at 20℃, which was supposed to yield a 92% r.h. at that temperature. Varying concentrations of sulfate acid were tested as humidity control agents at 5～35℃, with each yielding a stable humidity in the perspex chamber regardless of temperature. The stability of the humidity control system was also discussed when the solutions of sulfate acid were used as humidity control agents.

Key words: Humidity control, Modified device and methodology, Biological experiments, Karst, sap flow density, Triadica rotundifolia, Radermachera sinica, transpiration water consumption.