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%.