HUMIDITY SENSOR THEORY AND BEHAVIOR SENSOR CONSTRUCTION: Relative humidity sensors use an industrially proven thermoset polymer, three layer capacitance construction, platinum electrodes and except for high temperature versions (shown bottom), on-chip silicon integrated voltage output signal conditioning. (RHIC Sensor). In operation, water vapor in the active capacitor’s dielectric layer equilibrates with the surrounding gas. The porous platinum layer shields the dielectric response from external influences while the protective polymer over layer provides mechanical protection for the platinum layer from contaminants such as dirt, dust and oils. A heavy contaminant layer of dirt will slow down the sensor’s response time because it will take longer for water vapor to equilibrate in the sensor.
VOLTAGE OUTPUT: The RHIC sensor linear voltage output is a function of Vsupply, % RH and temperature. The output is ‘‘ratiometric,’’ i.e. as the supply voltage rises, the output voltage rises in the same proportion. A surface plot of the sensor behavior for temperatures between 0°C and 85°C is shown in the graph below. This surface plot is well approximated by a combination of two equations: 1.
A ‘‘Best Fit Line at 25°C,’’ or a similar, sensor specific equation at 25°C. The sensor independent ‘‘typical’’ Best Fit Line at 25°C (bold line in graph) is: Vout J Vsupply (0.0062 (% RH) + 0.16) A sensor specific equation can be obtained from an RH sensor printout. The printout equation assumes Vsupply J 5VDC and is included or available as an option on every sensor.
A sensor independent equation which corrects the % RH reading (from the Best Fit Line Equation) for temperature, T: True RH J (% RH)/(1.0546–.00216 T); T J °C Or True RH J (% RH)/(1.093–.0012 T); T J °F
TEMPERATURE & HUMIDITY EFFECTS: The output of all absorption based humidity sensors (capacitive, bulk resistive, conductive film, etc.) are affected by both temperature and %RH. Because of this, temperature compensation is used in applications which call for either higher accuracy or wider operating temperature ranges. When temperature compensating a humidity sensor, it is best to make the temperature measurement as close as possible to the humidity sensor’s active area, i.e. within the same moisture micro-environment. This is especially true when combining RH and temperature as a method for measuring dew point. Industrial grade Humidity and Dew Point instruments incorporate a 1000 ohm Platinum RTD on the back of the ceramic sensor substrate for unmatched temperature compensation measurement integrity. No on-chip signal conditioning is provided in these high temperature sensors.
The equations above match the typical surface plot (Best Fit Line at 25°C) or the actual surface plot (sensor specific equation at 25°C) to within the following tolerances: ±1% for T>20°C ±2% for 10°C