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Development of a fast response smart hydrogen temperature-humidity sensor
Sensors and Achutors B, 13-14 (1993) 700-702
700
Development of a fast response smart hydrogen temperature-humidity sensor Cheng-qun
Gui,
Guan-ping
Department of Ikision
Feng
and Yong-gui
Dong
Instrumenb and Mechanism, Tsinghua University, loo084 Beijing (China)
1. Introduction
v= zz+Au
Hydrogen has a high thermal capacity ratio and has been widely used as a cooling material in large turbine generators. In this case, it is very important to measure and control the temperature and humidity of the hydrogen. Up to now, Assmann psychrometers have been used to measure the humidity of hydrogen. This method has some disadvantages: (i) the humidity measuring accuracy is much affected by the pressure and flow velocity of hydrogen which flows out from the sampling pipe; (ii) the response time is so long that too much HZ sample gas is let out into the air, which is dangerous because a mixture of H, and air is explosive when the ratio of hydrogen is between 4% and 74.2%. In this paper, a new kind of fast response smart sensor has been developed to measure the temperature and humidity of hydrogen. First the principles of the sensor are introduced. Then the design and fabrication of the sensor are described. Finally, test results and conclusions are given.
(1)
F=zv-Al we obtain
v=
($ ) +A
2,
(2)
where V, I and 2 are voltage, current intensity and impedance of the electric circuit of this transducer, respectively; F, w and z are force, vibration velocity and mechanical impedance of the mechanical vibration system of this transducer respectively; A is the piezoelectric modulus. The mechanical impedance z of a quartz crystal resonator is mainly caused by the outer damping of it. Outer damping includes air damping, surface friction and support damping. The air damping is most sensitive to the relative humidity of the air in a special resonance frequency range [2]. So the relative humidity of the air and its variation will be determined by air damping and its variation which can be measured by measuring V and AV of the quartz crystal oscillating circuit.
2. Principles 2.1. Quartz crystal resonant dumping humidity sensor First, we analyse the quartz crystal oscillator as a piezoelectric transducer. The equivalent circuit of the piezoelectric transducer is shown in Fig. 1. [l]. Electrical energy is transduced into mechanical energy by the piezoelectric oscillator, then the mechanical energy is transduced into acoustic energy. When no outer force is acting on the quartz crystal, from the fundamental equations of this electric-acoustic transducer 2 11
v-AV
A
L AI-F
Iv
B!.L!I Fig. 1. Equivalent circuit of a piezoelectric
09254005/93/$6.00
transducer.
2.2. p-n junction temperahue sensor The voltage drop on a p-n junction diode which varies with temperature T (K) is given by
where 17 and V, are constants which depend on the material and fabrication process of the p-n junction diode; k=8.63 X 10e5 eV/K is Boltzmann’s constant; q= 1.6X 10-l’ C is the electric charge. Taking a silicon semiconductor p-n junction diode as an example, V,=1.72 V. Assuming that V,=O.65 V, T=300 K and q-3.5, then we obtain -dV, =-2mV/K dT
8 1993 - Elscvier Sequoia. All rights reserved
ml 3. Experiments
A function schematic diagram of a T-H measuring systemis shownin Fig.2. In this system,the temperature sensor circuit and humidity sensor circuit have been made into one thin-&n integrated circuit. The temperature sensor circuit involvesa bridge and an amplifier.The circuit principle diagram is shown in Fig. 3. The humidity sensor includes a quartz crystal oscillator, detector, tilter and amplifier.The circuit is shown in Fig. 4. A structure diagram of the sensor is shown in Fig. 5. The thin-filmintegrated circuit is fixed inside of the housing. The sensitive elements are set in a gas room
Fig. 6. r.h.-VOcharacteristic ewe.
Fig. 7. T-V, characteristic curve.
Fig. 2. Function schematic diagram of a T-H measuring system.
Fig. 3. Electric circuit of temperature sensor.
in front of the housing.The gas room is sealed separately from the circuit. The hydrogen gas flows into the gas room through an admissionport which has an oil and dirt filter. The Hz gas flowsout of the gas room through 8 symmetricalexit holes around the lower part of the gas room. This sensor has been tested by the China National Standard Materials Research Center (NSMRC).The results show that the measuring range and accuracyare: r.h: 5-99%, f 2%; 7’~O-50“C, f 0.5%. Hysteresiserror is less than 1% r.h., and response time is less than 4 s. The input-output characteristic curve of the humidity sensor is shown in Fig. 6. The input-output curve of the temperature sensor is shown in Fig. 7.
4. conclPsIons Fii. 4. Electric circuit of humidity sensor.
Fii. 5. Structure schematic diagram.
Problemsin measuringthe temperature and humidity of H, gas in a hydrogen cooling generator need to be solved urgently. The generally used Assmannpsychrometer is not suitable in this case, because its response time is so long that too much H, gas is released into the air. The fast response smart sensor introduced in this paper is suitable for measuring the temperature and humidity of Hz gas. The humidity sensor is constructed according to the resonant damping humidity sensitive theory. A p-n junction diode is used as the
702
temperature sensor. Advantages such as fast response, low hysteresis and long operation life are achieved.
Acknowledgement This project is supported by the National Natural Science Fund of China.
References
1 ZhangFuxue,Piezoelecrriciry,NationalDefenc.eIndustryPress, Beijing, China, 1984. 2 Gui Chengqun, Feng Guanping and Dong Yonggui, Study on resonant damping humidity measuring method and sensor, 92 Ha&n Chbaa-Japan ScL &wzp. El&rom~c Sensing Techrwl., 1992, p. 592-596.
700
Development of a fast response smart hydrogen temperature-humidity sensor Cheng-qun
Gui,
Guan-ping
Department of Ikision
Feng
and Yong-gui
Dong
Instrumenb and Mechanism, Tsinghua University, loo084 Beijing (China)
1. Introduction
v= zz+Au
Hydrogen has a high thermal capacity ratio and has been widely used as a cooling material in large turbine generators. In this case, it is very important to measure and control the temperature and humidity of the hydrogen. Up to now, Assmann psychrometers have been used to measure the humidity of hydrogen. This method has some disadvantages: (i) the humidity measuring accuracy is much affected by the pressure and flow velocity of hydrogen which flows out from the sampling pipe; (ii) the response time is so long that too much HZ sample gas is let out into the air, which is dangerous because a mixture of H, and air is explosive when the ratio of hydrogen is between 4% and 74.2%. In this paper, a new kind of fast response smart sensor has been developed to measure the temperature and humidity of hydrogen. First the principles of the sensor are introduced. Then the design and fabrication of the sensor are described. Finally, test results and conclusions are given.
(1)
F=zv-Al we obtain
v=
($ ) +A
2,
(2)
where V, I and 2 are voltage, current intensity and impedance of the electric circuit of this transducer, respectively; F, w and z are force, vibration velocity and mechanical impedance of the mechanical vibration system of this transducer respectively; A is the piezoelectric modulus. The mechanical impedance z of a quartz crystal resonator is mainly caused by the outer damping of it. Outer damping includes air damping, surface friction and support damping. The air damping is most sensitive to the relative humidity of the air in a special resonance frequency range [2]. So the relative humidity of the air and its variation will be determined by air damping and its variation which can be measured by measuring V and AV of the quartz crystal oscillating circuit.
2. Principles 2.1. Quartz crystal resonant dumping humidity sensor First, we analyse the quartz crystal oscillator as a piezoelectric transducer. The equivalent circuit of the piezoelectric transducer is shown in Fig. 1. [l]. Electrical energy is transduced into mechanical energy by the piezoelectric oscillator, then the mechanical energy is transduced into acoustic energy. When no outer force is acting on the quartz crystal, from the fundamental equations of this electric-acoustic transducer 2 11
v-AV
A
L AI-F
Iv
B!.L!I Fig. 1. Equivalent circuit of a piezoelectric
09254005/93/$6.00
transducer.
2.2. p-n junction temperahue sensor The voltage drop on a p-n junction diode which varies with temperature T (K) is given by
where 17 and V, are constants which depend on the material and fabrication process of the p-n junction diode; k=8.63 X 10e5 eV/K is Boltzmann’s constant; q= 1.6X 10-l’ C is the electric charge. Taking a silicon semiconductor p-n junction diode as an example, V,=1.72 V. Assuming that V,=O.65 V, T=300 K and q-3.5, then we obtain -dV, =-2mV/K dT
8 1993 - Elscvier Sequoia. All rights reserved
ml 3. Experiments
A function schematic diagram of a T-H measuring systemis shownin Fig.2. In this system,the temperature sensor circuit and humidity sensor circuit have been made into one thin-&n integrated circuit. The temperature sensor circuit involvesa bridge and an amplifier.The circuit principle diagram is shown in Fig. 3. The humidity sensor includes a quartz crystal oscillator, detector, tilter and amplifier.The circuit is shown in Fig. 4. A structure diagram of the sensor is shown in Fig. 5. The thin-filmintegrated circuit is fixed inside of the housing. The sensitive elements are set in a gas room
Fig. 6. r.h.-VOcharacteristic ewe.
Fig. 7. T-V, characteristic curve.
Fig. 2. Function schematic diagram of a T-H measuring system.
Fig. 3. Electric circuit of temperature sensor.
in front of the housing.The gas room is sealed separately from the circuit. The hydrogen gas flows into the gas room through an admissionport which has an oil and dirt filter. The Hz gas flowsout of the gas room through 8 symmetricalexit holes around the lower part of the gas room. This sensor has been tested by the China National Standard Materials Research Center (NSMRC).The results show that the measuring range and accuracyare: r.h: 5-99%, f 2%; 7’~O-50“C, f 0.5%. Hysteresiserror is less than 1% r.h., and response time is less than 4 s. The input-output characteristic curve of the humidity sensor is shown in Fig. 6. The input-output curve of the temperature sensor is shown in Fig. 7.
4. conclPsIons Fii. 4. Electric circuit of humidity sensor.
Fii. 5. Structure schematic diagram.
Problemsin measuringthe temperature and humidity of H, gas in a hydrogen cooling generator need to be solved urgently. The generally used Assmannpsychrometer is not suitable in this case, because its response time is so long that too much H, gas is released into the air. The fast response smart sensor introduced in this paper is suitable for measuring the temperature and humidity of Hz gas. The humidity sensor is constructed according to the resonant damping humidity sensitive theory. A p-n junction diode is used as the
702
temperature sensor. Advantages such as fast response, low hysteresis and long operation life are achieved.
Acknowledgement This project is supported by the National Natural Science Fund of China.
References
1 ZhangFuxue,Piezoelecrriciry,NationalDefenc.eIndustryPress, Beijing, China, 1984. 2 Gui Chengqun, Feng Guanping and Dong Yonggui, Study on resonant damping humidity measuring method and sensor, 92 Ha&n Chbaa-Japan ScL &wzp. El&rom~c Sensing Techrwl., 1992, p. 592-596.