Relative Humidity Sensor (RHS)

The Humidity sensor measures relative humidity with 0 – 100% range in temperature range of 203 K – 323 K, with accuracy better than 10%RH and resolution of 1%RH.

The humidity sensor is located on one of the REMS booms positioned on the MSL Rover mast.

The Humidity sensor is provided by the Finnish Meteorological Institute, and it is based on proprietary technology of Vaisala, Inc. Humidity observations make use of Vaisala Humicap® relative humidity sensor heads and Thermocap® temperature sensor head. The Humicap® sensor heads contain an active polymer film that changes its capacitance as function of relative humidity with 0 – 100%RH measurement range. The capacitance of the film is also temperature dependent, so accurate temperature measurements by Thermocap® are needed in order to interpret the humidity results correctly. The Humicap® sensor heads also have Pt heating resistors that are used periodically to heat the sensor heads to spur away possible contaminants that can affect the capacitance.

The control of the sensor is based on a low-power transducer controlled by a dedicated ASIC, which is in turn controlled by an FPGA in the REMS Instrument Control Unit. The transducer electronics and the sensor heads are placed on a single multi-layer PCB protected by a metallic Faraday cage. The PCB is supported by mechanical interface mounted on the REMS Boom. The Faraday cage is perforated to allow sufficient ventilation, and covered with white PTFE filter to protect the sensor heads from dust.

The general characteristics of the Humidity sensor are given:

• Dimensions 40 x 40 x 25 mm
• Mass 15 g
• Power consumption 15 mW
• Data rate 208 bits/s, 1.4 Mbits/sol
• Survival temperature 150 – 400 ºK
• Operational temperature 203 – 323 ºK
• Measurement range 0 – 100% RH
• Typical accuracy ± 8%RH in -70°C, ± 5%RH in -40°C, ± 3%RH in -20°C
• Resolution 1% RH