Types of Humidity Sensors and Their Field of Applications

In this work I present three types of humidity sensors and their field of applications. Most commonly used humidity sensors are based on capacitive or resistive measurement. All these types of humidity sensors have a comparable design, which uses an insulated substrate, electrode structures, and a sensing material. The choice of a suitable sensor fabrication (in case of technical specification) depends on the operating conditions. The impact of humidity plays an important role in all areas of human life such as biology or automated industrial processes because water vapor is a natural component of the air.

Humidity sensors are used in intelligent systems for monitoring soil moisture in the field of agriculture or for the monitoring of corrosion and erosion in infrastructures. Furthermore, humidity sensors are used for the human comfort problems in household applications. Due to the various fields of application of humidity sensors and the associated different requirements, there are various sensor principles.

Based on the measuring units, these sensors are divided into two main groups: Relative Humidity (abbreviated RH) sensors and Absolute Humidity (abbreviated AH) sensors. These sensor types are called hygrometric sensors. However, in many humidity measurement applications, relative humidity measurement is preferred because the measurement of relative humidity is simpler and thus less expensive and is widely used in the areas of indoor air quality and comfort problems.

To make the humidity sensors flexible for a wide range of application, the following requirement, such as a short response time, small hysteresis and a good sensitivity over a wide range of humidity and temperature are set for these sensors. Since the most commonly used method is the relative humidity measurement, the relative humidity is explained in the following. In general, the humidity is defined as the amount of water vapor in an atmosphere of air. Since the relative humidity is a temperature dependent variable, it is customary in hygrometry to measure the humidity together with the temperature. The relative humidity is given in percent and determined as follows: where: pw: water vapor pressure, ps: saturation pressure at the same given temperature in Bar.

Humidity sensors based on the change of their electrical properties are divided into two groups: resistive-type and capacitive-type. The construction of capacitive sensors and resistive sensors are comparable, but the measuring principle is different. The capacitive-type sensors are based on the change of their dielectric properties, whereas the resistive- type sensors are based on the change of their conductivity. Both sensor types have a pair of electrodes on a substrate coated with a humidity sensitive layer. The adsorption of water vapor causes a change in the dielectric constant of the material (capacitive-type) and this leads to a change in the capacitance between the electrodes, or a change in the conductivity of the material (resistive-type) whereby the resistance changes.

As substrate, an alumina substrate was selected. On this substrate are the electrodes, for this an interdigital structure (or comb structure) with intervals of 0.15 mm was chosen. The humidity-sensitive layer consists of a mixed aqueous solution of styrene-sulfonate monomers, crosslinking agents and vinyl polymers which are spin-cast onto the substrate. Since styrene-sulfonate are polymerized and crosslinked by ultraviolet irradiation, the coated film is irradiated with ultraviolet light in a nitrogen atmosphere. For protection, the humidity-sensitive layer is covered with a moisture-permeable film. This protective film serves to suppress the influence, such as cigarette smoke, oil and other impurities, and to protect the humidity-sensitive film from them. The size of the sensor is 5 mm x 7 mm.

Using a thermostatic humidity generator, the resistance was measured at various relative humidities. For the measurement, the sensor was connected to a load resistor and an AC voltage of less than 3 V was applied. The accuracy of the generated humidity in the thermostatic test chamber is better than 2% RH. The sensor shows a high sensitivity over the relative humidity and as expected a logarithmic behavior and has the advantage of being linear in the range from 30 % to 100 %. Since many sensors were produced on the same substrate in, the response characteristics are comparable and show the same behavior.

The curve with the solid line shows the response for the sensor with a protective layer and the curve with the dotted line shows the response for the sensor without a protective layer. The response time is measured for a quick change of relative humidity from 30% to 90% and vice versa. For the sensor with a protective layer, the response time for adsorption and desorption is a few seconds. For the sensor without a protective layer, the response time is 100 seconds for adsorption and 150 seconds for desorption .

This sensor is called “Humicape” and was developed by Vaisala in Finland and is used in many humidity-measuring instruments, such as radio-sondes. As substrate, a glass substrate was selected. On this substrate, the lower twin electrodes are attached by indium evaporation. The thin-film humidity-sensitive material used is cellulose acetat with a thickness of about 1 µm. On top is the upper electrode which is made by gold evaporation. This upper electrode has a thickness of about 10 nm to 20 nm and is porous enough for the transport of water vapor. The upper electrode, which acts as a counter electrode to the lower twin electrodes, results in a series connection of two capacitances. This construction has the advantage that the difficulties in contacting the thin upper electrode are eliminated.

The capacitance is approximately proportional to the ambient humidity in the range from 0% to 100%. The sensor has a good accuracy and a response time of about 1 s to reach 90% of the steady-state value.

As substrate, an alumina substrate was selected. On this substrate are the electrodes, for this an interdigital gold structure (or comb structure) with a thickness of 8 µm to 10 µm was chosen. The humidity-sensitive layer was prepared in with different mixing ratios of GTMAC (glycidyl trimethyl ammonium chloride), PPGDE (polypropylene glycol diglycidyl) and MTHPA (methyl tetrahydrophthalic anhydride). Figure 7 shows the response characteristics of the sensor at 25 °C and 1 kHz for a mixed ratio of GTMAC/PPGDE/MTHPA = 100/0/70. For the measurements, an AC voltage of 1 V was applied between the electrodes. The impedance of the sensor was measured in the range from 30% to 100%.

The curve for absorption and desorption shows a proportional behavior. For the determination of the hysteresis, two dotted lines in the range of +- 2% RH are shown in figure 2. For the hysteresis of the sensor, this results in a value of <2% RH. Figure 8 shows the response time of the sensor. This sensor has a response time of 55 s for adsorption and approximately the same response time for desorption. Table II shows the comparison of the different sensors presented. The resistive-type humidity sensor shows a linear behavior from 30% to 100%, so the sensor can be used in this humidity range. In addition, this sensor has a protective film, which protects the sensor from environmental influences. This point is an advantage for a wide range of applications but a disadvantage for the response time. The capacitive-type humidity sensor shows an approximately linear behavior from 0% to 100%, so the sensor can be used for the whole range. The porous upper electrode which can be considered as a protective layer. The response time of this sensor is 1 s to reach 90% of the steady-state value. The impedance-type humidity sensor shows a linear behavior in the range from 30% to 90%, so the sensor can be used in this humidity range with a small hysteresis of <2% RH. The response time of the impedance-type humidity sensor is 55 s.

The table can be used to select the sensor for the application where the focus is on response time or humidity range. In this paper, three types of humidity sensors were presented. Humidity plays an important role in all areas, so it has to be monitored. Due to the different areas of application, the humidity range and the response time plays an important role here. In areas where the sensor is exposed to environmental influences and the humidity to be monitored is in the range from 30% to 100% the resistive-type humidity sensor can be used. If a fast response time is required, then the sensor is not suitable for these areas and the capacitive sensor can be selected. The capacitive-type humidity sensor is also suitable in areas where the humidity range is to be monitored in the range from 0% to 100%. In areas where the humidity is in the range from 30% to 90% and the range is fluctuating, then the impedance-type humidity sensor can be used with a low hysteresis of <2% and a response time of 55 s for adsorption and desorption.