Development trend of the hottest humidity sensor 0

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The development trend of humidity sensors

in industrial and agricultural production, meteorology, environmental protection, national defense, scientific research, aerospace and other departments, it is often necessary to measure and control the environmental humidity. But among the conventional environmental parameters, humidity is the most difficult to measure accurately. The method of measuring humidity with dry wet bulb hygrometer or hair hygrometer has long been unable to meet the needs of the development of modern science and technology. This is because measuring humidity is much more complex than measuring temperature. Temperature is measured independently, while humidity is affected by other factors (atmospheric pressure, clamping force decreases, temperature). In addition, the standard of humidity is also a problem. Humidity calibration equipment produced abroad is very expensive

in recent years, great progress has been made in the field of humidity sensor research and development at home and abroad. Humidity sensor is developing rapidly from simple humidity sensor to integrated, intelligent and multi parameter detection, which creates favorable conditions for the development of a new generation of humidity/temperature measurement and control system, and also improves the humidity measurement technology to a new level

1 characteristics of humidity sensor

humidity sensor is the simplest humidity sensor. Humidity sensors are mainly divided into resistance type and capacitance type

1.1 humidity sensitive resistor

the characteristic of humidity sensitive resistor is that the substrate is covered with a film made of humidity sensitive material. When the water vapor in the air is adsorbed on the humidity sensitive film, the resistivity and resistance value of the element change. Using this characteristic, humidity can be measured. There are many kinds of humidity sensors, such as metal oxide humidity sensors, silicon humidity sensors, ceramic humidity sensors, etc. The advantage of humidity sensitive resistor is high sensitivity, and the main disadvantage is poor linearity and interchangeability of products

1.2 humidity sensitive capacitance

humidity sensitive capacitance is generally made of polymer film capacitance. Commonly used polymer materials include polystyrene, polyimide, cool acid acetate fiber, etc. When the environmental humidity changes, the dielectric constant of the humidity sensitive capacitance changes, so that its capacitance also changes, and its capacitance change is directly proportional to the relative humidity. The main advantage of humidity sensitive capacitor is smart, and the light utilization of graphene is civil oriented, which can quickly realize productization and bring high cost performance to consumers. It has high utilization sensitivity, good product interchangeability, fast response speed, small humidity hysteresis, easy to manufacture, easy to realize miniaturization and integration, and its accuracy is generally lower than that of humidity sensitive resistor. The main manufacturers of humidity sensitive capacitors abroad include Humirel company, Philips company, Siemens company, etc. Taking the sh1100 humidity sensitive capacitor produced by Humirel company as an example, its measurement range is (1% ~ 99%) RH, and the capacitance at 55% RH is 180pf (typical value). When the relative humidity changes from 0 to 100%, the variation range of capacitance is 163pf ~ 202pf. The temperature coefficient is 0.04pf/℃, the humidity hysteresis is ± 1.5%, and the response time is 5S

in addition to resistive and capacitive humidity sensors, there are electrolyte ion humidity sensors, weight humidity sensors (using the change of the weight of the humidity sensing film to change the oscillation frequency), light intensity humidity sensors, surface acoustic wave humidity sensors, etc. The linearity and anti pollution of the humidity sensor are poor. When detecting the environmental humidity, the humidity sensor should be exposed to the environment to be measured for a long time, which is easy to be polluted and affect its measurement accuracy and long-term stability

2 performance characteristics and product classification of integrated humidity sensors

at present, the main foreign manufacturers and typical products of integrated humidity sensors are Honeywell (hih-3602, hih-3605, hih-3610), Humirel (HM1500, hm1520, HF3223, htf3223), and sensiron (SHT11, SHT15). These products can be divided into the following three types:

2.1 linear voltage output integrated humidity sensor

typical products are hih3605/3610 and hm1500/1520. Its main characteristics are constant voltage power supply, built-in amplification circuit, which can output volt grade voltage signal proportional to relative humidity, fast response, good repeatability and strong anti pollution ability

2.2 linear frequency output integrated humidity sensor

the typical product is HF3223. It adopts a modular structure and belongs to a frequency output integrated humidity sensor. The output frequency is 8750hz (type value) at 55% RH. When the upper humidity changes from 10% to 95%, the output frequency decreases from 9560hz to 8030hz. This kind of sensor has the advantages of good linearity, strong anti-interference ability, easy to be equipped with digital circuit or single chip microcomputer, and low price

2.3 frequency/temperature output integrated humidity sensor

the typical product is htf3223. In addition to the function of HF3223, it also adds a temperature signal output, and uses a negative temperature coefficient (NTC) thermistor as a temperature sensor. When the ambient temperature changes, its resistance value also changes correspondingly and is led out from the NTC end. The temperature value can be measured with a secondary instrument

3 single chip intelligent temperature/temperature sensor

in 2002, sens universal experimental machine iron company took the lead in developing SHT11 and SHT15 intelligent temperature/temperature sensors in the world, with an overall dimension of only 7.6 (mm) × 5(mm) × 2.5 (mm), the volume is similar to that of the match head. Before leaving the factory, each sensor has made a precision standard in the temperature chamber, and the standard coefficient is programmed into the corresponding program and stored in the calibration memory. During the measurement process, the relative humidity can be automatically calibrated. They can not only accurately measure the relative temperature, but also measure the temperature and dew point. The range of measuring relative temperature is 0 ~ 100%, the resolution is 0.03% RH, and the maximum accuracy is ± 2% RH. The measuring temperature range is -40 ℃ ~ +123.8 ℃, and the resolution is 0.01 ℃. The accuracy of dew point measurement is ± 1 ℃. When measuring humidity and temperature, the bits of a/d converter can reach 12 bits and 14 bits respectively. The method of reducing the resolution can improve the measurement rate and reduce the power consumption of the chip. Sht11/15 has good interchangeability, fast response speed and strong anti-interference ability. It does not need external components. It is suitable for various single-chip computers and can be widely used in medical equipment and temperature/humidity regulation systems

the chip contains relative humidity sensor, temperature sensor, amplifier, 14 bit a/d converter, calibration memory (E2PROM), volatile memory (RAM), status register, cyclic redundancy check code (CRC) register, two-wire serial interface, control unit, heater and low voltage detection circuit. The measurement principle is to use two sensors to generate the signals of relative humidity and temperature respectively, then amplify them and send them to the a/d converter for analog-to-digital conversion, calibration and error correction, and finally send the data of relative humidity and temperature to the μ C。 In view of the non-linear relationship between the relative humidity reading value output by sht11/15 and the measured relative humidity, in order to obtain accurate data of relative humidity, we must use μ C carries out nonlinear compensation for the reading value. In addition, when the ambient temperature Ta ≠ +25 ℃, it is also necessary to compensate the temperature of the relative humidity sensor

there is a heater inside the chip. When the heater is powered on at the second position "1" of the status register, the temperature of the sensor will increase by about 5 ℃, and the power supply current will also increase by 8Ma (using +5v power supply). Using the heater can achieve the following three functions: ① by comparing the relative humidity and temperature values measured before and after heating, we can determine whether the sensor works normally; ② Using heater in humid environment can avoid sensor condensation; ③ A heater is also required to measure the dew point

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