Investigation on measurement principles ofhumidity sensors Dennis LepkeInstitute of Micro-Sensors,-Actuators and –Systems (IMSAS)Bremen, [email protected]
deAbstract—In this work I present three types ofhumidity sensors and their field of applications. Most commonly used humiditysensors are based on capacitive or resistive measurement. All these types ofhumidity sensors have a comparable design, which uses an insulated substrate,electrode structures, and a sensing material. The choice of a suitable sensorfabrication (in case of technical specification) depends on the operatingconditions.
I. IntroductionThe impact of humidity plays an importantrole in all areas of human life such as biology or automated industrialprocesses because water vapor is a natural component of the air 1,4. Humiditysensors are used in intelligent systems for monitoring soil moisture in thefield of agriculture or for the monitoring of corrosion and erosion ininfrastructures. Furthermore, humidity sensors are used for the human comfortproblems in household applications 1.
Due to the various fields ofapplication of humidity sensors and the associated different requirements, thereare various sensor principles 1,2. Table 1 shows five examples of the areasof application and the associated operating temperature and humidity range inthese areas 2. Based on the measuring units, these sensors are divided intotwo main groups: Relative Humidity (abbreviated RH) sensors and AbsoluteHumidity (abbreviated AH) sensors. These sensor types are called hygrometricsensors.
However, in many humidity measurement applications, relative humidity measurementis preferred because the measurement of relative humidity is simpler and thusless expensive and is widely used in the areas of indoor air quality andcomfort problems 1. To make the humidity sensors flexible for a wide range ofapplication, the following requirement, such as a short response time, smallhysteresis and a good sensitivity over a wide range of humidity and temperatureare set for these sensors 4.TABLE I. Examples of the application areas 2 Application Operating Temperature °C Humidity range RH% Automobile: car windowa -20 – 80 50-100 Domestic electric appliance: Drier for clothing 80 0-40 Industry: Electric device manufacturing 5-40 0-50 Medical service: Medical apparatus 10-30 80-100 Measurement: Hygrometer -5-100 0-100 a. Prevention of dew condensation II. TheorySince the most commonly used method is therelative humidity measurement, the relative humidity is explained in thefollowing. In general, the humidity is defined as the amount of water vapor inan atmosphere of air 1.
Since the relative humidity is a temperaturedependent variable, it is customary in hygrometry to measure the humiditytogether with the temperature. The relative humidity is given in percent anddetermined as follows: 1,4 (1)where: pw: water vaporpressure, ps: saturation pressure at the same given temperature inBar 1,4. III. Types ofhumidity sensorsHumidity sensorsbased on the change of their electrical properties are divided into two groups:resistive-type and capacitive-type 3. The construction of capacitive sensorsand resistive sensors are comparable, but the measuring principle is different.The capacitive-type sensors are based on the change of their dielectricproperties, whereas the resistive- type sensors are based on the change oftheir conductivity. 4 Both sensor types have a pair of electrodes on asubstrate coated with a humidity sensitive layer.
The adsorption of water vaporcauses a change in the dielectric constant of the material (capacitive-type)and this leads to a change in the capacitance between the electrodes, or achange in the conductivity of the material (resistive-type) whereby theresistance changes. 5A. Resistive-typehumidity sensorA thin-film resistive-typehumidity sensor is presented in 6. Figure 1 shows the schematic structure ofthe sensor. As substrate, an alumina substrate was selected.
On this substrateare the electrodes, for this an interdigital structure (or comb structure) withintervals of 0.15 mm was chosen. The humidity-sensitive layer consists of amixed aqueous solution of styrene-sulfonate monomers, crosslinking agents andvinyl polymers which are spin-cast onto the substrate.
Since styrene-sulfonateare polymerized and crosslinked by ultraviolet irradiation, the coated film isirradiated with ultraviolet light in a nitrogen atmosphere. For protection, thehumidity-sensitive layer is covered with a moisture-permeable film. Thisprotective film serves to suppress the influence, such as cigarette smoke, oiland other impurities, and to protect the humidity-sensitive film from them. Thesize of the sensor is 5 mm x 7 mm 6.
Figure 1. Schematicstructure of a thin-film resistive-type humidity sensor. 6Figure2 shows the response characteristics of the sensors at 25 °C with an operatingfrequency of 1 kHz. Using a thermostatic humidity generator, the resistance wasmeasured at various relative humidities. For the measurement, the sensor wasconnected to a load resistor and an AC voltage of less than 3 V was applied.
Theaccuracy of the generated humidity in the thermostatic test chamber is betterthan 2% RH. The sensor shows a high sensitivity over the relative humidity andas expected a logarithmic behavior and has the advantage of being linear in therange from 30 % to 100 %. Since many sensors were produced on the samesubstrate in 6, the response characteristics are comparable and show the samebehavior 6. Figure 2.
Responsecharacterics of the sensor (at 25 °C and 1 kHz).6Figure3 shows the response curves for two kinds of samples. The curve with the solidline shows the response for the sensor with a protective layer and the curvewith the dotted line shows the response for the sensor without a protectivelayer. The response time is measured for a quick change of relative humidityfrom 30% to 90% and vice versa. For the sensor with a protective layer, theresponse time for adsorption and desorption is a few seconds. For the sensorwithout a protective layer, the response time is 100 seconds for adsorption and150 seconds for desorption 6.Figure 3.
Response curvesfor two kinds of samples (solid line : Sensor with protective film, dotted line:Sensor without protective film).6 B. Capacitive-typehumidity sensorA thin-filmcapacitive-type humidity sensor is presented in 1 and 2. Figure 4 shows theschematic structure of the sensor. This sensor is called ‘Humicape’ and was developedby Vaisala in Finland and is used in many humidity-measuring instruments, suchas radio-sondes. As substrate, a glass substrate was selected.
On thissubstrate, the lower twin electrodes are attached by indium evaporation. Thethin-film humidity-sensitive material used is cellulose acetat with a thicknessof 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 porousenough for the transport of water vapor 1,2. The upper electrode, which actsas a counter electrode to the lower twin electrodes, results in a seriesconnection of two capacitances. This construction has the advantage that thedifficulties in contacting the thin upper electrode are eliminated. 2.Figure 4.
Schematicstructure of a thin-film capacitive-type humidity sensor. 2Figure 5 shows the responsecharacteristics of the sensor for different frequencies. The capacitance isapproximately 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 reach90% of the steady-state value 1,2.
Figure 5. Responsecharacteristics of the ‘Humicape’ humidity sensors for different frequencies.2C. Impedance-type humidity sensor An impedance-typehumidity sensor is presented in 7.
Figure 6 shows the schematic structure ofan impedance-type humidity sensor. As substrate, an alumina substrate wasselected. On this substrate are the electrodes, for this an interdigital goldstructure (or comb structure) with a thickness of 8 µm to 10 µm was chosen. Thehumidity-sensitive layer was prepared in 7 with different mixing ratios ofGTMAC (glycidyl trimethyl ammonium chloride), PPGDE (polypropylene glycoldiglycidyl) and MTHPA (methyl tetrahydrophthalic anhydride) 7. Figure 6. Schematicstructure of a impedance-type humidity sensor. 7Figure 7 shows theresponse characteristics of the sensor at 25 °C and 1 kHz for a mixed ratio ofGTMAC/PPGDE/MTHPA = 100/0/70.
For the measurements, an AC voltage of 1 V wasapplied between the electrodes. The impedance of the sensor was measured in therange from 30% to 100%. The curve forabsorption and desorption shows a proportional behavior. For the determinationof the hysteresis, two dotted lines in the range of +- 2% RH are Figure 7. Responsecharacteristics of the sensor. Filled circle: absorption curve, non-filledcircle: desorption curve, dotted lines: range of +- 2% RH (at 25 °C and 1 kHz).7shown in figure 2.For the hysteresis of the sensor, this results in a value of <2% RH 7.
Figure 8 shows theresponse time of the sensor. This sensor has a response time of 55 s foradsorption and approximately the same response time for desorption 7. Figure 8. Response time ofthe sensor. 7 IV. ComparisonTable II shows the comparison of the different sensors presented. Theresistive-type humidity sensor shows a linear behavior from 30% to 100%, so thesensor can be used in this humidity range.
In addition, this sensor has aprotective film, which protects the sensor from environmental influences. Thispoint is an advantage for a wide range of applications but a disadvantage forthe response time. The capacitive-type humidity sensor shows an approximatelylinear 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. Theresponse 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 from30% to 90%, so the sensor can be used in this humidity range with a smallhysteresis of <2% RH. The response time of the impedance-type humidity sensoris 55 s.
TABLE II. Comparison of the differentsensor-types Type Humidity range Hysteresis Response time resisitive 30-100 – >100 s capacitive 0-100 – 1 s impedance 30-90 <2% RH 55 s Thetable can be used to select the sensor for the application where the focus ison response time or humidity range. V.
ConclusionIn this paper, three types of humidity sensors were presented.Humidity plays an important role in all areas, so it has to be monitored. Dueto the different areas of application, the humidity range and the response timeplays an important role here. In areas where the sensor is exposed toenvironmental influences and the humidity to be monitored is in the range from30% to 100% the resistive-type humidity sensor can be used. If a fast responsetime is required, then the sensor is not suitable for these areas and the capacitivesensor can be selected.
The capacitive-type humidity sensor is also suitable inareas 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 isfluctuating, then the impedance-type humidity sensor can be used with a lowhysteresis of <2% and a response time of 55 s for adsorption and desorption.References1 H. Farahani, R. Wagiran, and M. N. Hamidon, "HumiditySensors Principle, Mechanism, and Fabrication Technologies: A ComprehensiveReview", Sensors, vol.
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