Humidity sensor-device used to sense or measure humidity

As we know water content in the air is very important for our well being. Humidity is an important factor for operating certain equipments such as electronic circuits, electrostatic sensitive components, high voltage devices etc. It has to be assured to maintain relative humidity near 50% at normal room temperatures of about 20-25 degreeC. To detect moisture contents hygrometer is used. Generally, sensors for moisture, humidity, and dew temperature can be capacitive, conductive, oscillating, or optical.

The term moisture generally refers to the water content of any material, but for practical reasons, it is applied only to liquids and solids, whereas the term humidity is reserved for the water vapor content in gases. Let us understand definitions of few of the important terms before we will see types of humidity sensor.

Moisture: the amount of water contained in a liquid or solid by absorption that can be removed without altering its chemical properties.

Humidity ratio: the mass of water vapor per unit mass of dry gas.

Absolute humidity: the mass m of water vapor per unit volume v of wet gas: dw =m/v
It is nothing but density of water vapor component. The unit of absolute humidity is grams per cubic meter, or grains per cubic.

Relative humidity: It is ratio of the actual vapor pressure of the air at any temperature to the maximum of saturation vapor pressure at the same temperature. Relative humidity can be expressed in percentage as follows:
H = 100 * (Pw/Ps)
Where, Pw is the partial pressure of water vapor and Ps is the pressure of saturated water vapor at given temperature. The value of H expresses the vapor content as a percentage of the concentration required to cause the vapor saturation.

Dew-Point temperature: the temperature at which the partial pressure of the water vapor present would be at its maximum, or saturated vapor condition, with respect to equilibrium with a plain surface of ice. The dew point is the temperature at which relative humidity is 100%. In other words, the dew point is the temperature that the air must reach for the air to hold the maximum amount of moisture it can.

Let us examine basics of various humidity sensors such as Capacitive thin-film humidity sensor, conductive humidity sensor, Thermal Conductivity Sensor, Optical Hygrometer and Oscillating Hygrometer etc.

Capacitive thin-film humidity sensor

An air-filled capacitor may serve as a relative humidity sensor because moisture in the atmosphere changes air electrical permitivity according to the following equation.

k = 1 + (211/T)* ( P + ((48*Ps)/T)*H ) * 10 -6

T = Absolute temp. in K
P = Pressure of moist air in mm Hg
Ps = Pressure of saturated water vapor at temp. T in mm Hg
H = relative humidity in%

capacitive humidity sensor

From the equation it is imperative that the dielectric constant of moist air and, therefore, the capacitance are proportional to the relative humidity. Instead of air, the space between the capacitor plates can be filled with an appropriate isolator whose dielectric constant changes significantly upon being subjected to humidity.

The capacitive sensor may be formed of a hygroscopic polymer film with metallized electrodes deposited on the opposite sides.

The block diagram of the capacitive measurement system where the dielectric constant of the sample changes the frequency of the oscillator. This method of moisture measurement is quite useful in the process control of pharmaceutical products. figure-1 depicts working of capacitive humidity sensor.

In the design of capacitive thin film humidity sensor, R and C components are used. When releative humidity increases, the distributed surface resistance drops and equivalent capacitance between terminals increases. As capacitance depends on frequency for low humidity measurement frequency will be chosen to about 100 Hz and for high humidity measurement it is selected between 1-10KHz.

conductive humidity sensor

conductive humidity sensor

Another phenomenon used in humidity sensor is measurement of changes in the resistivity of conductor. As we know resistance of nonmetal conductors depend upon water content. The concept is employed in the design of resistive humidity sensor referred also as hygristor.

As shown in the figure , The sensor contains a material of relatively low resistivity which changes significantly under varying humidity conditions. The material is deposited on the top of two interdigitized electrodes to provide a large contact area. When water molecules are absorbed by the upper layer, resistivity between the electrodes changes and can be measured by an electronic circuit.

Thermal Conductivity Sensor

thermal humidity sensor

Absolute humidity sensor can be developed using two self heating thermistors. Using the thermal conductivity of gas to measure humidity can be accomplished by a thermistor-based sensor.

Two tiny thermistors Rt1 and Rt2 are supported by thin wires to minimize thermal conductivity loss to the housing. The left thermistor is exposed to the outside gas through small venting holes, and the right thermistor is hermetically sealed in dry air. Both thermistors are connected into a bridge circuit (R1 and R2), which is powered by voltage + E. The thermistors develop self-heating due to the passage of electric current. Their temperatures rise up to 170 DegreeC over the ambient temperature. Initially, the bridge is balanced in dry air to establish a zero reference point. The output of this sensor gradually increases as absolute humidity rises from zero. At about 150 g/m3, it reaches the saturation and then decreases with a polarity change at about 345 g/m3 .

Optical Hygrometer

The basic idea behind the optical hygrometer is the use of a mirror whose surface temperature is precisely regulated by a thermoelectric heat pump. The mirror temperature is controlled at the threshold of the formation of dew. Sampled air is pumped over the mirror surface, and if the mirror temperature crosses a dew point, it releases moisture in the form of water droplets. The reflective properties of the mirror change at water condensation because water droplets scatter light rays. This can be detected by an appropriate photodetector.

Oscillating Hygrometer

The idea behind the oscillating hygrometer is similar to that behind the optical sensor. The difference is that the measurement of the dew point is made not by the optical reflectivity of the surface, but rather by detecting the changing mass of the chilled plate. The chilled plate is fabricated of a thin quartz crystal that is a part of an oscillating circuit. This implies the other name for the sensor as piezoelectric hygrometer, because the quartz plate oscillation is based on the piezoelectric effect.

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