Humidity Sensors: Types and Working Principles

Humidity sensors are devices used to sense or measure the amount of water vapor in the air. As we know, the water content in the air is crucial for our well-being. Humidity plays a significant role in the operation of sensitive equipment like electronic circuits, electrostatic components, and high-voltage devices. Maintaining a relative humidity of around 50% at normal room temperatures (20-25°C) is often necessary. Hygrometers are used to detect moisture levels.

Generally, sensors that measure moisture, humidity, and dew temperature can be capacitive, conductive, oscillating, or optical. The term “moisture” usually refers to the water content in liquids and solids, while “humidity” is reserved for the water vapor content in gases.

Let’s define some important terms before exploring different types of humidity sensors:

• Moisture: The amount of water contained in a liquid or solid 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’s essentially the density of the water vapor component. The unit is grams per cubic meter (g/m³) or grains per cubic foot.

• Relative Humidity (H): The ratio of the actual vapor pressure of the air at a given temperature to the maximum saturation vapor pressure at the same temperature. It’s expressed as a percentage:

  H = 100 * (Pw/Ps)

  Where:

  • Pw = Partial pressure of water vapor
  • Ps = Pressure of saturated water vapor at a given temperature

  H represents the vapor content as a percentage of the concentration needed for 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. It’s the temperature at which the relative humidity reaches 100%. In other words, it’s the temperature the air must reach to hold the maximum amount of moisture it can.

Now, let’s examine the basics of various humidity sensors:

1. Capacitive Thin-Film Humidity Sensor

An air-filled capacitor can function as a relative humidity sensor because moisture in the atmosphere changes the air’s electrical permittivity. This change is governed by the following equation:

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

Where:

• T = Absolute temperature in Kelvin (K)
• P = Pressure of moist air in mm Hg
• Ps = Pressure of saturated water vapor at temperature T in mm Hg
• H = Relative humidity in %

The equation indicates that the dielectric constant of moist air, and therefore the capacitance, is proportional to the relative humidity. Instead of air, the space between the capacitor plates can be filled with an insulator whose dielectric constant changes significantly with humidity. The capacitive sensor can be made of a hygroscopic polymer film with metallized electrodes on opposite sides.

The dielectric constant of the sample changes the frequency of the oscillator. This measurement method is useful in the process control of pharmaceutical products. Figure-1 depicts the working principle of a capacitive humidity sensor.

Capacitive thin-film humidity sensors utilize R and C components. As relative humidity increases, the distributed surface resistance drops, and the equivalent capacitance between terminals increases.

Capacitance is frequency-dependent. For low humidity measurements, a frequency of around 100 Hz is chosen. For high humidity measurements, it’s selected between 1-10 kHz.

2. Conductive Humidity Sensor

Another principle used in humidity sensing is measuring changes in a conductor’s resistivity. The resistance of nonmetal conductors depends on water content. This concept is used in resistive humidity sensors, also known as hygristors.

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

3. Thermal Conductivity Sensor

Absolute humidity sensors 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, rising up to 170°C above 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 saturation and then decreases with a polarity change at about 345 g/m³.

4. Optical Hygrometer

The fundamental 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 dew formation. Sampled air is pumped over the mirror surface, and if the mirror temperature crosses the 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 change can be detected by an appropriate photodetector.

5. Oscillating Hygrometer

The concept behind the oscillating hygrometer is similar to that of the optical sensor. The difference is that the dew point measurement is made by detecting the changing mass of the chilled plate, rather than the optical reflectivity of the surface. The chilled plate is made of a thin quartz crystal that’s part of an oscillating circuit. This leads to the alternative name, piezoelectric hygrometer, because the quartz plate oscillation is based on the piezoelectric effect.