Humidity sensor. Electronic Materials II (2004) Principle of ceramic humidity sensor

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1 Humidity sensor Low Humidity High Humidity Principle of ceramic humidity sensor humidity increase Chemisorption (n=1) Physisorption (n>2) proton conduction resistance decrease R(Ω) Brief History 1783 De Saussure: suggest hair hygrometer 1895 Sresneversky: elucidate the principle of hair hygrometer 1912 Whiplle: Modification of the equation RH(%)

2 Humidity sensor: Principle Physisorbed state Chemisorbed state

3 Humidity sensor: Hair Hygrometer The human hair cells will indicate the level of moisture in the air by expanding and contracting. When their air is moist, the hair will expand and lengthen, making the pointer point down. When the air is dry, the hair will contract and shorten, making the pointer point up. When you make your hygrometer observations each day, you should make a mark to indicate where the pointer points. Over time, you'll be able to see the humidity patterns that will help you forecast the weather.

4 Humidity sensor: Hair Hygrometer: Problems? This is a human head hair of Caucasian origin. Caucasian hairs come in the widest variety of colors, can be of fine to medium coarseness and are generally straight or wavy. In addition, the shafts vary from round to oval in cross section. Finally, color pigments are fine- to mediumsized and are evenly distributed throughout the shaft. This is a human head hair of Asian origin. Such hair is generally coarse, straight and circular in cross section. Its diameter is wider than the hair of other racial groups, and the outer layer of the hair, the cuticle, is usually significantly thicker. The medulla, or inner layer of cells, is continuous and wide. In addition, the hair shaft contains pigment particles that are generally larger than those of Caucasian hairs, and often appear to be grouped in patchy areas. Finally, the hair may have a reddish appearance, a product of its pigment. This is a human head hair of Afro-Caribbean origin. Such hairs are generally curly or kinky, and have a flattened cross section. Larger than those of other racial groups, its pigment particles are grouped in clumps of different sizes and shapes and may be so dense that they render the hair opaque. Furthermore, the hair shaft may vary or seem to vary in diameter because of its flattened nature and the way it settles on the microscope slide. The hairs are different each other according to their genetic origins.

5 Humidity sensor: Humidity General <The relationship between absolute and relative humidity> absolute humidity relative humidity = X 100 (%) capacity absolute humidity: the amount of water/unit volume of gas(g/m 3 ) capacity: the amount it is capable of holding at that temperature RH 804 e s D = T P o D: absolute humidity (g/m 3 ) RH: relative humidity (%) T: temperature ( o C) e s : saturated vapor pressure P o : standard pressure

6 Humidity sensor: Heat Index The cooling effect by sweating of human being is retarded by increasing humidity in air. Heat index was defined for alarming the real degree of hotness. The reason why the humidity sensor is so important in HVAC application.

7 Humidity sensor: Chilled Mirror Hygrometer Thermoelectric Cooler cools the polished mirror till the dew point - find the RH value by means of searching the dew point 1. Mirror is illuminated by high intensity light emitting diode(led) 2. Quantity of reflected light from the mirror surface is detected by a photoresistor or optical detector below dew point - moisture condense on the mirror surface - the amount of light received by the sensor drastically decrease by scattering Advantage 1. Most accurate and reliable (has been used for precision laboratory calibrations and standards) Disadvantages 1. Sensitive to the contamination (heterogeneous nucleation) 2. The difficulties of measuring the condensate surface temperature 3. High cost

8 Humidity sensor: Chilled Mirror Hygrometer

9 Humidity sensor: Classification Type Materials Principle Humidity T operation Response time Comment Ceramic MgCr 2 O 4 -TiO 2 TiO 2 -V 2 O 5 ZnCr 2 O 4 -LiZnVO 4 Al 2 O 3 thin film H + conduction by physisorption H + conduction by physisorption H + conduction by physisorption C by physisorption 0-100%RH o C %RH o C 30-90%RH 0-50 o C ppm 25 o C < 10 sec. < 10 sec. < 2 min. < 10 sec. self-cleaning by heating self-cleaning by heating no cleaning (stable OH - radical) small concentration of absolute humidity Polymer Conductive Polymer Hair or cotton ribbon H + conduction by physisorption length change by humidity 30-90%RH 0-50 o C 20-80%RH o C < 2 min min. Humidistat for HVAC application

10 Humidity sensor: Equations r K = 2γM ρrtln(p s /P) r K : Kelvin radius(maximum radius of pore for capillary condensation) γ: surface tension of water (72.75 dyn/cm at 20 o C) M: molecular weight of water ρ: density of water R: gas constant T: temperature P s : water-vapor pressure at saturation P: water-vapor pressure RH P P s /P r K condensation till the larger pore resistance smaller r K easy condensation at the low RH larger r K condensation at the higher Rh

11 Humidity sensor: Control Parameter 1. Linearity - Pore size and distribution (micro, meso, macro,.) 2. Hysteresis - control the amount of chemisorption or physisorption. 3. Time drift of resistance by gradual chemisorption or physisorption - The compensation with each other - Self heating at o C to clean the remnant surface radical. C + weak P R(Ω) Chemisorption R(Ω) Thermal Cleaning C + P Physisorption C + strong P RH(%) time

12 Humidity sensor: Capacitive Type - Detect capacitance increase due to the physisorbed water layer - linear sensitivity in the wide range of humidity - sensing material : porous silicon or porous polymer - lack of long-term stability in a hot and wet atmosphere in the case of polymer M. Matsuguch, T. Kuroiwa, T. Miyagishi, S. Suzuki, T. Ogura and Y. Sakai, Sensors and Actuators B: Chemical, 52(1-2), 53(1998)

13 Humidity sensor: Capacitive Type Example

14 Humidity sensor: Capacitive Type Example

15 Humidity sensor: Capacitive Type Example (Thin Film)

16 Humidity sensor: Resistive-Type Example

17 Humidity sensor: Applications and Examples Application 1. Microwave oven 2. Air conditioner 3. Humidifier & Dehumidifier 4. HVAC 5. Food industry 6. Bio application (incubator) 7. Dryer 8. Green house 9. Circuit protection 10. Weather monitoring Other Candidate Materials 1. TiO 2 2. TiO 2 -SnO 2 3. TiO 2 -SnO 2 -Al 2 O 3 4. Al 2 O 3 5. ZrO 2 -MgO (conduction change by chemisorption) Example: MgCr 2 O 4 -TiO 2 ceramics - average grain size: 1 µm - specific surface area: 0.1 m 2 /g - porosity: 25%; pore size: µm - cleaning by heating at 500 o C for 5 sec. T.Nitta, Chemical sensor Technology, Vol.1, pp (1988)