What Is a Humidity Probe?

Human survival and social activities are closely related to humidity. With the development of modernization, it is difficult to find an area unrelated to humidity. Due to different application fields, the technical requirements for humidity sensors are also different. From a manufacturing point of view, the same humidity sensor has different materials and structures and different processes. Its performance and technical indicators (such as accuracy) are very different, so the price is also very different. For users, when selecting a humidity sensor, they must first figure out what kind of sensor is needed; what grade of product to buy when their own financial resources allow it, and weigh the relationship between "necessity and possibility" so as not to blindly Act. From our interactions with users, I think there are several issues worth noting.

Chinese name: Humidity Sensor
Foreign name: humidity sensor

Measurement Segment: Low-humidity section, medium-humidity section, high-humidity section
Measuring range: 0-100% RH

Humidity sensor selection

Humidity sensor measurement range

As with measuring weight and temperature, selecting a humidity sensor must first determine the measurement range. Except for the meteorological and scientific research departments, the temperature and humidity measurement and control generally do not require 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 to control. The measurement range and the control range are collectively called the use range. Of course, for those who do not need to engage in a measurement and control system, they can simply choose a general-purpose humidity meter.

Humidity sensor measurement accuracy

As with the measurement range, measurement accuracy is also the most important indicator of the sensor. Every increase-a percentage point. For the sensor, it is the last step, even the last grade. To achieve different accuracy, the manufacturing costs vary widely, and the selling prices vary widely. For example, an imported low-cost humidity sensor is only a few dollars, and a full-humidity humidity sensor for calibration is a few hundred dollars, a difference of nearly a hundred times. Therefore, users must be tailored, and they should not blindly pursue "high, precise, and sharp."

Manufacturers often give the accuracy of their humidity sensors in segments. For example, the middle 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 a value at a specified temperature (such as 25 ° C). Such as using humidity sensors at different temperatures. The displayed value 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. Each change in temperature is 0.1 ° C. A humidity change (error) of 0.5% RH will be produced. If it is difficult to achieve constant temperature in the application, it is inappropriate to propose too high a humidity measurement accuracy. Because the humidity changes with temperature, the accuracy of measuring humidity will lose practical significance. Therefore, the humidity control must first control the temperature, which is why a large number of applications are often temperature and humidity-physical sensor rather than simply a humidity sensor.

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

Humidity sensor with above accuracy. Corresponding temperature sensor. The temperature measurement accuracy must be more than ± 0.3 , at least ± 0.5 . And the requirement of accuracy higher than ± 2% RH is probably difficult to achieve even the standard humidity generator for calibrating the sensor, let alone the sensor itself. The article from the Humidity Chamber of the National Standards Research Center states: "Even at 20-25 ° C, it is still difficult to achieve an accuracy of 2% RH."

Humidity sensor principle

^{[1] The} humidity sensor is the simplest humidity sensor. There are two types of humidity-sensitive elements: resistance and capacitance.

Humidity-sensitive capacitors are generally made of polymer film capacitors. Commonly used polymer materials include polystyrene, polyimide, and butyric acetate fiber. When the ambient humidity changes, the dielectric constant of the humidity sensitive capacitor changes, so that its capacitance also changes, and its capacitance change is proportional to the relative humidity.

The accuracy of the electronic humidity sensor can reach 2-3% RH, which is higher than that of wet and dry bulbs.

The humidity sensor has poor linearity and pollution resistance. When detecting the humidity of the environment, the humidity sensor must be exposed to the environment to be tested for a long time, and it is easily contaminated to affect its measurement accuracy and long-term stability. This method is not as good as a wet and dry bulb method. The following briefly introduces various humidity sensors.

1.Lithium chloride humidity sensor

(1) Resistance type lithium chloride hygrometer

The first lithium chloride electric humidity sensor based on the resistance-humidity principle was developed by FWDunmore of the American Bureau of Standards. This kind of element has high precision, simple structure and low price, and is suitable for a series of advantages such as normal temperature and humidity measurement and control.

The measurement range of the lithium chloride element is related to the lithium chloride concentration and other components of the moisture-sensitive layer. The effective humidity sensing range of individual components is generally within 20% RH. For example, the 0.05% concentration corresponds to a humidity range of approximately (80 to 100)% RH, and the 0.2% concentration corresponds to a range of (60 to 80)% RH. It can be seen that when measuring a wide range of humidity, components with different concentrations must be used together. The number of hygrometer combinations that can be used for full-range measurement is generally five. The measurable range of the lithium chloride hygrometer using the component combination method is usually (15 to 100)% RH. Some foreign products claim that their measurement range is up to 2 to 100)% RH.

(2) Dew point lithium chloride hygrometer

The dew-point type lithium chloride hygrometer was first developed by Forboro Corporation in the United States, and a lot of research work has been done in China and many other countries since then. This kind of hygrometer is similar to the above-mentioned resistance type lithium chloride hygrometer, but its working principle is completely different. In short, it works by using the saturated water vapor pressure of a saturated aqueous solution of lithium chloride as a function of temperature.

2.Carbon humidity sensor

Carbon humidity sensor was first proposed by EKCarver and CWBreasefield in the United States in 1942. Compared with commonly used sounding devices such as hair, casing and lithium chloride, carbon humidity sensor has fast response, good repeatability and no impact The advantages of eclipse effect and narrow hysteresis ring are remarkable. China's meteorological department started the development of carbon humidity sensors in the early 1970s and achieved positive results. The measurement uncertainty does not exceed ± 5% RH, the time constant is 2 to 3s at positive temperature, and the hysteresis is generally 7 %, The specific resistance stability is also good.

3.Alumina hygrometer

The outstanding advantages of alumina sensors are that the volume can be very small (for example, the humidity sensor used in radiosondes is only 90 m thick and 12 mg in weight), the sensitivity is high (the lower measurement limit is -110 ° C), and the response speed is fast (typically 0.3 s to 3s), the measurement signal is directly output in the form of electrical parameters, which greatly simplifies the data processing program, and so on. It is also suitable for measuring moisture in liquids. The above characteristics are exactly what some measurement fields in industry and meteorology want. It is therefore considered to be one of several acceptable sensors for high-altitude atmospheric detection. It is precisely because of these characteristics that people have a strong interest in this method. However, unfortunately, despite the tireless efforts of professionals in many countries to improve the performance of the sensor, they are exploring the practical conditions such as the process conditions for producing products with stable quality and improving the stability of performance.

There has never been a major breakthrough in this regard. So far, sensors have generally been used only under specific conditions and limited ranges. In recent years, this method has begun to emerge in the field of low frost point measurement in the industry.

4.Ceramic humidity sensor

In the field of humidity measurement, the measurement of low and high humidity and its low and high temperature conditions is still a weak link so far, and the humidity measurement technology under high temperature conditions is the most backward. In the past, ventilated wet and dry bulb hygrometers are almost the only method that can be used under this temperature condition, and this method also has various problems in practical use, which is not satisfactory. On the other hand, the progress of science and technology requires more and more occasions for measuring humidity at high temperatures, such as cement, metal smelting, food processing, and other industrial processes that involve process conditions and quality control. Therefore, since the 1960s, many countries have begun to develop humidity sensors suitable for measurement under high temperature conditions. Considering the use conditions of the sensor, people naturally look at the direction of exploration on certain inorganic substances that have both water absorption and high temperature resistance. Practice has proven that ceramic elements not only have moisture-sensitive characteristics, but also can be used as temperature-sensing elements and gas-sensing elements. These characteristics make it very likely to be a promising multifunctional sensor. Teri, Fukushima, Nitta and others have taken quite a successful step in this regard. They developed multifunctional sensors called "Wet Porcelain-Type II" and "Wet Porcelain-Type III" in 1980. The former can measure and control temperature and humidity. It is mainly used for air conditioning. The latter can be used to measure humidity and various organic vapors such as alcohol. It is mainly used in food processing.

The above are several types of sensors that are more commonly used. In addition, there are other humidity sensors developed according to different principles, which will not be introduced here one by one.

Humidity sensor time drift and temperature drift

Almost all sensors have time drift and temperature drift. Since the humidity sensor must be in contact with water vapor in the atmosphere, it cannot be sealed. This determines that its stability and life are limited. Under normal circumstances, the manufacturer will indicate that the valid use time of a calibration is 1 year or 2 years, and it is responsible for recalibration when it expires. Users are advised to consider the channels for re-calibration in the future when selecting sensors. Do not be greedy for cheap or superstitious foreign goods and ignore the after-sales service.

Temperature drift was mentioned in the previous section. When selecting a humidity sensor, consider the temperature range of the application, and see if the selected sensor can work normally at the specified temperature, and whether the temperature drift exceeds the design index. The user should be reminded that the temperature coefficient of the capacitive humidity sensor is a variable, and it varies with the use temperature and humidity range. This is because the dielectric coefficients of water and polymers are not synchronized with temperature changes, and the temperature coefficient mainly depends on the dielectric coefficients of water and moisture-sensitive materials. Therefore, the temperature coefficient of the capacitive humidity sensor is Non-constant. Capacitive humidity sensors have the smallest temperature coefficient in the normal temperature and middle humidity sections, and the temperature drift in the low and middle humidity sections is negligible at 5-25 ° C. But when it is used in high temperature and high humidity area or negative temperature and high humidity area, we must consider the influence of temperature drift and make necessary compensation or correction.

Field department temperature () humidity (% RH)

Textile spinning mill 23 60

Weaving factory 18 85

Pharmaceutical and pharmaceutical factory 10 30 50 60

Operating room 23 26 50 60

Light Industry Printing Factory 23 27 49 51

Cigarette factory 21 24 55 65

Match factory 18 22 50

Electronic semiconductor 22 30 45

Computer room 20 30 40 70

Communication cable inflation -10 30 0 20

Food beer fermentation 4 8 50 70

Agricultural Fine Breeding 15 40 40 75

Artificial greenhouse 5 40 40 100

Storage of fruits frozen -3 5 80 90

Underground vegetable cellar-3--1 70-80

Cultural relics preservation 16 18 50 55

Note: Use range in different fields (% RH / )

Relationship between Humidity Sensor and Traditional Method

As early as the 18th century, humans invented wet and dry bulbs and hair hygrometers, and electronic humidity sensors have been in recent decades. Especially in the past 20 years, it has developed rapidly. The alternation of old and new things is closely related to the change of people's ideas. Because the prices of wet and dry bulbs and hair hygrometers are still significantly lower than the humidity sensors, some people do not recognize the price of electronic humidity sensors. It seems that when people who are used to brooms switch to vacuum cleaners, they always feel that spending a few hundred dollars to buy a vacuum cleaner is not worth it. It is better to spend a few dollars to buy a broom.

Because the traditional method of moisture measurement is so impressed in people's minds, some people have formed the inherent concept that only a wet and dry bulb hygrometer is accurate. Some users use dry and wet bulb hygrometers to compare the humidity sensors they just bought. If they find that the displayed values are different, they immediately think that the humidity sensors are inaccurate. Note that the accuracy of wet and dry bulbs is only 5% to 7% RH, which is not only lower than the electronic humidity sensor, but also depends on the accuracy of the two dry bulb and wet bulb thermometers; the hygrometer must be in a ventilated state: When the water quality and wind speed meet certain requirements, the specified accuracy can be achieved. Humidity sensor manufacturers use standard humidity generators to calibrate one by one before the products leave the factory. The most commonly used shunt standard humidity generator is used for calibration. Therefore, it is hoped that users need to adopt the same method when calibration is needed, and avoid using low-accuracy instruments to calibrate or compare high-precision sensors.

Humidity sensor considerations

The humidity sensor is non-hermetic. In order to protect the accuracy and stability of the measurement, it should be avoided in an acidic, alkaline and organic solvent-containing atmosphere. Also avoid use in dusty environments. In order to correctly reflect the humidity of the space to be measured, you should also avoid placing the sensor too close to the wall or in a dead corner where air is not flowing. If the room being tested is too large, multiple sensors should be placed.

Some humidity sensors have high requirements for the power supply, otherwise the measurement accuracy will be affected. Or the sensors may interfere with each other and may not even work. When using, according to technical requirements, provide a suitable power supply that meets the accuracy requirements.

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

Because humidity sensors have a certain degree of dispersion, both imported and domestic sensors need to be debugged and calibrated one by one. Most of them need to be re-calibrated after the humidity sensor is replaced, which is especially important for humidity sensors with 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. We should fully understand the structure of the humidity sensor when using it. Some considerations.

There are not many types of humidity sensors, but no matter what kind of humidity sensor you are using, you still need to pay attention to the above details. Not only all sensors of humidity sensors have their considerations during use. We are using When you should first read the instruction manual and the manufacturer has consulted related issues, in order to use it better.

Humidity sensor development trend

Introduce the characteristics of humidity-sensitive components, focus on the performance characteristics and product classification of integrated humidity sensors, single-chip intelligent humidity / temperature sensors, and finally give the technical indicators of typical products of integrated 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 environmental humidity. But among conventional environmental parameters, humidity is the most difficult parameter to accurately measure. The method of measuring humidity with a wet and dry bulb hygrometer or a hair hygrometer has long been unable to meet the needs of modern technological development. This is because measuring humidity is more complicated than measuring temperature. Temperature is measured independently, but humidity is affected by other factors (atmospheric pressure, temperature). In addition, humidity standards are a challenge. The humidity calibration equipment produced abroad is very expensive.

In recent years, great progress has been made in the field of humidity sensor development at home and abroad. Humidity sensors are rapidly developing from simple moisture-sensitive elements to integrated, intelligent, and multi-parameter detection, creating favorable conditions for the development of a new generation of humidity / temperature measurement and control systems, and also raising the humidity measurement technology to a new level.

Humidity sensor characteristics

The humidity sensor is the simplest humidity sensor. There are two types of humidity-sensitive elements: resistive and capacitive.

Humidity sensor

The characteristic of the humidity sensitive resistor is to cover the substrate with a film made of a moisture-sensitive material. When water vapor in the air is adsorbed on the moisture-sensitive film, the resistivity and resistance of the element change. You can measure humidity. There are many types of moisture-sensitive resistors, such as metal oxide special moisture-sensitive resistors, silicon moisture-sensitive resistors, and ceramic moisture-sensitive resistors. The advantage of the humidity sensor is high sensitivity, the main disadvantage is the poor linearity and poor interchangeability of the product.

Humidity sensor humidity sensitive capacitor

Humidity-sensitive capacitors are generally made of polymer film capacitors. Commonly used polymer materials include polystyrene, polyimide, and acid acetate fiber. When the ambient humidity changes, the dielectric constant of the humidity sensitive capacitor changes, so that its capacitance also changes, and its capacitance change is proportional to the relative humidity. The main advantages of humidity-sensitive capacitors are high sensitivity, good product interchangeability, fast response speed, small amount of hysteresis, easy manufacturing, and easy miniaturization and integration. Its accuracy is generally lower than that of humidity-sensitive resistors. The main manufacturers of humidity-sensitive capacitors abroad include Humirel, Philips, and Siemens. Taking the SH1100 type humidity sensitive capacitor produced by Humirel as an example, the measurement range is (1% to 99%) RH, and the capacitance at 180% is 180pF (typical value). When the relative humidity changes from 0 to 100%, the range of capacitance change is 163pF to 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 in the weight of the moisture-sensitive membrane to change the oscillation frequency), light intensity humidity sensors, and surface acoustic waves. Humidity sensor and so on. The humidity sensor has poor linearity and pollution resistance. When detecting the humidity of the environment, the humidity sensor must be exposed to the environment to be tested for a long time, and it is easily contaminated to affect its measurement accuracy and long-term stability.

Humidity sensor features

At present, the major manufacturers and typical products of integrated humidity sensors abroad are Honeywell (HIH-3602, HIH-3605, HIH-3610), Humirel (HM1500, HM1520, HF3223, HTF3223), Sensiron (SHT11, SHT15 type). These products can be divided into the following three types:

Linear voltage output integrated humidity sensor

Typical products are HIH3605 / 3610 and HM1500 / 1520. Its main features are constant voltage power supply and built-in amplifier circuit, which can output volt-level voltage signals proportional to relative humidity. It has fast response speed, good repeatability and strong anti-pollution ability.

Linear frequency output integrated humidity sensor

The typical product is HF3223. It uses a modular structure and belongs to a frequency output integrated humidity sensor. The output frequency is 8750Hz (type value) at 55% RH. When the relative humidity changes from 10% to 95%, the output frequency is reduced from 9560Hz to 8030Hz. . This sensor has the advantages of good linearity, strong anti-interference ability, easy to match digital circuits or single-chip computers, and low price.

Frequency / temperature output integrated humidity sensor

Typical products are HTF3223. In addition to its HF3223 function, it also adds a temperature signal output terminal, using a negative temperature coefficient (NTC) thermistor as a temperature sensor. When the ambient temperature changes, its resistance value changes accordingly and is led out from the NTC terminal. The temperature value can be measured with a secondary meter.

Single-chip intelligent humidity / temperature sensor

In 2002, Sensiron took the lead in successfully developing the SHT11 and SHT15 intelligent humidity / temperature sensors in the world. Its external dimensions are only 7.6 (mm) x 5 (mm) x 2.5 (mm), and its volume is similar to a match head. Before leaving the factory, each sensor has been precision-calibrated in the temperature chamber, and the standard coefficient is compiled into a corresponding program and stored in the calibration memory. The relative humidity can be automatically calibrated during the measurement process. Not only can they accurately measure relative temperature, they can also measure temperature and dew point. The relative temperature range is 0 to 100%, the resolution is 0.03% RH, and the highest accuracy is ± 2% RH. The measurement temperature range is -40 ° C to + 123.8 ° C, and the resolution is 0.01 ° C. The accuracy of measuring dew point is <± 1 . When measuring humidity and temperature, the number of A / D converter bits can reach 12 and 14 respectively. The method of reducing the resolution can increase the measurement rate and reduce the power consumption of the chip. SHT11 / 15's products have good interchangeability, fast response speed, strong anti-interference ability, do not need external components, adapt to various microcontrollers, and can be widely used in medical equipment and temperature / humidity adjustment 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 string Line interface, control unit, heater and low voltage detection circuit. Its measurement principle is to first use two sensors to generate signals of relative humidity and temperature, and then amplified them and send them to the A / D converter for analog / digital conversion, calibration and error correction. Finally, the relative humidity is transmitted through a two-wire serial interface. And temperature data are sent to C. In view of the non-linear relationship between the relative humidity readings output by SHT11 / 15 and the measured relative humidity, in order to obtain accurate data on relative humidity, C must be used to non-linearly compensate the readings. In addition, when the ambient temperature TA + 25 ° C, the relative humidity sensor needs to be temperature compensated.

There is a heater inside the chip. When the second bit of the status register is set to "1", the heater is powered on, which can increase the temperature of the sensor by about 5 ° C and increase the power supply current by 8 mA (using a + 5V power supply). Use the heater to achieve the following three functions: By comparing the relative humidity and temperature measured before and after heating, you can determine whether the sensor is working properly; using the heater in a humid environment to avoid condensation on the sensor; measuring dew point A heater is also required.

Dew point is also an important parameter in humidity measurement, which indicates the temperature at which condensation occurs initially during water vapor cooling. To calculate the dew point, Sensirion also offers users a program "SHT xdp.bsx" for measuring dew points. This program can be used to control the on and off of the internal heater, and then calculate the dew point based on the measured temperature and relative humidity values. When you run this program on the command response interface, the prompt ">" is displayed on the computer screen. The user first enters the letter "S" from the keyboard, and then enters the corresponding number to obtain the following results:

When you enter the number "1", measure and display the Celsius temperature dgC = xx.x;

When the number "2" is entered, the relative humidity% RH = xx.x is measured and displayed;

When entering the number "3", turn on the heater and increase the temperature of the sensor by 5 ° C;

When the number "4" is entered, the heater is turned off to cool down the sensor;

When the number "5" is entered, the dew point temperature dpC = xx.x is displayed.

Humidity sensor technical indicators

The measurement range of the integrated humidity sensor can generally reach 0 to 100%. However, some manufacturers limit the measurement range to 10% to 95% in order to ensure the accuracy index. When designing a + 3.3V low-voltage power humidity / temperature test system, SHT11 and SHT15 sensors can be selected. The working current of this sensor during the measurement phase is 550 A, and the average working current is 28 A (12 bits) or 2 A (8 bits). When powering on, it defaults to Sleep Mode, and the supply current is only 0.3A (typical). As long as there is no new command after the measurement, it will automatically return to sleep mode, which can minimize chip power consumption. In addition, they have a low voltage detection function. When the power supply voltage is lower than + 2.45V ± 0.1V, the 6th bit of the status register is updated immediately, which makes the chip inoperable, which plays a protective role.

Humidity sensor packaging method

Due to the limitation of its working principle, the humidity sensor must adopt a non-hermetic packaging form, that is, the packaging shell must be left with a contact hole or a contact window that communicates with the outside world, so that the moisture-sensitive part of the humidity-sensitive chip and the moisture in the air can be very good. contact. At the same time, in order to prevent the moisture-sensitive chip from being polluted by dust or impurities in the air, some protection measures need to be taken. At present, the main method is to use metal dust cover or polymer porous membrane for protection. The following describes the different packaging forms of several humidity sensors ^[2] .

1.Transistor case (TO) package

The schematic diagram of the package structure is shown in Figure 1 [1]; at present, it is a relatively common method to package moisture-sensitive components with TO-type packaging technology. There are two types of TO packaging technology: metal packaging and plastic packaging. The metal package first fixes the moisture-sensitive chip at the center of the housing base, and can be cured by epoxy resin.Then, the Au wire or other metal is welded with a thermocompression welding machine or an ultrasonic welding machine in the welding area and the terminal of the moisture-sensitive chip. The tube cap is sleeved on the flange around the base, and the tube cap and the edge of the base are welded firmly by resistance welding or circular parallel welding. A small hole or a small window is opened at the top or side of the metal tube cap, so that the moisture-sensitive chip can contact the air. According to different humidity-sensitive chips and performance requirements, you can consider adding a layer of metal dust cover to extend the service life of the humidity sensor ^[2] .

2.Single Inline Package (SIP) package

Single in-line package (SIP) is also commonly used to package humidity sensors. The number of output pins of a humidity-sensitive chip is generally only a few [1], so I / O pins on the substrate can be led to one side, and "card" leads (base) plated with Ni, Ag, or Pb-Sn The material is mostly Kovar alloy) stuck on the I / O pad of the substrate, and the card lead is immersed in the molten Pb-Sn groove for reflow soldering, and the solder joint is firmly welded. According to the needs, there are two kinds of pitches of the card leads: 2.54 mm and 1.27 mm. Usually the leads are connected into a strip shape, and then cut into a single card lead after welding. Generally, the substrates of the assembled components need to be coated and protected. The simplest is to impregnate a layer of epoxy resin and then cure. Finally, it is protected by plastic packaging, burrs are repaired, and packaging is completed ^[2] .

The socket of the single in-line package occupies a small area of the substrate and is easy to insert and remove. The SIP process is simple and easy to implement, suitable for multiple varieties and small batch production, and easy to replace and repair one lead at a time ^[2] .

3. Small outline package (SOP)

The small outline package (SOP) method is another method of packaging a humidity sensor. SOP is developed from the deformation of the dual in-line package (DIP), which bends the DIP's in-line pins outward to 90 °, and becomes a package suitable for surface-mount technology (SMT). SOP is basically all plastic packaging.The packaging process is as follows: first, the moisture-sensitive chip is bonded to the lead frame with conductive glue or epoxy resin, cured by the resin, the moisture-sensitive chip is fixed, and then the welding area on the moisture-sensitive chip The bonding area with the lead frame pins is legally connected with a lead key. Then put it into a plastic mold for film-plastic encapsulation. After exiting the mold, it will be trimmed to remove plastic burrs and bend the outer pins of the frame. The surface of the plastic casing is opened with a small window in contact with the air, and an air filter film is attached to block impurities such as dust, thereby protecting the humidity-sensitive chip. Compared with the TO and SIP packages, the SOP package is much smaller in size and lighter in weight. The SOP packaged humidity sensor has good long-term stability, small drift, low cost, and easy to use. At the same time, it is suitable for SMT, which is a relatively good packaging method ^[2] .

4. Other package forms

The external support frame is made of polymer compounds and is cast with a pre-designed mold. Its design fully considers the space structure to ensure that the moisture-sensitive chip and air can fully contact. The humidity-sensitive chip is directly inserted into the outer frame along the slideway, and then fixed. Insert the external lead from the other end of the outer frame, and connect it to the pad of the humidity-sensitive chip (it can also be left floating), and then connect the humidity-sensitive chip with the external lead by the conductive adhesive thermosetting method. Finally, both sides of the outer frame are covered with air filter film. The filter membrane is a porous membrane made of polytetrafluoroethylene, which allows air to penetrate into the sensor and blocks dust and water droplets ^[2] .

This kind of humidity sensor package is different from the traditional humidity sensor package.It does not use the traditional wire bonding method to connect the external lead and the humidity-sensitive chip, but directly connects the external lead of the humidity-sensitive chip, thereby avoiding the internal lead. Causes the failure problem. At the same time, its package size is small, the sensor performance is stable, and it can work for a long time. However, its external frame production requirements are relatively high, and the process is relatively complicated ^[2] .

5. Mixed package of humidity sensor and other sensors

Many times, humidity sensors are not packaged separately, but integrated with other sensors such as temperature sensors, wind speed sensors or pressure sensors, as well as back-end processing circuits, in a hybrid package to meet the corresponding functional requirements. The packaging process is as follows: first, the moisture-sensitive chip is bonded to the substrate with conductive adhesive or epoxy resin, and the resin is cured to fix the moisture-sensitive chip. Then, the bonding pads on the humidity-sensitive chip and the substrate bonding pads are legally connected by wire bonds. Then cover the shell (Crystal polymer can be selected as the material). The surface of the shell is provided with a small window in contact with the air, so that the humidity-sensitive element and the temperature-sensitive element chip are in full contact with the air, and the other parts are isolated from the air and sealed for protection. The small window is affixed with an air filter film to prevent contamination by impurities ^[2] .

The LCC package has no pins, so both parasitic capacitance and parasitic inductance are small. At the same time, it also has the advantages of excellent electrical and thermal properties, small package size, suitable for SMT, etc. ^[2] .

National Standard for Humidity Sensors

GB-T15768-1995 General Specification for Capacitive Humidity Sensor and Humidity Sensor

GBT 11605-2005 humidity measurement method

JJF 1012-1987 terminology for commonly used humidity measurement

JJF 1076-2001 Humidity Sensor Calibration Specification

JJF 1101-2003 Temperature and Humidity Calibration Specification for Environmental Testing Equipment

JJG 205-2005 Verification Regulations for Mechanical Thermo-hygrometer

JJG 499-2004 Verification Regulations for Precision Dewpoint Meter

JJG 500-2005 Electrolytic Humidity Meter Verification Regulations

JJG 826-1993 secondary standard split type humidity generator

JJG 899-1995 Verification Method for Low Moisture Analyzer of Petroleum

Installation method of humidity sensor

1.Wall mounting method 1

2.Wall mounting method 2

3.Air duct type installation method 1

Air duct type installation method 1 4.Air duct type installation 2

5.Air duct type installation method 3

6.Three way pipeline installation

7.Three way pipeline installation 2

Humidity sensor performance judgment method

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 determination. It is best to buy two or more humidity sensor products of the same type and the same manufacturer at one time. The more the more, the more the problem is explained. Put them together and compare the test output values. Under relatively stable conditions, observe the consistency of the test. . If further testing is required, it can be recorded at intervals within 24 hours. Generally, there are three types of humidity, temperature, and temperature in one day. The consistency and stability of the product can be observed more comprehensively, including temperature compensation characteristics.

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

3. Test both open and closed cases of the product. Compare whether it is consistent and observe its thermal effect.

4. Test the product at high and low temperatures (according to the instruction manual standard), and return to the normal state to compare the test with the records before the experiment, check the temperature adaptability of the product, and observe the product consistency.

The performance of the product should ultimately be based on the formal and complete testing methods of the quality inspection department. Saturated salt solution is used for calibration, and brand-name products can also be used for comparison and testing. The product should also be calibrated in the long-term use process to judge the quality of the humidity sensor more comprehensively. ^[3]

Humidity sensor use

1.Use of humidity sensor

Humidity sensors are used for humidity measurement. There are many representation methods based on the definition of humidity. This article defines humidity sensors as electronic sensitive components / devices that measure the relative humidity of the environment. ^[4]

2. Classification of humidity sensors

Carbon film humidity sensor

Metal oxide ceramic humidity sensor

Electrolyte humidity sensor-lithium chloride humidity sensor

Polymer humidity sensorPolymer humidity sensor

Polymer Humidity Sensor-Polymer Humidity Capacitor (Popular)

Infrared humidity sensor

Microwave humidity sensor

Ultrasonic humidity sensor

and many more

Humidity Sensor Product Brand

Polyma, Schneider, Siemens, Mitsubishi, Panasonic, HLP Germany, Shenrong Japan, Humirel France, Syhitech Korea, Honeywell USA

Humidity sensor market prospects

Consulting company INTECHNOCONSULTING's sensor market report shows that the global sensor market capacity in 2008 was 50.6 billion US dollars, and the global sensor market is expected to reach more than 60 billion US dollars in 2010. The survey shows that Eastern Europe, Asia Pacific, and Canada have become the fastest growing regions of the sensor market, while the United States, Germany, and Japan are still the regions with the largest sensor market distribution. As far as the world is concerned, the fastest growing sensor market is still the automotive market, and the second is the process control market, which is optimistic about the prospect of the communication market.

Some sensor markets such as pressure sensors, temperature sensors, flow sensors, and level sensors have shown the characteristics of mature markets. The market scales of flow sensors, pressure sensors, and temperature sensors are the largest, accounting for 21%, 19%, and 14% of the entire sensor market, respectively. The main growth of the sensor market comes from emerging sensors such as wireless sensors, MEMS (Micro-Electro-MechanicalSystems) sensors, and biosensors. Among them, the compound annual growth rate of wireless sensors in 2007-2010 is expected to exceed 25%.

At present, the global sensor market is showing a rapid growth trend among constantly changing innovations. Relevant experts pointed out that the main technologies in the sensor field will be extended and improved on the existing basis, and countries will compete to accelerate the development and industrialization of new-generation sensors, and competition will become increasingly fierce. The development of new technologies will redefine the future sensor market, such as the emergence of new sensors such as wireless sensors, fiber optic sensors, smart sensors and metal oxidation sensors, and market share expansion.

Humidity sensor market analysis

There are many domestic and international humidity sensor products on the domestic market. Capacitive humidity sensors are more common. The types of moisture-sensitive materials are mainly polymer polymers, lithium chloride, and metal oxides. Capacitive humidity sensors have the advantages of fast response, small size, good linearity, and stability. Some foreign products also have high-temperature performance. However, the products that achieve the above performance are mostly foreign famous brands, and the prices are more expensive. Some low-cost products of capacitive humidity sensors sold on the market often do not reach the above-mentioned level, and the linearity, consistency, and repeatability are not very satisfactory. Below 30% RH, the humidity-sensing section above 80% RH is seriously deformed. Some products use single-chip microcomputer compensation and correction, which causes the "step" jump of humidity, reduces the accuracy, and has the disadvantages of poor consistency and poor linearity. No matter the high-grade or low-grade capacitive humidity sensor, the long-term stability is not ideal. Most of the long-term use drifts seriously. The capacitance of the humidity-sensitive capacitor changes to pF level, and the change of 1% RH is less than 0.5pF. It often causes errors of tens of RH%. Most capacitive humidity sensors do not have the performance of working at temperatures above 40 ° C, and often fail and damage. ^[5]

Capacitive humidity-sensitive elements are also lacking in corrosion resistance, and often require higher environmental cleanliness. Some products also have failures such as light failure and static failure. Metal oxides are ceramic moisture-sensitive resistors. Advantages, but in the dust environment, the ceramic pores will be blocked and the components will fail. It is often treated by electric dust removal, but the effect is not ideal, and it cannot be used in flammable and explosive environments. Its surface structure has the weakness of "natural aging", the impedance is unstable, and the metal-oxide ceramic moisture resistance also has the weakness of poor long-term stability. Lithium chloride humidity thermistor has the most outstanding advantage is the long-term stability is very strong, so through strict process production, the manufactured instrument and sensor products can achieve higher accuracy, strong stability is the product has good linearity , Precision and consistency, is a reliable guarantee of long-term service life. Long-term stability of lithium chloride moisture-sensitive elements Other moisture-sensitive materials cannot yet be replaced.