Humidity Sensor

1 measuring range

As with measuring weight and temperature, selecting the humidity sensor first determines the measurement range. In addition to the meteorological and scientific research departments, the measurement of temperature and humidity generally does not require full wet-range (0-100% RH) measurements. In today's information age, sensor technology is closely linked with computer technology and automatic control technology. The purpose of the measurement is to control, and the measurement range and control range are collectively called the range of use. Of course, for those who do not need to implement a measurement and control system, the direct selection of a universal hygrometer is all that is required.

2 measurement accuracy

As with the measurement range, the measurement accuracy is the most important indicator of the sensor. Each increase - a percentage point. For the sensor, it is the last step, even the last one. Because to achieve different accuracy, the manufacturing costs vary greatly, and the price difference is very far. For example, an imported low-cost humidity sensor is only a few dollars, and a full-humidity humidity sensor for calibration is several hundred dollars, a difference of nearly 100 times. Therefore, users must tailor their clothing to avoid blindly pursuing “high, fine, sharp”.

Manufacturers often segment the accuracy of their humidity sensors. For example, the medium and low humidity sections (0-80%RH) are ±2%RH, while the high humidity sections (80-100%RH) are ±4%RH. And this accuracy is a value at a specified temperature (eg, 25°C). Use humidity sensors at different temperatures. The indication also considers the effect of temperature drift. As we all know, relative humidity is a function of temperature, and the temperature affects the relative humidity in a given space. Each temperature changes by 0.1°C. A humidity change (error) of 0.5% RH will be generated. If it is difficult to use a constant temperature in the occasion of use, it is not suitable to propose excessively high humidity measurement accuracy. Because the humidity changes with temperature, the accuracy of measuring humidity will lose its practical significance. Therefore, we must first control the temperature control of humidity, which is a large number of applications are often temperature and humidity - body sensors instead of simply the sake of humidity sensor.

In most cases, the accuracy of ±5% RH is sufficient if there is no accurate temperature control method or if the space under test is unsealed. For ±3%RH for precise control of constant temperature and humidity in the local space, or when it is necessary to track and record changes in humidity at any time

Accuracy humidity sensor above. The corresponding temperature sensor. The accuracy of temperature measurement should be ±0.3°C or more, at least ±0.5°C. Accuracy higher than ±2% RH is not even a standard humidity generator for calibration sensors, let alone the sensor itself. According to the article from the humidity chamber of the National Standards Material Research Center, “The relative humidity measurement instrument is still difficult to achieve 2% RH accuracy even at 20-25°C.”

Principle 3

Humidity sensor is the simplest humidity sensor. Humidity sensors are mainly resistive and capacitive.

Humidity resistance is characterized by covering the substrate with a film made of a moisture sensing material. When the water vapor in the air adsorbs on the moisture sensing film, the resistance and the resistance of the element change, and this characteristic is utilized. Measure humidity.

Humidity-sensitive capacitors are generally made of polymer film capacitors. Commonly used polymer materials include polystyrene, polyimide, and cellulose acetate. When the humidity of the environment changes, the dielectric constant of the humidity sensitive capacitor changes, and its capacitance also changes. The amount of capacitance change is proportional to the relative humidity.

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

Humidity sensor has poor linearity and anti-pollution. When detecting the humidity of the environment, the humidity sensor must be exposed to the environment to be measured for a long period of time. It is easy to be contaminated and affect its measurement accuracy and long-term stability. There is no wet and dry bulb measurement method in this area. The following is a brief introduction to various humidity sensors.

1, lithium chloride humidity sensor

(1) Resistive lithium chloride hygrometer

The first lithium chloride electro-humid sensing element based on the principle of resistance-humidity characteristics was developed by FW Dunmore of the US Bureau of Standards. This kind of component has higher precision, at the same time the structure is simple and inexpensive, and it is suitable for a series of advantages such as measurement and control at room temperature and humidity.

The measurement range of the lithium chloride element is related to the lithium chloride concentration of the moisture sensitive layer and other components. The effective moisture range of individual components is generally within 20% RH. For example, a concentration of 0.05% corresponds to a humidity range of approximately (80 to 100)% RH, and a concentration of 0.2% corresponds to a concentration of (60 to 80)% RH. This shows that when measuring a wide range of humidity, it is necessary to use different concentrations of components together. Hygrometers that can be used for full-scale measurements typically have a combined number of five components. Lithium chloride hygrometers that use a component assembly method typically have a measurable range of (15 to 100)% RH. Some foreign products claim that the measurement range is reachable ( 2 to 100)%RH.

(2) Dew-point lithium chloride hygrometer

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

2, carbon humidity sensor

The carbon humidity sensor was first proposed by EK Carver and CW Breasefield in the United States in 1942. Compared to commonly used sounding elements such as hair, casing, and lithium chloride, the carbon humidity sensor has fast response, good repeatability, and no rush. The advantages of the eclipse effect and the narrow hysteresis loop are impressive. China's meteorological department developed the carbon humidity sensor in the early 1970s and achieved positive results. Its measurement uncertainty is no more than ±5%RH, the time constant is 2~3s at positive temperature, and the delay is generally 7 About %, the specific resistance stability is also better.

3, alumina moisture meter

The outstanding advantage of alumina sensors is that the volume can be very small (for example, 90μm thick, 12mg weight for sonde moisture sensors), high sensitivity (lower limit of measurement -110°C dew point), and fast response (usually 0.3 Between s and 3s), the measurement signal is directly output in the form of electrical parameters, which greatly simplifies the data processing procedure, and so on. In addition, it is also suitable for measuring the moisture in liquids. The above features are exactly what some measurement fields in industry and weather are hoping for. It is therefore considered to be one of several desirable sensors for high-altitude atmospheric exploration. It is precisely because of these characteristics that people have a keen interest in this method. However, it is regrettable that even though professionals in many countries have made unremitting efforts to improve the performance of sensors, they are exploring important process-related issues such as the technological conditions for the production of stable quality products and the improvement of performance stability.

There has been no major breakthrough. Therefore, so far, sensors can only be used in specific conditions and within a limited range. In recent years, this approach has begun to emerge in industry with low frost point measurements.

4, ceramic humidity sensor

In the field of humidity measurement, measurement of low humidity and high humidity and its measurement at low temperature and high temperature is still a weak link so far, and humidity measurement technology under high temperature conditions is the most backward. In the past, the ventilated wet bulb hygrometer was almost the only method that could be used under this temperature condition, and this method has various problems in practical use and is unsatisfactory. On the other hand, the progress of science and technology requires more and more occasions to measure humidity at high temperatures, such as the measurement and control of humidity in many industrial processes involving process conditions and quality control such as cement, metal smelting, and food processing. Therefore, since the 1960s, many countries have begun to research and develop humidity sensors that are suitable for measurements at high temperatures. Taking into account the conditions of use of sensors, it is natural for people to focus their exploration on certain inorganic substances that are both water-absorbent and resistant to high temperatures. Practice has proven that ceramic components not only have moisture sensitive properties, but also serve as temperature sensing components and gas sensing components. These features make it highly likely to be a promising multi-function sensor. Temples such as Temple Day, Fukushima, and Nitta have already taken a very successful step in this regard. In 1980 they developed multi-function sensors called "Wet Porcelain - Type II" and "Wet Porcelain - Type III". The former can measure the temperature and humidity, mainly used for air conditioning, the latter can be used to measure humidity and a variety of organic vapors such as alcohol, mainly for food processing.

The above types are used in several types of sensors, and there are other humidity sensors developed based on different principles, which are not described here.

4 o'clock and temperature drift

Almost all sensors are drifted and temperature drifted. Since the humidity sensor must be in contact with atmospheric moisture, it cannot be sealed. This determines its stability and longevity is limited. Under normal circumstances, the manufacturer will mark the effective use time of 1 calibration as 1 year or 2 years and will be responsible for recalibration. Ask the user to consider the channel to be recalibrated in the future when selecting the sensor. Do not try to be cheap or superstitious and ignore the after-sales service.

The temperature drift has already been mentioned in the previous section. Select the humidity sensor to consider the temperature range of the application, to 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 α of the capacitive humidity sensor is a variable that varies with the use temperature and humidity range. This is because the change of the dielectric constant of water and polymer with temperature is not synchronized, and the temperature coefficient α mainly depends on the dielectric constant of water and moisture sensing material, so the temperature coefficient of the capacitive humidity sensor is not Very few. Capacitive humidity sensor has the lowest temperature coefficient in the normal temperature and middle humidity section. When the temperature is 5-25°C, the temperature drift in the middle and low humidity sections is negligible. However, when used in a high-temperature, high-humidity zone or in a negative temperature and high humidity zone, it is necessary to consider the effects of temperature drift and perform necessary compensation or correction.

Field Sector Temperature (°C) Humidity (%RH)

Textile Spinning Factory 23 60

Weaving factory 18 85

Pharmaceutical and Pharmaceutical Factory 10 to 30 50 to 60

Operating room 23 to 26 50 to 60

Light Industry Printing Factory 23～ 27 49～51

Cigarette factory 21 to 24 55 to 65

Match Factory 18-22 50

Electronic semiconductor 22 30 ~ 45

Computer room 20 to 30 40 to 70

Communication cable charging -10～30 0～20

Food Beer Fermentation 4～8 50～70

Breeding 15 to 40 40 to 75

Artificial shed 5 to 40 40 to 100

Storage fruit frozen -3 ~ 5 80 ~ 90

Underground vegetable cellar -3～ -1 70～ 80

Cultural relics 16 to 18 50 to 55

Note: Range of use in different areas (%RH/°C)

Relationship with traditional methods for measuring humidity

As early as the 18th century, humans invented the wet and dry ball and hair hygrometer, and the electronic humidity sensor was in recent decades. Especially in the past 20 years it has developed rapidly. The alternation of old and new things is very much related to people’s change of ideas. As the prices of wet and dry bulbs and hair hygrometers are still significantly lower than those of humidity sensors, some people have not recognized the price of electronic humidity sensors. Just like people who used to use a broom to switch to vacuum cleaners, they always felt that spending a few hundred yuan to buy a vacuum cleaner was not a good idea. It would be better to spend a few dollars to buy a broom.

Because the traditional method of measuring moisture is too deep in people's minds, some people have formed the inherent concept that only wet and dry hygrometers are accurate. Some users use a wet-bulb hygrometer to compare the newly-acquired humidity sensors. If they find different values, they immediately think that the humidity sensor is not accurate. The accuracy of wet and dry bulbs is only 5% to 7%RH, not only lower than the electronic humidity sensor, but also depends on the accuracy of the two dry thermometers and wet bulbs; the hygrometer must be in a ventilated state: only gauze water jacket, When the water quality and the wind speed meet certain requirements, the required accuracy can be achieved. Humidity sensor manufacturers must use standard humidity generators to calibrate each branch before it leaves the factory. The most commonly used standard humidity generator is used for calibration. Therefore, it is hoped that the user also adopts the same method when the calibration is needed, so as to avoid calibrating or comparing the sensor with a low accuracy.

5 matters needing attention

Humidity sensors are non-hermetic. To protect the accuracy and stability of measurements, they should be avoided in acidic, alkaline, and organic solvent-containing atmospheres. Also avoid using in dusty environments. In order to correctly reflect the humidity of the space to be measured, it is also necessary to avoid placing the sensor too close to the wall or in a corner where air does not flow. If the room being measured is too large, multiple sensors should be placed.

Some humidity sensors require a relatively high power supply, otherwise it will affect the measurement accuracy. Or the sensors can interfere with each other and they can't even work. When using, technical requirements should be provided to provide a suitable power supply that meets the accuracy requirements.

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

Due to the decentralization of moisture sensors, both imported and domestic sensors must be calibrated on a branch-by-station basis. Most need to re-calibrate the calibration after replacing the humidity sensor, especially for the humidity sensor with high measurement accuracy.

Humidity sensors are now being widely used. Humidity sensors can monitor humidity in the environment and have important applications in food protection, environmental testing, etc. We should fully understand that the structure of the humidity sensor is already in use when using the humidity sensor. Some notes in the.

There are not many types of humidity sensors, but no matter what kind of humidity sensors are used, they should pay attention to the above details. Not only all the sensors of humidity sensors have their attention during use, we are using them. You should first read the questions that are related to the manufacturer's consultation before using the manual.

6 development trends

In industries such as industrial and agricultural production, meteorology, environmental protection, national defense, scientific research and aerospace, it is often necessary to measure and control the environmental humidity. However, in conventional environmental parameters, humidity is the most difficult parameter to measure accurately. The method of measuring humidity using a wet and dry hygrometer or hair hygrometer can no longer meet the needs of modern technology development. This is because the measurement humidity is much more complex than the measurement temperature. The temperature is measured independently and the humidity is affected by other factors (atmospheric pressure, temperature). In addition, the humidity standard is also a problem. The humidity calibration equipment produced abroad is very expensive.

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

7 Features

Humidity sensor is the simplest humidity sensor. Humidity sensors are mainly resistive and capacitive.

Humidity resistance

Humidity resistance is characterized by covering the substrate with a film made of a moisture sensing material. When the water vapor in the air adsorbs on the moisture sensing film, the resistance and the resistance of the element change, and this characteristic is utilized. Measure humidity. Humidity-sensitive resistors have many types, such as metal oxide humidity-sensitive resistors, silicon moisture-sensitive resistors, and ceramic moisture-sensitive resistors. Humidity resistance has the advantage of high sensitivity, the main drawback is the poor interchangeability of linearity and product.

Humidity capacitance

Humidity-sensitive capacitors are generally made of polymer film capacitors. Common polymer materials include polystyrene, polyimide, and acid acetate fibers. When the humidity of the environment changes, the dielectric constant of the humidity sensitive capacitor changes, and its capacitance also changes. The amount of capacitance change is proportional to the relative humidity. The main advantages of the humidity sensitive capacitor are high sensitivity, good product interchangeability, fast response speed, low hysteresis of humidity, ease of manufacture, easy implementation of miniaturization and integration, and its accuracy is generally lower than the humidity sensitive resistance. Humirel, Philips, and Siemens are the main manufacturers of moisture-sensitive capacitors in foreign countries. Taking the SH1100 type humidity sensitive capacitor manufactured by Humirel as an example, the measurement range is (1% to 99%) RH, and the capacitance at 55% RH is 180 pF (typical). When the relative humidity changes from 0 to 100%, the capacitance changes from 163 pF to 202 pF. The temperature coefficient is 0.04pF/°C, the humidity hysteresis is ±1.5%, and the response time is 5s.

In addition to resistive and capacitive humidity sensors, there are electrolyte ion-type humidity sensors, weight-type humidity sensors (the use of changes in the weight of the moisture-sensitive film to change the oscillation frequency), light-type humidity sensors, surface acoustic waves Humidity sensor and so on. Humidity sensor has poor linearity and anti-pollution. When detecting the humidity of the environment, the humidity sensor must be exposed to the environment to be measured for a long period of time. It is easy to be contaminated and affect its measurement accuracy and long-term stability.

8 Features

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

Linear voltage output type integrated humidity sensor

Typical products are HIH3605/3610, HM1500/1520. Its main feature is the use of constant voltage power supply, built-in amplifier circuit, can output volt-level voltage signals proportional to relative humidity, fast response, good repeatability, and strong anti-pollution ability.

Linear frequency output integrated humidity sensor

The typical product is HF3223 type. It adopts modular structure and belongs to frequency output type 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 decreases from 9560Hz to 8030Hz. . This kind of sensor has the advantages of good linearity, strong anti-interference ability, easy to match digital circuit or single-chip microcomputer, and low price.

Frequency/temperature output type integrated humidity sensor

The typical product is the HTF3223 type. In addition to the functions of the 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 accordingly and is derived from the NTC terminal, and the secondary instrument can be used to measure the temperature value.

Monolithic intelligent humidity/temperature sensor

In 2002, Sensiron Company took the lead in developing the SHT11 and SHT15 intelligent humidity/temperature sensors in the world. Its external dimensions are only 7.6 (mm) × 5 (mm) × 2.5 (mm), and its volume is similar to the match head. Before leaving the factory, each sensor has been made in the temperature chamber of the precision standard, the standard coefficient is compiled into a corresponding program stored in the calibration memory, the relative humidity can be automatically calibrated during the measurement process. Not only do they measure relative temperatures accurately, they 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 dew point accuracy is <±1°C. The number of A/D converters can be up to 12 bits and 14 bits when measuring humidity and temperature, respectively. Using a method that reduces resolution can increase the measurement rate and reduce the power consumption of the chip. SHT11/15's products have good interchangeability, fast response, and strong anti-interference ability. They do not require external components and can be used in various medical devices and temperature/humidity control systems.

Internal chip contains relative humidity sensor, temperature sensor, amplifier, 14-bit A/D converter, calibration memory (E2PROM), volatile memory (RAM), status register, cyclic redundancy check (CRC) register, two-wire string Row interface, control unit, heater and low voltage detection circuit. The measurement principle is to use two sensors to generate relative humidity and temperature signals, and then 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 is sent to μC. Since the relative humidity reading of the SHT11/15 output is non-linear relative to the measured relative humidity, in order to obtain accurate data on relative humidity, μC must be used for non-linear compensation of the reading. 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. Turning the heater on when the second bit of the status register is set to "1" increases the temperature of the sensor by approximately 5°C and increases the supply current by 8mA (using a +5V supply). The use of heaters can achieve the following three functions: 1 by comparing the measured relative humidity before and after heating and temperature values, can determine whether the sensor is working properly; 2 use a heater in a humid environment, to avoid condensation of the sensor; 3 measuring dew point It also requires the use of a heater.

The dew point is also an important parameter in the humidity measurement and it represents the temperature at which dew condensation initially occurs during vapor cooling. In order to calculate the dew point, Sensirion also provides the user with a program "SHT xdp.bsx" for measuring the dew point. This program can be used to control the internal heater on and off, and then calculate the dew point based on the measured temperature and relative humidity values. When you run this program on the command response screen, 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 result:

When the number "1" is entered, the Celsius temperature dgC=xx.x is measured and displayed;

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

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

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

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

9 Technical Specifications

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% for accuracy. When designing a +3.3V low voltage power supply humidity/temperature test system, SHT11 and SHT15 sensors are available. The sensor operates at 550μA during the measurement phase and has an average operating current of 28μA (12 bits) or 2μA (8 bits). The default is Sleep mode at power-up, and the supply current is only 0.3μA (typ). After the measurement is completed, as long as there is no new command, it automatically returns to the sleep mode, which can minimize the power consumption of the chip. In addition, they also 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 immediately updated, making the chip inoperable, thus playing a protective role.

10 Packaging Method

Due to the limitation of the working principle of the humidity sensor, a non-sealed package must be adopted, that is, a contact hole or a contact window is required to be connected to the outside world of the packaged tube, so that the humidity sensing 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 contaminated by dust or impurities in the air, some protection measures need to be taken. At present, the main means is to use metal dust cover or polymer porous membrane for protection. The following describes the different package types of several humidity sensors.

Transistor Housing (TO) Package

A schematic diagram of the package structure is shown in Figure 1 [1]. At present, packaging a humidity sensor with a TO package technology is a relatively common method. TO type packaging technology has two kinds of metal packaging and plastic packaging. The metal package firstly fixes the moisture sensitive chip in the center of the housing base, and can be cured by epoxy resin; then the Au wire or other metal can be used in the welding area and terminal of the moisture sensitive chip by a heat press welder or an ultrasonic welder. The filaments are connected; finally, the cap is put on the flange around the base, and the cap is welded to the edge of the base by resistance welding or circular parallel welding. The top or side of the metal cap has a small hole or small window so that the moisture sensitive chip can contact the air. According to different moisture sensitive chips and performance requirements, consider adding a metal dust cover to extend the life of the humidity sensor [2] .

2. Single in-line package (SIP) package

Single in-line packages (SIPs) are also commonly used to package humidity sensors. Humidity chip output pin number is generally only a few [1], which can be the substrate on the I / O pin to the side, with Ni plating, Ag plating or Pb-Sn plating "card" lead (base The material is mostly Kovar alloy) stuck in the I/O pad of the substrate, and the card lead is immersed in the molten Pb-Sn bath for reflow and the solder joints are soldered. According to requirements, the pitch of the card lead is 2.54 mm and 1.27 mm. Usually, the leads are connected into strips. After welding, they are cut into individual card leads. It is usually also necessary to apply a protective coating to the substrate on which the components have been assembled. The simplest is to impregnate a layer of epoxy resin and then cure it. Finally plastic protection, repair burr, complete the package [2] .

Single-in-line packaged sockets occupy a small area of ​​the substrate and are easily plugged in. The SIP process is simple, suitable for multi-species, small-volume production, and facilitates lead-by-lead replacement and rework [2] .

3. Small Outline Package (SOP)

The Small Outline Package (SOP) method is another method of packaging a humidity sensor. The SOP evolved from the dip-in-package (DIP) variant, which bends the DIP's straight-in pins outward by 90° to become a surface mount technology (SMT) package. Basically, all SOPs are plastic packages. The encapsulation process is as follows: first, the moisture sensitive chip is bonded to the lead frame with conductive adhesive or epoxy resin, and the resin is cured to fix the moisture sensitive chip, and then the soldering area on the humidity sensitive chip is fixed. The bond area with the lead frame pins is legally connected with the wire bond. Then it is put into a plastic mold for film-plastic packaging. After the mold has been trimmed and trimmed, the plastic burr is removed, and the pins outside the frame are bent and formed. The plastic housing has a small window that is in contact with the air and is affixed with an air filter membrane to block dust and other impurities, thereby protecting the moisture sensitive chip. Compared to the TO and SIP packages, the SOP package has a much smaller form factor and a lighter weight. The humidity sensor of the SOP package has good long-term stability, low drift, low cost, and easy to use. At the same time suitable for SMT, is a relatively good packaging method [2] .

4. Other package forms

The external support frame is formed of a polymer compound and is cast from a pre-designed mold. Its design takes full account of the space structure and ensures that the moisture sensitive chip and the air are fully in contact. The moisture sensitive chip is inserted directly into the outer frame along the slide and then fixed. The outer lead is inserted from the other end of the outer frame, and is connected to the soldering area of ​​the moisture sensitive chip (it can also be left unconnected), and then the moisture sensitive chip and the outer lead are connected by a conductive adhesive thermosetting method. Finally, air filters are applied to the front and back of the outer frame. Filtration Membrane A porous membrane made of polytetrafluoroethylene that allows air to penetrate into the sensor while blocking dust and water droplets [2] .

The humidity sensor package is different from the traditional humidity sensor package, it does not use the traditional wire bonding method to connect the outer lead and moisture sensitive chip, but directly connect the humidity sensitive chip lead, thus avoiding the inner lead The cause of 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, it requires a relatively high degree of production of external frames and the technology is relatively complex [2] .

5. Humidity sensor and other sensors mixed package

Many times, the humidity sensor is not packaged separately, but is integrated with other sensors such as temperature sensors, wind speed sensors or pressure sensors, and back-end processing circuits to meet the corresponding functional requirements. The packaging process is as follows: the humidity sensitive chip is firstly adhered to the substrate with a conductive adhesive or an epoxy resin, and the moisture-sensitive chip is fixed by the resin. Then, the pad on the moisture sensitive chip and the substrate bonding area are legally connected by wire bonds. Then cover the shell (material can choose crystal polymer). The surface of the housing is provided with a small window in contact with the air, so that the moisture-sensitive element and the temperature-sensitive element chip and the air are in full contact, and the other parts are isolated from the air and sealed. The window is affixed with an air filter membrane to prevent contamination of impurities [2] .

Since the LCC package has no pins, the parasitic capacitance and parasitic inductance are small. At the same time it also has excellent electrical properties and thermal properties, small package size, suitable for SMT and other advantages [2] .

11 national standards

edit

GB-T15768-1995 General specification for capacitive humidity sensor and humidity sensor

GBT 11605-2005 humidity measurement method

JJF 1012-1987 commonly used humidity measurement terminology

JJF 1076-2001 humidity sensor calibration specification

JJF 1101-2003 Environmental Test Equipment Temperature and Humidity Calibration Specifications

JJG 205-2005 Mechanical Thermohygrometer Verification Procedure

JJG 499-2004 Precise dewpoint verification procedure

JJG 500-2005 electrolysis hygrometer verification procedures

JJG 826-1993 Secondary Standard Shunt Humidity Generator

JJG 899-1995 Petroleum low moisture analyzer verification method

12 uses

1, the 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 the humidity sensor as an electronic sensitive element/device that measures the relative humidity of the environment.

2, the classification of humidity sensor

Carbon film humidity sensor

Metal oxide ceramic humidity sensor

Electrolyte Humidity Sensor - Lithium Chloride Humidity Resistor

Polymer Humidity Sensors - Polymer Humidity Resistors

Polymer Humidity Sensor - Polymer Humidity Capacitor (Popular)

Infrared humidity sensor

Microwave humidity sensor

Ultrasonic humidity sensor

and many more

13 product brand

Bao Lima, Schneider, Siemens, Mitsubishi, Panasonic, Germany and Germany HLP, Japan God Rong, France Humirel, South Korea Syhitech, United States Honeywell

14 market prospects

The sensor market report of the consulting company INTECHNOCONSULTING shows that in 2008, the global sensor market capacity was 50.6 billion U.S. dollars, and the global sensor market in 2010 is expected to reach more than 60 billion U.S. dollars. The survey shows that Eastern Europe, Asia Pacific, and Canada have become the fastest growing areas in the sensor market, while the United States, Germany, and Japan are still the regions with the largest distribution of sensor markets. Worldwide, the fastest-growing sensor market is still the automotive market, the second is the process control market, optimistic about the communications market.

Some sensor markets such as pressure sensors, temperature sensors, flow sensors, and level sensors have demonstrated the features of a mature market. Flow sensors, pressure sensors, and temperature sensors are the largest in the market, accounting for 21%, 19%, and 14% of the entire sensor market. The major growth in the sensor market comes from emerging sensors such as wireless sensors, MEMS (Micro-Electro-Mechanical Systems) sensors, and biosensors. Among them, the wireless sensor's compound annual growth rate in 2007-2010 is expected to exceed 25%.

At present, the global sensor market shows a rapid growth trend in the ever-changing innovation. Relevant experts pointed out that the major technologies in the field of sensors will be extended and improved on an existing basis. 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 market of sensors, such as the emergence and market share of new sensors such as wireless sensors, fiber optic sensors, smart sensors and metal oxide sensors.

15 Market Analysis

There are many domestic and foreign humidity sensor products on the domestic market. Capacitive humidity sensors are more common. The types of moisture sensing materials are mainly high-molecular 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 brands, and the prices are relatively expensive. Some of the low cost products of capacitive moisture sensors sold on the market often fail to reach the above-mentioned levels, and their linearity, consistency, and repeatability are not ideal. Below 30% RH, 80% RH or more of the wet section is seriously deformed. Some products use SCM compensation correction, so that the humidity "step" jump, so that the accuracy is reduced, there is a lack of consistency, 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 of drift is serious. The capacitance of the humidity sensitive capacitor changes to pF level, the change of 1% RH is less than 0.5pF, and the change of capacitance value changes. It often results in errors of several tens of percent RH. Most capacitive humidity sensors do not have the performance to operate at temperatures above 40°C and tend to fail and fail.

Capacitive humidity sensors are also less resistant to corrosion and often require higher cleanliness of the environment. Some products also have light failure, electrostatic failure, etc. Metal oxides are ceramic moisture sensitive resistors with the same humidity sensitive capacitance. Advantages: However, under the dust environment, ceramic pores are blocked and the components are ineffective. The method of energization and dust removal is often used. However, the effect is not ideal and can not be used in flammable and explosive environments. The alumina wet sensing materials cannot be overcome. The weakness of the surface structure "natural aging", the instability of the impedance, and the humidity resistance of the metal oxide ceramics also have the disadvantage of poor long-term stability. Lithium chloride humidity sensitive resistors have the most outstanding advantage of long-term stability, so through the strict process production, the manufactured instrument and sensor products can achieve high accuracy, and the stability is strong. The product has good linearity. , precision and consistency are reliable guarantees for long-term service life. The long-term stability of lithium chloride humidity sensors does not replace other moisture-sensitive materials.