Abstract—In requirements, there are various sensor principles

Abstract—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 has 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 1,4. 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 1. Due to the various fields of
application of humidity sensors and the associated different requirements, there
are various sensor principles 1,2. Table 1 shows five examples of the areas
of application and the associated operating temperature and humidity range in
these areas 2. 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 1. 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 4. 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 1. 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: 1,4 where: pw: water vapor
pressure, ps: saturation pressure at the same given temperature in
Bar 1,4. Humidity sensors
based on the change of their electrical properties are divided into two groups:
resistive-type and capacitive-type 3. 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. 4 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. 5 A thin-film resistive-type
humidity sensor is presented in 6. Figure 1 shows the schematic structure of
the sensor. 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 6. Figure
2 shows the response characteristics of the sensors at 25 °C with an operating
frequency of 1 kHz. 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 6, the response characteristics are comparable and show the same
behavior 6. Figure
3 shows the response curves for two kinds of samples. 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 6. A thin-film
capacitive-type humidity sensor is presented in 1 and 2. Figure 4 shows the
schematic structure of the sensor. 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 1,2. 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. 2 Figure 5 shows the response
characteristics of the sensor for different frequencies. The capacitance is
approximately proportional to the ambient humidity in the range from 0% to 100%
2. The sensor has a good accuracy and a response time of about 1 s to reach
90% of the steady-state value 1,2. An impedance-type
humidity sensor is presented in 7. Figure 6 shows the schematic structure of
an impedance-type humidity sensor. 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 7 with different mixing ratios of
GTMAC (glycidyl trimethyl ammonium chloride), PPGDE (polypropylene glycol
diglycidyl) and MTHPA (methyl tetrahydrophthalic anhydride) 7. 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 7. 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 7. 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 1,2. 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.