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RH+T Wet and Dry Bulb Controller


RHC301 is a relative-humidity (RH) controller that uses wet and dry bulb temperatures to measure and display RH. A superior version of RHC201, it can apply a maximum or minimum function on RH as well as calculate dew point temperature. It receives two 4 - 20 mA or RTD inputs for wet and dry bulb temperatures within the range of -20 to 100 °C. The RH controller has a large two-color triple LED display to indicate a large variety of parameters. The upper display can indicate the calculated RH, peak RH (maximum) or valley RH (minimum). The middle can indicate dry bulb temperature or setpoint, while the lower can indicate wet bulb temperature, dew point temperature or the difference between dry and wet bulb temperatures. 15 additional LEDs indicate relay status, the process or measured value displayed, temperature units used (Celsius or Fahrenheit) and communication status. Operators program up to 4 setpoints easily via membrane buttons on the front of RHC301. The controller has an accuracy of ± 2 RH with a Class A Pt100 RTD and ± 0.5 RH for linear input. RHC301’s electromagnetic relays trigger humidification (heating), dehumidification (cooling), or any of six alarms - full-scale high and low, deviation high and low, inband and outband. Relays can be assigned to RH, Tdry, Twet, Tdew, Tdiff, water alarm or sensor fail. It transmits two isolated output signals which can be RH, Tdry, Twet, Tdew or Tdiff to other devices such as a Programmable Logic Controller (PLC) or Solid-State Relay (SSR). Signal isolation protects your devices and machines downstream of our instruments from damaging fluctuations in the input signal. RHC301 is conventionally an ON-OFF controller that outputs a discrete signal, but can also be used in Time-Proportioning Mode which turns the actuator on and off for periods proportional to the desired control effort, to emulate a continuous actuator that can generate a control effort between 0 and 100%. By including input terminals in the excitation loop, RHC301 provides a 24 V DC power supply for two-wire 4-20 transmitters.

  • 2 x 4 - 20 mA or RTD inputs for dry and wet temperatures
  • Max, Min functions for RH
  • 2 or 4 setpoints for RH / Tdry / Twet / Tdew / Tdiff / Water alarm / Sensor fail
  • 85 - 265 V AC or 20 - 35 V DC supply
  • 24 V DC transmitter supply
  • 2 x Isolated 0/4 - 20 mA or 0 - 10 V DC retransmission outputs corresponding to RH / Tdry / Twet / Tdew / Tdiff
  • Water alarm annunciator
  • Dew point temperature


  • humidity measurement
  • humidity control



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