In industrial and agricultural production, meteorology, environmental protection, national defense, scientific research, aerospace and other departments, often need a measurement and control of environmental humidity.
But in a regular environment parameter, humidity is difficult to accurately measure a parameter.
With psychrometer or hair moisture meter to measure the humidity method, already cannot satisfy the needs of the development of modern science and technology.
This is because the measuring humidity measuring temperature is much more complex than quantity, temperature is a independent were measured, and the humidity is affected by other factors,
Atmospheric pressure, temperature)
The influence of.
In addition, the standard of humidity is also a problem.
Foreign production of humidity calibration equipment is very expensive.
In recent years, both at home and abroad has made great progress in the field of research and development of humidity sensor.
Moisture sensor is from simple moisture sensor to integration, intelligent, the direction of the rapid development of multi-parameter detection, for the development of a new generation of humidity/temperature measurement and control system has created favorable conditions, and the humidity measurement technology to a new level.
1 the characteristics of moisture sensor moisture sensor is * simple humidity sensor.
Main resistive, capacitive moisture sensor two kinds big.
1 humidity-sensitive resistance humidity sensitive resistor is the feature of coating on the substrate with a sense of wet membrane materials, when the water vapor in the air adsorption on feeling wet film, components of resistivity and resistance value changes, use this feature to measure humidity.
There are many kinds of humidity sensitive resistor, such as metal oxide, humidity sensitive resistor, silicon humidity sensitive resistor, ceramic humidity-sensitive resistance etc.
Humidity sensitive resistor is the advantage of high sensitivity, the main drawback is linearity and poor compatibility of the product.
2 humidity-sensitive capacitance humidity-sensitive capacitance is generally made of polymer film capacitance, commonly used polymer material with polystyrene, polyimide, cool acid cellulose acetate, etc.
When the environment humidity change, humidity-sensitive capacitance of the dielectric constant change, make its capacitance also produces change, the capacitance variation is proportional to the relative humidity.
Main advantage of humidity-sensitive capacitance is high sensitivity, good product compatibility, fast response speed, small amount of humidity hysteresis, easy to manufacture, easy to realize miniaturization and integration, its precision lower average specific humidity sensitive resistor.
Foreign production of humidity-sensitive capacitance of the main manufacturers have Humirel company, Philips, Siemens and so on.
With Humirel company SH1100 humidity-sensitive capacitance type as an example, its measuring range is (
1% ~ 99%)
Electric capacity of RH, when 55% RH for 180 pf (
When the relative humidity varying from 0 to 100%, the change of capacitance range is 163 ~ 202 pf pf.
Temperature coefficient is 0.
04 pf / ℃, humidity hysteresis of plus or minus 1.
5%, the response time of 5 s.
Besides resistive, capacitive moisture sensor, and ionic electrolyte moisture sensor type, weight, moisture sensor (
Use feeling wet film weight change to change the oscillation frequency)
Type, light intensity, moisture sensor, surface acoustic wave (saw) moisture sensor, etc.
Moisture sensor linearity and stain resistance is poor, when testing environment humidity, moisture sensor to long-term exposure to the environment, under test is very easy to be polluted and affect the measurement accuracy and long-term stability.
2 the performance characteristics of integrated humidity sensor and the product classification at present, foreign production of integrated humidity sensor main manufacturers and typical products of Honeywell (respectively
The type 3610)
, Humirel company (
HM1500, HM1520, HF3223, HTF3223 type)
, Sensiron company (
SHT11, SHT15 type)
These products can be divided into three types: 2.
1 linear output voltage integrated humidity sensor products are typical HIH3605/3610, HM1500/1520.
Its main characteristic is USES the constant voltage power supply, a built-in amplifier circuit, to output and the relative humidity is proportional relationship of grade v voltage signal, response speed, good repeatability, strong ability to resist pollution.
2 linear output frequency integration for HF3223 humidity sensor typical products.
It adopts modular structure, belongs to the frequency output type integrated humidity sensor, at 55% RH on the output frequency of 8750 hz (
When on humidity change from 10% to 95%, the output frequency will reduce from 9560 hz to 8030 hz.
This kind of sensor has good linearity, strong anti-interference ability, easy to match the digital circuit or single chip microcomputer, and low price.
3 frequency/temperature output of typical products integrated humidity sensor HTF3223 type.
It besides have HF3223 function, but also increased the temperature signal output terminal, with negative temperature coefficient (
Thermal resistor as temperature sensor
When the environmental temperature change, its resistance value also corresponding change and from the NTC end, deserve to go up secondary instrument can measure temperature.
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