Humidity Sensor: Classification, Package and Application

Catalog

I Classification

There are mainly two types of humidity sensors: resistive and capacitive.

• Resistive Humidity Sensors

The substrate is covered with a film made of a 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 will change. Using this feature, we can measure the humidity.

There are many types of resistive humidity sensors, such as metal oxide resistive humidity sensors, silicon resistive humidity sensors, ceramic resistive humidity sensors, etc. The main advantages of resistive humidity sensors are high sensitivity, good product interchangeability, fast response speed, small hysteresis of humidity, easy manufacturing, easy to achieve miniaturization and integration, and its accuracy is generally lower than that of humidity sensitive resistors.

Resistive humidity sensors have high sensitivity, but the linearity and product interchangeability are poor.

Figure 1. Schematic View of Resistive Humidity Sensor

• Capacitive Humidity Sensors

Capacitive humidity sensors are generally made of polymer film capacitors, and commonly used polymer materials include polystyrene, polyimide, and acetate. When the environment humidity changes, the dielectric constant of the capacitive humidity sensor changes, and its capacitance also changes. The amount of capacitance change is proportional to the relative humidity.

The resistive humidity sensor has high sensitivity, good product interchangeability, fast response speed, and small hysteresis of humidity, but its accuracy is generally lower than that of resistive humidity sensors. Besides, it's easy for manufacturing, and easy to achieve miniaturization and integration.

Figure 2. Structure of capacitive humidity sensor

Generally, the humidity sensor has poor linearity and anti-pollution properties. When detecting environmental humidity, the humidity sensor must be exposed to the environment to be tested for a long time, which is easily contaminated and affects its measurement accuracy and long-term stability. In this respect, there is no better method of wet and dry bulb measurement. The following is a brief introduction to various humidity sensors.

1. Lithium Chloride Humidity Sensor

(1) Resistive Lithium Chloride Hygrometer

The first lithium chloride humidity sensor based on the principle of resistance-humidity characteristics was developed by F.W. Dunmore of the American Bureau of Standards. This kind of element has high accuracy, simple structure, and low price, and is suitable for measurement and control at normal temperature and humidity.

The measurement range of the lithium chloride element is related to the lithium chloride concentration and other components of the humidity-sensitive layer. The effective humidity sensing range of a single element is generally within 20%RH. For example, the humidity sensing range corresponding to a concentration of 0.05% is approximately (80-100)%RH, and the range corresponding to a concentration of 0.2% is (60-80)%RH.

Therefore, when measuring a wide humidity range, we must use components of different concentrations. The number of components of the hygrometer combination that can be used for full-range measurement is generally 5, and the measurement range of the lithium chloride hygrometer using the component combination method is usually (15～100)%RH. Some foreign products claim that the measurement range can reach ( 2～100)%RH.

(2) Dew Point Type Lithium Chloride Hygrometer

The dew point type lithium chloride hygrometer was first developed by the Forboro company in the United States. This type of hygrometer is similar to the above-mentioned resistive lithium chloride hygrometer, but its working principle is completely different. In short, it works with the characteristic that the water vapor pressure of a lithium chloride saturated water solution

2. Carbon Humidity Sensor

The carbon humidity sensor was first proposed by EKCarver and CWBreasefield in the United States in 1942. Compared with the lithium chloride humidity sensor, the carbon humidity sensor has a fast response, good repeatability. And its advantages of no erosion effect and narrow hysteresis ring are remarkable.

Figure 3. Carbon Humidity Sensor

3. Alumina Hygrometer

The outstanding advantage of the aluminum oxide sensor is its small volume(the humidity sensor used for the radiosonde is only 90μm thick and 12mg weight), high sensitivity (the lower measurement limit is -110℃ dew point), and fast response speed(generally 0.3 s to 3s). Besides, the measurement signal is directly output in the form of electrical parameters, which greatly simplifies the data processing procedures. In addition, it is also suitable for measuring moisture in liquids.

The above characteristics are exactly what is desired in certain measurement fields in industry and meteorology. Therefore, it is considered to be one kind of sensors suitable for high-altitude atmospheric exploration.

However, it is regrettable that although professionals in many countries have made unremitting efforts to improve the performance of sensors, there has never been a major breakthrough in exploring the process conditions for producing products with stable quality and performance.

Therefore, so far, the alumina hygrometer can usually only be used under specific conditions and a limited range.

4. Ceramic Humidity Sensor

In the field of humidity measurement, the measurement of low humidity and high humidity under low and high temperatures is still a weak link so far, among which the humidity measurement technology under high temperatures is the most backward.

In the past, the vented wet and dry bulb hygrometer is almost the only method that can be used under high temperatures, which also has various problems in actual use. However, with the advance in science and technology, there are more and more occasions requiring to measure humidity at high temperatures, such as many industrial processes of humidity measurement and control like cement, metal smelting, food processing, etc...

Therefore, since the 1960s, many countries have begun to develop humidity sensors suitable for measurement under high temperatures. Considering the use conditions of the sensor, people naturally focus on certain inorganic substances that are both water-absorbing and high-temperature resistant.

Ceramic components are not only humidity-sensitive but also can be used as temperature-sensitive and gas-sensitive components. These characteristics make it very likely to become a promising multi-functional sensor in the future.

Figure 4. Optical Micrograph of Ceramic Humidity Sensor

II Selection of Humidity Sensors

1. Measuring range

As with the measurement of weight and temperature, we must first determine the measurement range in the selection of the humidity sensor. Except for meteorological and scientific research departments, temperature and humidity measurement and control generally do not require full humidity (0-100%RH) measurement.

Today, in this information age, sensor technology is closely integrated with computer technology and automatic control technology to control. Of course, for users who do not need to engage in measurement and control systems, a universal hygrometer is sufficient.

2. Measurement Accuracy

Measurement accuracy is also an important indicator of the sensor. To achieve different precisions, their manufacturing costs vary greatly, and their prices vary widely. For example, an imported low-cost humidity sensor is only a few dollars, and a full-humidity-range humidity sensor for calibration is hundreds of dollars. Therefore, users must choose according to actual circumstances.

Manufacturers often give the accuracy of their humidity sensors in sections. For example, the middle and low humidity sections (0-80%RH) are ±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 ℃). Besides, if we use the humidity sensor at different temperatures, we should also consider the effect of temperature drift.

Time Drift&Temperature Drift

Almost all sensors have time drift and temperature drift. Since the humidity sensor must be in contact with moisture in the atmosphere, it cannot be sealed, which means its stability and life is limited. Under normal circumstances, the manufacturer will indicate that the effective time of one calibration is 1 year or 2 years, and is responsible for re-calibration when it expires.

Also, when selecting a humidity sensor, we should see whether the selected sensor can work normally at the specified temperature and whether the temperature drift exceeds the design index. The temperature coefficient α of the capacitive humidity sensor is a variable, which varies with the temperature and humidity range. This is because the permittivity of water and high-molecular polymers changes asynchronously with temperature, and the temperature coefficient α mainly depend on the permittivity of water and humidity-sensitive materials.

The capacitive humidity sensor has the smallest temperature coefficient at normal temperature and mid-humidity. When it is 5-25℃, the temperature drift at mid-low humidity is negligible. However, when it is used in high temperature and high humidity areas or minus-temperature and high-humidity areas, it is necessary to consider the effect of temperature drift and make necessary compensation or correction.

Figure 5. Effect of Chemical Exposure on Humidity Sensor Readings

As we all know, relative humidity is a function of temperature, and temperature greatly affects the relative humidity in a space. The temperature changes by 0.1℃, the humidity will change by 0.5%RH. If it is difficult to achieve a constant temperature, it is not appropriate to require a high humidity measurement accuracy. Because if the humidity changes with the temperature, the accuracy of the humidity measurement will lose its practical significance. Therefore, we must first control the temperature, which is why a large number of applications are often temperature-humidity sensors rather than simply humidity sensors.

In most cases, if there is no precise temperature control method, or the measured space is unsealed, an accuracy of ±5%RH is sufficient. For local spaces that require precise control of constant temperature and humidity, or where it is necessary to track and record changes in humidity at any time, we should choose one over±3%RH accuracy.

III Performance Judgement

When the actual calibration of the humidity sensor is difficult, the performance of the humidity sensor can be judged and checked by some simple methods.

1. Consistency judgment. It is best to purchase more than two humidity sensor products of the same type and the same manufacturer at one time. Connect them together and compare the output values under relatively stable conditions to test the consistency.

For further testing, it can be recorded at intervals of 24 hours. Generally, there are three types of humidity and temperature status一high, medium, and low一within a day. You can observe the consistency and stability of the product more comprehensively, including temperature compensation characteristics.

2. Use the mouth to breathe out or use other humidification methods to humidify the sensor, observe its sensitivity, repeatability, humidification and dehumidification performance, resolution, and the highest range of the product, etc.

3. Test the product in two cases: open box and closed box. Observe whether the thermal effect is consistent.

4. Test the product in the high-temperature state and low-temperature state (according to the instruction standard), and test again when it returns to the normal state. Compare the records to check the consistency and the temperature adaptability of the product.

The performance of the product is ultimately based on the formal and complete testing methods of the quality inspection department. The saturated salt solution is used for calibration, and brand-name products can also be used for comparison and detection. Besides, it's necessary to have a long-term calibration in order to comprehensively judge the quality of the humidity sensor.

IV Packaging Method

Due to the limitation of its working principle, the humidity sensor can not be sealed. Instead, the package is required to leave a contact hole or a contact window linking to the outside, so that the humidity-sensitive part of the humidity sensor and the moisture in the air can well contact with each other.

At the same time, in order to prevent the humidity sensor from being polluted by dust or impurities in the air, some protective measures are needed. At present, the main method is to use a metal dust cover or polymer porous membrane for protection. The following introduces different packaging types of several humidity sensors.

1. Transistor Outline(TO) Package

The schematic diagram of the packaging structure is shown in Figure. At present, it is a relatively common method to package humidity sensors with TO-type packaging technology. There are two kinds of TO packaging technology: metal packaging and plastic packaging.

Figure 6. TO Package

In the metal package:

• First, fixes the humidity sensor to the center of the housing base with epoxy resin ;

• Then connect the Au wire or other metals with a hot welding machine or an ultrasonic welding machine in the welding area of the humidity sensor;

• Finally put the tube cap on the flange around the base, and weld the cap and the edge of the base firmly by resistance welding or circular parallel welding method. A small hole or a small window is opened on the top or side of the metal tube cap, so that the humidity sensor can contact with the outside air.

According to the performance requirements of different humidity sensors, you can consider adding a metal dust cover to extend their service lives

2. Single In-Line Package (SIP)

Single in-line package (SIP) is also commonly used to package humidity sensors. Generally, the humidity sensor only has several output pins, so we can first lead the I/O pins on the substrate to one side, then fix the Ni, Ag or Pb-Sn plating lead on the I/O welding area of the substrate, and immersed it in the molten Pb-Sn groove for reflow soldering to weld the welding spot firmly.

The assembled substrates are coated for protection. The simplest is to impregnate a layer of epoxy resin and then solidify. Finally, protect with plastic packaging, repair burrs, and complete the package.

Figure 7. SIP

The socket of the single in-line package occupies a small area of the substrate, and the insertion is free. The SIP process is simple and easy to use, suitable for many varieties and small batch production, and it is convenient for the replacement and repair of individual leads.

3. Small Outline Package (SOP)

The small outline package(SOP) is another packaging method of the humidity sensor. SOP is developed from the dual in-line package (DIP). It bends the in-line pins of DIP to 90° and makes it become a package suitable for surface mount technology (SMT).

SOP is basically plastic packaging, and its packaging process is:

• First, stick the humidity sensor to the lead frame with a conductive adhesive or epoxy resin.

• Solidify the resin to fix the humidity sensor, and then connect the soldering area on the humidity sensor with the bonding area of the lead frame pin by wire bonding.

• Then put it into a plastic mold for film-plastic packaging. After the mold is removed, remove the burrs, and bend the outer leads of the frame. A small window in contact with the air is opened on the surface of the plastic casing, and an air filter film is attached to block impurities such as dust, thereby protecting the humidity sensor.

Compared with TO and SIP package, SOP package is much smaller and lighter. The humidity sensor in the SOP package has good long-term stability, small drift, low cost, and easy to use, which is suitable for SMT.

Figure 8. Small Outline Package

4. Mixed Package

In many cases, the humidity sensor is not packaged separately but integrated with other sensors such as temperature sensors, wind speed sensors or pressure sensors, and back-end processing circuits to meet certain functional requirements. The packaging process is as follows:

• First, bond the humidity sensor to the substrate with a conductive adhesive or epoxy resin, and solidify the resin to fix the humidity sensor.

• Then, the welding area on the humidity sensor and the bonding area of the substrate are connected by wire bonding.

• Next, cover the case (the covering material can be crystal polymer). A small window in contact with air is opened on the surface of the casing, so that the humidity sensor and the temperature sensor are fully in contact with the air, while the other parts are isolated from the air. An air filter film is attached to the small window to prevent impurities.

5. Other Packaging Method

The external support frame is formed by a polymer compound and cast with a pre-designed mold. The design fully considers the space structure to ensure that the humidity sensor can fully contact the air.

• First, insert the humidity sensor directly into the outer frame along with the slide.

• Insert the outer lead from the other end of the outer frame and connect it to the soldering area of the humidity sensor (it can also be suspended), and then connect the humidity sensor and the outer lead with a conductive adhesive.

• Finally, air filter membranes are attached to both sides of the outer frame. The filter membrane is a porous membrane made of polytef, which allows air to penetrate into the sensor and blocks dust and water drops.

This humidity sensor packaging method is different from the traditional one. It does not use the traditional wire bonding method to connect the outer lead and the humidity sensor, but directly connects the outer lead of the humidity sensor, thereby avoiding the failure caused by inner leads. At the same time, its volume is small, the sensor performance with this packaging method is stable and can work for a long time. However, it has higher requirements for making external frames, and the process is relatively complicated.

V Application

The work of any industry is inseparable from the air, and the humidity of the air is directly related to work, life, and production, making the monitoring and control of humidity more and more important. The application of the humidity sensor is mainly in the following aspects:

1. Climate Monitoring

Weather monitoring and forecast are significant to industrial and agricultural production, military and people's lives, and scientific experiments. Therefore, the humidity sensor is an indispensable humidity measuring device. For example, the resin humidity sensor has been used in the meteorological balloon humidity measuring instrument.

2. Greenhouse Farming

The greenhouse is widely applied in modern agriculture, forestry, and animal husbandry industries. Humidity control in greenhouses is as important as temperature control. Controlling humidity within a suitable range for the growth of crops, trees, livestock, and poultry is good for reducing pests and diseases and increasing production.

Figure 9. Humidity Sensor in Greenhouse

3. Industrial Production  

In the textile, electronics, precision machinery, ceramic industry, and other sectors, air humidity directly affects the quality and output of the products, which must be effectively monitored and regulated.

4. Goods Storage

Different goods have different adaptability to the environment. If the humidity is too high or too low, the goods will lose their original performance. For example, in areas with high humidity, the damage of electronic products in the warehouse is serious. Non-metallic parts will mold and deteriorate, and metal parts will corrode and rust.

5. Protection for Precise Instruments  

Many precise instruments and equipment have high requirements on the working environment, and the environmental humidity must be controlled within a certain range to ensure normal operation and improve work efficiency and reliability. For example, the working humidity of the program-controlled telephone switchboard is preferably 55% ± 10%. If the temperature is too high, the insulation performance will be affected. If it is too low, static electricity will be easily generated, which will affect the normal operation.

VI Matters Needing Attention

1. The humidity sensor is not sealed. In order to protect the accuracy and stability of the measurement, it should not be used in the atmosphere of acid, alkali, and organic solvents and dusty environments. In order to correctly reflect the humidity of one space, the sensor can not be placed in a dead corner that is too close to the wall and poorly ventilated. If the room under test is too large, we should use multiple sensors.

2. Some humidity sensors have relatively high requirements on the power supply, otherwise, it will affect the measurement accuracy or the sensors may interfere with each other and can't work at all. Therefore, we should choose a suitable power supply according to the technical requirements.

3. When the sensor needs to perform long-distance signal transmission, signal attenuation might occur. When the transmission distance exceeds 200m, a humidity sensor with a frequency output signal is recommended.

Because humidity sensors all have a certain degree of dispersion, both imported and domestic sensors need to be calibrated. Most of them need to be re-calibrated after being replaced, especially for humidity sensors with relatively high measurement accuracy.

Humidity sensors are now widely used in food protection and environmental detection, but no matter what the situation is, it is necessary to pay attention to the above details before we use them.

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