What is a resistive humidity sensor?
Resistive humidity sensors measure the variation of the electrical impedance of a hygroscopic medium such as a conductive polymer, salt or a treated substrate.
Resistive sensors are based on an interdigitated or two-wire winding. After depositing a hydroscopic polymer coating, their resistance changes inversely with humidity. The change in impedance is usually an inverse exponential relationship to humidity.
Resistive sensors generally consist of noble metal electrodes deposited on a substrate by photoresist techniques or electrodes wound on a plastic or glass cylinder. The substrate is covered with a salt or a conductive polymer. Alternatively, the substrate can be treated with activating chemicals such as acid.
The sensor absorbs water vapor and the ionic functional groups are dissociated, which increases the electrical conductivity. The response time of most resistive sensors varies from 10 to 30 s to reach 63% of the actual value. The impedance range of typical resistive elements ranges from 1 ohms to 000 ohms.
Most resistive sensors use a symmetrical AC excitation voltage without DC bias to prevent sensor bias. This response can be linearized by analog or digital methods. Typical variable resistance ranges from a few kilohms to 100 Mohms. The nominal excitation frequency is 30 Hz to 10 kHz.
Sensor calibration and accuracy
The "resistive" sensor is not purely resistive in that the capacitive effects make the response an impedance measurement. A distinct advantage of resistive RH sensors is their interchangeability, generally within plus or minus 2% RH, which allows electronic signal conditioning circuits to be calibrated by a resistance at a fixed RH point. This eliminates the need for humidity calibration standards, so resistive humidity sensors are generally field replaceable.
The accuracy of individual resistive humidity sensors can be confirmed by testing in an RH calibration chamber or by a computerized DA system referenced in a standardized humidity controlled environment. The nominal operating temperature of the resistive sensors varies from -40 degrees C to 100 degrees C.
In residential and commercial environments, the life expectancy of these sensors is> 5 years, but exposure to chemical vapors and other contaminants such as oil mist can cause premature failure. Another disadvantage of some resistive sensors is their tendency to shift values when exposed to condensation if a water soluble coating is used.
Resistive humidity sensors have significant temperature dependencies when installed in an environment with large temperature fluctuations. Simultaneous temperature compensation is integrated for more precision. The small size, low cost, interchangeability and long-term stability make these resistive sensors suitable for use in control and display products for industrial, commercial and residential applications.