Humidity controller (PID box)

A humidity controller in conjunction with a T/RH sensor and heater is used to maintain the humidity of the sample air at a desired setpoint RH. A basic aerosol system will typically have two heaters, each controlled by a humidity controller in the PID box: (i) the inlet stack heater and (ii) the 'touch-up' heater. The humidity controller allows power to be applied to the heater when the RH increases above a desired setpoint.

The stack heater design is described here. This heater gently heats the sample stream to help lower sample humidity. Another ‘touch-up’ heater is deployed directly upstream of the impactor box to lower the humidity of the sample air to <40%.

Blue rectangle indicates location of 'touch-up' heater, pink circle indicates location of RH sensor. The RH sensor is connected to humidity controller in PID box.
PID box for RH and flow control - setpoint on first humidity controller (for stack heater) is RH=55%, setpoint on second humidity controller (for 'touch-up' heater) is RH=40%.