What is Humidity Sensor?

The humidity sensor is one of many sensors, and in this article I will tell you what is a temperature sensor? What the hell is it for? How to choose the temperature sensor?

What is Humidity Sensor?

A humidity sensor, also known as a hygrometer, is a device used to measure and monitor the moisture content or relative humidity of the surrounding air. It is a vital component in various applications where maintaining the appropriate humidity level is crucial, such as indoor environments, industrial processes, agriculture, and weather forecasting.

Figure: Wireless Humidity Sensor Product Map

principle

Humidity includes the humidity of gases and the humidity of solids. The humidity of the gas refers to the content of water vapor in the atmosphere. The measurement method has absolute humidity, that is, the weight of water vapor contained in each cubic meter of gas under standard conditions (0°C, 1 atmosphere), that is, the water vapor density; relative humidity, That is, the ratio of the partial pressure of water vapor actually contained in a certain volume of gas to the maximum partial pressure of water vapor that the gas can contain at the same temperature; or the moisture content, that is, the mass of water vapor contained in per kilogram of dry air. Of these, relative humidity is the most commonly used. The humidity of a solid is the percentage of moisture contained in a substance, that is, the ratio of the mass of moisture contained in a substance to its total mass.

The humidity sensor, which is made of the characteristics that water molecules have a large dipole moment, is easy to adsorb on the surface of a solid and penetrate into the interior of a solid, is called a water molecule affinity type humidity sensor. The wet process changes its own performance to form different types of humidity sensors; the humidity sensor that has nothing to do with the affinity of water molecules is called a non-water molecule affinity sensor. The main measurement principles are: using the heat conduction of humid air and dry air The difference is used to measure the humidity; the microwave is used to propagate in the air containing water vapor, and the water vapor absorbs the microwave to cause a certain energy loss, and the energy lost in transmission is related to the humidity in the ambient air to measure the humidity; the water vapor can absorb Infrared rays of a specific wavelength are used to measure the humidity in the air.

Lithium chloride is a stable ionic inorganic salt that does not decompose, volatilize, or deteriorate in the atmosphere. Its moisture absorption has a certain functional relationship with the relative humidity of the air. With the increase or decrease of the relative air humidity, the moisture absorption of lithium chloride also changes. When the lithium chloride solution absorbs water vapor, the number of conductive ions increases, thus resulting in a decrease in resistance; otherwise, the resistance increases. This measurement method, which converts the relative humidity of the air into its resistance value, is called hygroscopic humidity measurement. The sensor of the lithium chloride resistance hygrometer works according to this principle.

Different types of humidity sensors

1. Water molecule affinity humidity sensor

According to the different materials used, water molecule affinity type humidity sensors are divided into the following four categories

1) Electrolyte type: Taking lithium chloride as an example, a pair of electrodes are made on an insulating substrate and coated with a lithium chloride salt gel film. Lithium chloride is highly deliquescent, and produces ion conduction, and the resistance decreases with the increase of humidity.

2) Ceramic type: Generally, metal oxides are used as raw materials to make a porous ceramic through ceramic technology. It is made by utilizing the resistance value of porous ceramics sensitive to water vapor in the air.

3) Polymer type: a moisture-sensitive element is prepared by coating or attaching a layer of organic polymer moisture-sensitive film on an insulating substrate by a spin coater. Its basic features are wide sources of materials, simple manufacturing process, no heating and cleaning, suitable for mass production, wide application range and strong practicability. In addition, it has excellent performance and can be used for the measurement of a wide range of humidity, with small hysteresis hysteresis, fast response, small temperature coefficient and long service life. In the research field of polymer humidity sensors, polymer electrolytes have been widely concerned by researchers in the past few years due to their special advantages such as easy processing, low price, fast response and high sensitivity. However, its disadvantage is that due to its solubility in water, it will affect its moisture-sensitive properties under high humidity when applied.

4) Single crystal semiconductor type: The material used is mainly silicon single crystal, which is made by semiconductor technology. Made of diode humidity sensitive devices and MOSFET humidity sensitive devices and so on. It is characterized by easy integration with semiconductor circuits.

2. Non-water molecule affinity humidity sensor

The difference between the heat conduction of humid air and dry air can be used to measure the humidity, and the thermistor humidity sensor can be made; when microwave or ultrasonic wave is transmitted in the air containing water vapor, the correlation between the energy of transmission loss and the humidity in the ambient air To measure the humidity, it can be made into a microwave or ultrasonic humidity sensor; to measure the humidity in the air by using water vapor to absorb infrared rays of a specific wavelength, it can be made into an infrared absorption humidity sensor.

According to the different humidity sensitive materials, it can be divided into: semiconductor oxides, polymer materials, porous organic or inorganic materials, etc. Humidity sensor is the simplest humidity sensor, there are two main types: resistive and capacitive:

1) Resistive humidity sensor

The characteristic of the humidity sensitive resistor is that the substrate is covered with a film made of moisture-sensitive material. When the water vapor in the air is adsorbed on the moisture-sensitive film, the resistivity and resistance value of the element will change. Using this characteristic Humidity can be measured. The advantage of the humidity sensitive resistor is high sensitivity, but the disadvantage is poor linearity and product interchangeability. There are many types of humidity sensitive resistors, such as metal oxide humidity sensitive resistors, silicon humidity sensitive resistors, ceramic humidity sensitive resistors, etc. Porous ceramics are the most widely used moisture-sensitive materials.

Capacitive Humidity Sensor

2) Capacitive humidity sensor

Humidity-sensitive capacitors are generally made of polymer film capacitors. Commonly used polymer materials include polystyrene, polyimide, and cellulose acetate. When the ambient humidity changes, the dielectric constant of the humidity-sensitive capacitor changes, so that the capacitance also changes, and the capacitance change is proportional to the relative humidity. The main advantages of humidity sensitive capacitors are high sensitivity, good product interchangeability, fast response, small humidity hysteresis, easy manufacturing, easy miniaturization and integration, and their accuracy is generally lower than that of humidity sensitive resistors.

Capacitive Humidity Sensor

In addition to resistive and capacitive humidity sensors, there are electrolyte ion-type humidity sensors, weight-type humidity sensors (using changes in the weight of the moisture-sensitive film to change the oscillation frequency), quartz oscillator-type humidity sensors, and light-intensity-type humidity sensors. Humidity sensor, surface acoustic wave humidity sensor, etc.

Regardless of the type of humidity sensor, the basic form of its product is to coat a moisture-sensitive material on the substrate to form a moisture-sensitive film. After the water vapor in the air is adsorbed on the moisture-sensitive material, the parameters such as the mass, impedance, and dielectric constant of the element will change, thereby making a humidity-sensitive element. Therefore, the traditional humidity sensor also has the following problems:

1) Poor long-term stability. When detecting the ambient humidity, the humidity sensor has to be exposed to the environment to be tested for a long time, and it is easy to be polluted, which will affect its measurement accuracy and long-term stability.

2) The interchangeability of the sensor is poor, and the selectivity to gas is also poor.

3) The time for calibration and calibration is long and complicated. Because the output signal of such sensors is usually non-linear, which makes calibration difficult.

How to select a humidity sensor

1.Measuring range

Like measuring weight and temperature, the first thing to do when selecting a humidity sensor is to determine the measurement range. Except for meteorological and scientific research departments, those engaged in temperature and humidity measurement and control generally do not need full humidity range (0-100%RH) measurement. In today's information age, sensor technology is closely integrated with computer technology and automatic control technology. The purpose of measurement is control, and the measurement range and control range are collectively referred to as the use range. Of course, for users who do not need to engage in measurement and control systems, it is enough to directly choose a general-purpose humidity meter.

2.measurement accuracy

Like the measurement range, the measurement accuracy is the most important indicator of the sensor. Each increase - percentage points. For the sensor, it is a step up, or even a higher level. Because to achieve different precision, the manufacturing cost varies greatly, and the selling price also varies greatly. For example, an imported cheap humidity sensor is only a few dollars, while a full-humidity humidity sensor for calibration costs hundreds of dollars, a difference of nearly a hundred times. Therefore, users must tailor their clothes, and should not blindly pursue "high, precise, and sharp".

Manufacturers often give the accuracy of their humidity sensors in sections. For example, the medium and low humidity section (0-80%RH) is ±2%RH, while the high-humidity section (80-100%RH) is ±4%RH. And this accuracy is the value at a specified temperature (such as 25°C). Such as the use of humidity sensors at different temperatures. Its indication should also consider the influence of temperature drift. As we all know, relative humidity is a function of temperature, and temperature seriously affects the relative humidity in a given space. For every 0.1°C change in temperature. There will be a humidity change (error) of 0.5%RH. If it is difficult to achieve a constant temperature in the application occasion, it is not appropriate to propose an excessively high humidity measurement accuracy. Because the humidity is also erratic with the change of temperature, extravagant talk about the accuracy of humidity measurement will lose its practical significance. Therefore, the first thing to control the humidity is to control the temperature well, which is why a large number of applications are often temperature and humidity-integrated sensors rather than simply humidity sensors.

In most cases, if there is no precise temperature control means, or the measured space is not sealed, the accuracy of ±5%RH is enough. For local spaces that require precise control of constant temperature and humidity, or where humidity changes need to be tracked and recorded at any time, choose ±3%RH.

Relationship between humidity sensor and traditional humidity measurement method

As early as the 18th century, human beings invented dry wet bulbs and hair hygrometers, and electronic humidity sensors have been developed in recent decades. Especially in the past 20 years, it has developed rapidly. The alternation of new and old things has a lot to do with the change of people's ideas. Since the prices of wet and dry bulbs and hair hygrometers are still significantly lower than humidity sensors, some people do not recognize the price of electronic humidity sensors. Just like when people who are used to using brooms switch to vacuum cleaners, they always feel that it is not worthwhile to spend hundreds of dollars on a vacuum cleaner. It is not as easy to balance as spending a few dollars on a broom.

Because the traditional humidity measurement method is too deep in people's minds, some people have formed only dry wet bulb hygrometer. It is the accurate inherent concept. Some users use the wet and dry bulb hygrometer to compare the newly purchased humidity sensor. If they find that the indications are different, they immediately think that the humidity sensor is not accurate. It should be noted that the accuracy of dry and wet bulbs is only 5% to 7%RH, which is not only lower than that of electronic humidity sensors, but also depends on two thermometers itself; the hygrometer must be in a ventilated state: only when the gauze water jacket, water quality, and wind speed meet certain requirements can the specified accuracy be achieved. Humidity sensor manufacturers must use standard humidity generators to calibrate one by one before the products leave the factory, and the most commonly used shunt type standard humidity generators are used for calibration. Therefore, users are expected to use the same method when they need to calibrate, and avoid using instruments with low accuracy to calibrate or compare sensors with high accuracy.

Precautions

The humidity sensor is not hermetically sealed. In order to protect the accuracy and stability of the measurement, try to avoid using it in an atmosphere containing acidic, alkaline or organic solvents. Also avoid using it in a dusty environment. In order to correctly reflect the humidity of the space to be measured, it should also avoid placing the sensor too close to the wall or in a dead corner where there is no air circulation. If the room to be measured is too large, multiple sensors should be placed.

Some humidity sensors have relatively high requirements on the power supply, otherwise the measurement accuracy will be affected. Or the sensors interfere with each other and don't even work. When in use, a suitable power supply that meets the precision requirements shall be provided according to technical requirements.

When the sensor needs to perform long-distance signal transmission, pay attention to the attenuation of the signal. When the transmission distance exceeds 200m, it is recommended to use a humidity sensor with a frequency output signal.

Due to the dispersibility of humidity sensitive elements, both imported and domestic sensors need to be debugged and calibrated one by one. Most of the humidity sensors need to be re-adjusted and calibrated after replacing the humidity sensor, which is especially important for humidity sensors with relatively high measurement accuracy.

Humidity sensors are now being widely used. Humidity sensors can monitor the humidity in the environment very well. They have important applications in food protection and environmental detection. When using humidity sensors, we should fully understand the structure of humidity sensors. Some notes in .

There are not many forms of humidity sensors, but no matter what kind of humidity sensor is used, we still need to pay attention to the above details. Not only humidity sensors, but all sensors have their precautions during use. We are using When using it, you should first read the instruction manual and consult with the manufacturer about relevant issues in order to use it better.