Pressure Sensor

1 heavy type

Heavy-duty pressure sensors are one of the sensors, but we rarely hear about them. It is commonly used in transportation applications by monitoring pneumatics, light-load hydraulics, brake pressures, oil pressures, transmissions, and Pressure, hydraulics, flow, and fluid levels in key systems such as truck/trailer airlocks to maintain the performance of heavy-duty equipment.
The heavy-duty pressure sensor is a pressure measurement device that has a housing, a metal pressure interface, and a high-level signal output. Many sensors have a round metal or plastic housing and are tubular in appearance with a pressure port at one end and a cable or connector at the other end. Heavy-duty pressure sensors of this type are commonly used in extreme temperature and electromagnetic interference environments. Customers in the industrial and transportation sector use pressure sensors in the control system to measure and monitor the pressure of fluids such as coolants or lubricating oils. At the same time, it can also timely detect pressure spike feedback and find problems such as system blockages, so that solutions can be found instantly.
Heavy-duty pressure sensors are constantly evolving. In order to be able to use more complex control systems for heavy-duty pressure sensors, design engineers must increase sensor accuracy while reducing costs and facilitating practical applications.

Category 2

The basic principle of multi-sensor information fusion technology is just like the human brain's process of comprehensively processing information. Various sensors perform multi-level and multi-space information complementation and optimization and combination processing, and eventually produce a consistent interpretation of the observation environment. In this process, it is necessary to make full use of multi-source data for reasonable dominance and use. The ultimate goal of information fusion is based on separate observation information obtained by each sensor, and more useful information is derived through multi-level and multi-aspect combinations of information. This not only utilizes the advantages of multiple sensors working in tandem with each other, but also comprehensively processes data from other sources to improve the intelligence of the entire sensor system. [1]

Pressure sensors are the most widely used type of sensors. The traditional pressure sensor is mainly composed of a mechanical structure type device, and the pressure is indicated by the deformation of the elastic element. However, this structure has a large size and a heavy weight and cannot provide electrical output. With the development of semiconductor technology, semiconductor pressure sensors have also emerged. Its characteristics are small size, light weight, high accuracy and good temperature characteristics. In particular, with the development of MEMS technology, semiconductor sensors have developed toward miniaturization, and they have low power consumption and high reliability.

Diffused silicon pressure transmitter

The diffusion silicon pressure transmitter is manufactured by encapsulating isolated silicon piezoresistive pressure sensitive elements in a stainless steel housing. It can convert the sensed liquid or gas pressure into a standard electrical signal for external output. The DATA-52 series diffusion silicon pressure transmitter is widely used in on-site measurement and control of industrial processes such as power supply/drainage, thermal power, petroleum, chemical, and metallurgy. .

Diffused Silicon Pressure Transmitter DATA-52 Series

Diffused Silicon Pressure Transmitter DATA-52 Series

Performance:

Measuring medium: liquid or gas (no corrosion on stainless steel housing)

Range: 0-10MPa

Accuracy class: 0.1% FS, 0.5% FS (optional)

Stability performance: ±0.05% FS/year; ±0.1% FS/year

Output signal: RS485, 4~20mA (optional)

Overload capacity: 150% FS

Zero temperature coefficient: ±0.01%FS/°C

Full-scale temperature coefficient: ±0.02% FS/°C

Protection level: IP68

Ambient temperature: -10°C～80°C

Storage temperature: -40°C～85°C

Power supply: 9V ~ 36V DC;

Structural material: Shell: Stainless steel 1Cr18Ni9Ti

Sealing ring: fluorine rubber

Diaphragm: Stainless Steel 316L

Cable: φ7.2mm polyurethane special cable

Semiconductor pressure resistance type

The semiconductor piezo-resistive diffusion pressure sensor forms semiconductor deformation pressure on the surface of the sheet and deforms the sheet by an external force (pressure) to generate a piezo-electric impedance effect, thereby converting the impedance change into an electric signal.

Electrostatic capacity type

A capacitance type pressure sensor forms a capacitance by opposing a fixed pole of a glass and a movable pole of silicon, and converts a change in the capacitance generated by the deformation of the movable pole by an external force (pressure) into an electrical signal. (E8Y's operating principle is electrostatic capacity, other models use semiconductors).

3 can not avoid the error

When selecting the pressure sensor, we must consider his comprehensive accuracy. The accuracy of the pressure sensor is influenced by what kind of factors. In fact, there are many factors that cause sensor error. Here we note four unavoidable errors. This is the sensor's Initial error.

First offset error: Since the vertical offset of the pressure sensor remains constant throughout the entire pressure range, variations in the spread of the diffuser and laser trimming will produce offset errors.

Followed by the sensitivity error: the size of the error is proportional to the pressure. If the sensitivity of the device is higher than the typical value, the sensitivity error will be an increasing function of pressure. If the sensitivity is lower than the typical value, the sensitivity error will be a decreasing function of pressure. The reason for this error is the change in the diffusion process.

The third is the linearity error: This is a factor that has little effect on the initial error of the pressure sensor. The reason for this error is the physical nonlinearity of the silicon, but for the sensor with amplifier, it should also include the nonlinearity of the amplifier. The linear error curve can be a concave curve or a convex curve load cell.

The last is the lag error: In most cases, the lag error of the pressure sensor is completely negligible because of the high mechanical stiffness of the silicon wafer. Generally, only the hysteresis error is considered in the case where the pressure changes greatly.

The four errors of the pressure sensor can not be avoided. We can only choose high-precision production equipment, use high technology to reduce these errors, you can also carry out a certain error calibration at the factory, as much as possible to reduce the error to Meet the needs of customers.

4 wiring method

The wiring of the sensor has always been one of the most inquiries in the customer's procurement process. Many customers do not know how to connect the sensor. In fact, the wiring of various sensors is basically the same. The pressure sensor generally has a two-wire system and a three-wire system. Four-wire system, and there are five-wire system.
Two-wire pressure sensor system is relatively simple, the average customer knows how to wire, a line connecting the positive power supply, the other line is the signal line through the instrument connected to the negative power supply, this is the simplest, three-wire pressure sensor is in two A line was added to the wire system. This wire was directly connected to the negative pole of the power supply, which was a little more troublesome than the two-wire system. The four-wire pressure sensor is definitely two power inputs and the other two are signal outputs. Most of the four-wire system is a voltage output instead of a 4-20mA output. A 4-20mA pressure transmitter is called a two-wire system. Some of the pressure sensor's signal output is not amplified, and the full-scale output is only a few tens of millivolts. Some pressure sensors have an internal amplifier circuit and the full-scale output is 0~2V. As for how to receive the display meter, it depends on how large the range of the meter is. If there is a gear that is compatible with the output signal, it can be directly measured, or the signal adjustment circuit must be added. The five-wire pressure sensor is not much different from the four-wire system, and there are fewer five-wire sensors on the market. [2]

Thread type

There are many kinds of thread pressure sensor, common NPT, PT, G, M, are pipe threads.
NPT is an abbreviation for National (American) Pipe Thread. It belongs to the United States pressure sensor standard 60 degree taper pipe thread and is used in North America. The national standard can be found in GB/T12716-1991.
PT is the abbreviation of Pipe Thread. It is a 55 degree sealed taper thread and belongs to the family of the Wyeth pressure sensor thread. It is mostly used in European and Commonwealth countries. It is commonly used in the water and gas pipe industry and has a taper of 1:16. The national standard can be consulted GB/T7306-2000
G is a 55 degree non-threaded seal pipe thread, a family of Wyeth pressure transducer threads. Marked with G for a cylindrical thread. National standards can be consulted GB/T7307-2001
M is a metric common thread. For example, M20*1.5 means that the diameter is 20mm and the pitch is 1.5. If the customer has no special requirements, the pressure sensor is generally M20*1.5 thread.
In addition, the 1/4, 1/2, and 1/8 marks in the thread are the diameters of the thread size in inches. The in-line person usually refers to the thread size in points, one inch is equal to 8 points, 1/4 inch is 2 points, and so on. G seems to be the collective name for the pipe thread. The division of 55 and 60 degrees is a functional one, commonly known as a pipe circle. The thread is machined from a cylindrical surface.
ZG is commonly known as the tube cone, that is, the thread is processed by a conical surface. The general water pressure connector is like this, the old national standard is marked as Rc.
The metric thread is represented by the pitch. The US-British thread is expressed by the number of thread teeth per inch. This is the biggest difference in the pressure sensor thread. The metric thread is a 60-degree equal-toothed type, and the English-made thread is an isosceles 55-degree type. American thread 60 degrees. The metric thread is in metric units and the United States and British thread is in imperial units.
The pipe thread is mainly used for the connection of the pressure pipe, and the internal and external threads of the pipe thread are closely matched. The pressure sensor pipe thread has two kinds of straight pipe and tapered pipe. The nominal diameter refers to the diameter of the connected pressure pipe. Obviously the diameter of the thread is larger than the nominal diameter. 1/4, 1/2, 1/8 is the nominal diameter of the inch thread, in inches. [3]

5 installation problems

Install correctly

Usually the damage of the high temperature melt pressure sensor is caused by the improper installation position. If the sensor is forcibly installed in an excessively small hole or an irregularly shaped hole, the vibration film of the sensor may be damaged by the impact. , Choosing the right tool to process the mounting holes is beneficial to control the size of the mounting holes. In addition, the proper installation torque is conducive to the formation of a good seal, but if the installation torque is too high, it will easily lead to the slippage of the high-temperature melt pressure sensor, in order to prevent this This phenomenon occurs, and the anti-detachment compound is usually applied to the threaded portion of the sensor prior to installation.

1. Correct installation method of pressure sensor:

(1) Verify the frequency response of the pressure sensor under normal atmospheric pressure and standard temperature conditions with appropriate instruments.

(2) Verify the correctness of the pressure sensor code and the corresponding frequency response signal.

2. Determine the specific installation location

In order to determine the number and the specific installation location of the pressure sensor, it is necessary to consider each inflation section of the inflatable net.

(1) The pressure sensor must be installed along the cable, preferably at the cable gland.

(2) Each cable shall be equipped with not less than 4 pressure sensors, close to the two pressure sensors of the telephone exchange, and should not be separated by 200m.

(3) Install one at the beginning and at the end of each cable.

(4) One branch point should be installed for each cable. If two branch points are close to each other (less than 100 m), only one can be installed.

(5) One cable change method (heading, underground) should be installed

(6) For unbranched cables, because the cable pattern of the base line is consistent, the installation distance of the pressure sensor is not to be 500m, and the total number is not less than 4.

(7) In order to determine the point of failure of the pressure sensor, in addition to installing the pressure sensor at the starting point, 150 to 200m from the starting point, but also to install an additional one of course in the design, must consider the economic and technical factors, do not need to install Where pressure sensors are located, they should not be installed.

Inspection size

If the size of the mounting hole is not suitable, the high-temperature melt pressure sensor will be easily worn during the installation process. This will not only affect the sealing performance of the equipment, but also make the sensor unable to fully function, and may even cause potential safety hazards. . Only suitable mounting holes can avoid thread wear (thread industry standard 1/2-20 UNF2B). Mounting holes can often be used to test the mounting holes to make appropriate adjustments.

6 common terms

Pressure is one of the important parameters in industrial production. In order to ensure the normal operation of production, pressure must be monitored and controlled. The following are the common terms used when selecting pressure sensors:

Standard pressure

The pressure represented by the atmospheric pressure is greater than the atmospheric pressure, and the pressure less than the atmospheric pressure is called the negative pressure.

Absolute pressure

The absolute vacuum is the standard pressure.

Relative pressure

The size of the pressure for comparing objects (standard pressure).

Atmospheric pressure

Refers to atmospheric pressure. The standard atmospheric pressure (1 atm) corresponds to the pressure of a 760 mm mercury column.

vacuum

Refers to sub-atmospheric pressure conditions. 1 Torr = 1/760 atmosphere (atm).

Detection pressure range

Refers to the sensor pressure range.

Can withstand pressure

When the pressure is restored to the detection pressure, the pressure does not drop with its performance.

Round trip accuracy

When the pressure is increased or decreased at a constant temperature (23° C.), the pressure fluctuation value of the operating point obtained by outputting the reversed pressure value is removed at the full scale value of the detected pressure.

Accuracy

At a certain temperature (23°C), when the zero pressure and the rated pressure are added, the value deviating from the value of the output current specified value (4mA, 20mA) is removed with the full scale value. The unit is represented by %FS.

Linear

The analog output changes linearly with the detection pressure, but it deviates from the ideal straight line. The value that expresses this deviation as a percentage of the full scale value is called linear.

Hysteresis (linear)

Use a zero voltage and a rated voltage to draw an ideal straight line between the output current (or voltage) values, find the difference between the current (or voltage) value and the ideal current (or voltage) value as an error, and then determine the pressure rise and fall The value of the error. The value obtained by removing the maximum value of the absolute value of the above difference with the full-scale current (or voltage) value is the hysteresis. The unit is represented by %FS.

Hysteresis

The value obtained by subtracting the difference between the pressure at the output ON point and the pressure at the OFF point is a hysteresis with the full scale value of the pressure.

Non-corrosive gas

Refers to substances (nitrogen, carbon dioxide, etc.) and inert gases (argon, helium, etc.) contained in the air.

7 application areas

Pressure sensors are mainly used in: pressurized cylinders, superchargers, gas-liquid pressurized cylinders, gas-liquid superchargers, presses, compressors, air-conditioning refrigeration equipment and other fields.

1, applied to the hydraulic system

The pressure sensor is mainly used in the hydraulic system to complete closed loop control of force. When the control spool is suddenly moved, spike pressures that are several times the system operating pressure can be generated in a very short time. In typical walking machinery and industrial hydraulics, any pressure sensor can be quickly destroyed if it is not designed to take into account such extreme conditions. Need to use the impact of the pressure sensor, pressure sensor to achieve two main methods of shock resistance, one is to change the strain chip, the other is an external coil, the first method is generally used in the hydraulic system, mainly because Easy to install. Another reason is that the pressure sensor is also subject to uninterrupted pressure pulsation from the hydraulic pump.

2. Applied to safety control system

Pressure sensors are often used in safety control systems. The main area of ​​focus is the air compressor's own safety management system. There are many sensor applications in the field of safety control. As a very common sensor, pressure sensors are not surprisingly used in safety control systems.
The application in the field of safety control is generally considered from the aspect of performance. Considering from the consideration of price and the convenience of practical operation, it is proved that the effect of selecting a pressure sensor is very good. Pressure sensors use mechanical processing techniques to mount components and signal conditioners on a small chip. Therefore, the small size is also one of its advantages. In addition, the cheap price is another great advantage. To a certain extent it can improve the accuracy of system testing. In the safety control system, the pressure from the compressor is controlled to a certain extent by installing a pressure sensor in the piping of the air outlet. This is a certain protection measure and a very effective control system. When the compressor is normally started, if the pressure value does not reach the upper limit, the controller will open the air intake through the adjustment to make the device reach the maximum power.

3, applied to injection mold

Pressure sensors play an important role in injection molding. The pressure sensor can be installed in the injection molding machine nozzle, hot runner system, cold runner system and mold cavity, it can measure the plastic injection molding machine, filling mold, pressure and cooling process from the injection molding machine nozzle to Plastic pressure somewhere between the mold cavities.
4, used to monitor mine pressure

Sensor technology is one of the key technologies for mine pressure monitoring. On the one hand, we should correctly apply existing sensors to serve the mining industry; on the other hand, as sensor manufacturers, we must also develop and develop new pressure sensors to suit more mining applications. There are many kinds of pressure sensors, and based on the special circumstances of mine pressure monitoring, mining pressure sensors mainly include: vibrating wire pressure sensors, semiconductor piezoresistive pressure sensors, metal strain gauge pressure sensors, differential transformer pressure sensors and so on. These sensors have a wide range of applications in the mineral industry. Which sensors are used depends on the specific mining environment.

5, applied to promote sleep

Pressure sensor itself can not promote sleep, we just put the pressure sensor on the ground floor of the mattress, because the pressure sensor has high sensitivity, when people turn over, heartbeat and breathing and other related actions, the sensor will analyze this series of information to infer sleep What kind of state is the person sleeping in, and then through the analysis of the sensor, collecting the signal of the sensor to get the sleep data such as heartbeat and breathing rhythm, and finally composing all the data processing into a piece of track, of course, can compress your one night's sleep into one The first few minutes of music.

6, used in compressors, air conditioning cold equipment

Pressure sensors are commonly used in air presses and air conditioning and refrigeration equipment. These sensors are compact, easy to install, and typically have a special valve needle design for the pressure port.

8 performance parameters

There are many kinds of pressure sensors, and their performance is also quite different. How to choose a more suitable sensor to achieve economical and reasonable use.

Rated pressure range

The rated pressure range is a pressure range that satisfies the standard value. That is, between the highest and lowest temperatures, the sensor outputs a pressure range that meets the specified operating characteristics. In practice, the pressure measured by the sensor is within this range.

2. The maximum pressure range

The maximum pressure range is the maximum pressure that the sensor can withstand for long periods of time without causing a permanent change in the output characteristics. In particular, semiconductor pressure sensors generally reduce the rated pressure range significantly in order to improve the linearity and temperature characteristics. Therefore, it will not be damaged even if it is used continuously above the rated pressure. The general maximum pressure is 2-3 times the highest rated pressure.

3. Damage pressure

Damage pressure is the maximum pressure that can be applied to the sensor without damaging the sensor element or the sensor housing.

4. Linearity

Linearity is the maximum deviation of the linear relationship between sensor output and pressure over the operating pressure range.

5. Pressure Hysteresis

The difference between the sensor output when approaching a certain pressure from the minimum working pressure and the maximum working pressure at room temperature and within the working pressure range.

6. Temperature range

The pressure sensor temperature range is divided into compensation temperature range and operating temperature range. The compensation temperature range is due to the temperature compensation applied and the accuracy within the temperature range of the rated range. The operating temperature range is a temperature range that ensures that the pressure sensor can operate normally.

Technical parameters (range 15MPa-200MPa)

Parameter Unit Specification Parameter Unit Specification

Sensitivity mV/V 1.0±0.05 Sensitivity temperature coefficient ≤%F·S/10°C ±0.03

Non-linearity ≤%F·S ±0.02～±0.03 Working temperature range °C -20°C～+80°C

Hysteresis ≤%F·S ±0.02～±0.03 Input resistance Ω 400±10Ω

Repeatability ≤%F·S ±0.02～±0.03 Output resistance Ω 350±5Ω

Creep ≤%F·S/30min ±0.02 Safe overload ≤%F·S 150% F·S

Zero output ≤%F·S ±2 Insulation resistance MΩ ≥5000MΩ(50VDC)

Zero temperature coefficient ≤%F·S/10°C ±0.03 Recommended excitation voltage V 10V-15V

Scope of application

It is used to monitor various invasive blood pressures such as arterial pressure, central venous pressure, pulmonary artery pressure, left coronary pressure, and directly obtain the physiological parameter of blood pressure, which provides an objective basis for clinical diagnosis, treatment and prognosis estimation of the disease. .

Structural specifications

The medical grade polycarbonate and polyvinyl chloride are used as the main body of the sensor and the material of the pressure connection tube.

Package specifications are CH-DPT-248, CH-DPT-248II, CH-DPT-248III.

Installer

1) Turn on the monitor before connecting the pressure sensor system.

2) Use sterilization measures to open the package and confirm that all the interface safety seals and accessories such as the three-way valve work well.

Note: Do not tighten too tightly when connecting the connector.

Conventional/medical pressure sensor FOP-M

3) All the ports of the plug valve should be covered with a hole protective cap until the sensor system is filled with heparin physiological saline solution and bubbles are exhausted before it is replaced with a non-porous protective cap.

4) Connect the pressure sensor to the monitor and zero the monitor according to the monitor instructions.

Note: a) If it is impossible to zero, please replace the sensor and re-zero; if the zero adjustment is unsuccessful, please check whether the cable connection, monitor, etc. are normal. b) When installing the DPT-248II, CH-DPT-248III sensors, use color coding to identify the type of blood pressure: red --- arterial pressure; blue --- central venous pressure; yellow --- pulmonary artery pressure; green -- - left coronary artery pressure; white --- other.

5) Rinse the tubing with heparin saline, side by side despite the air in the road.

Note: There must be no air bubbles in the piping.

6) After all lines are filled with heparin saline, connect the sensor system to the human body.

9 Principle Applications

species

There are a wide variety of mechanical sensors, such as resistance strain gauge pressure sensors, semiconductor strain gauge pressure sensors, piezoresistive pressure sensors, inductive pressure sensors, capacitive pressure sensors, resonant pressure sensors, and capacitive acceleration sensors. But the most widely used is the piezoresistive pressure sensor, which has a very low price, high accuracy and good linearity.

understanding

In understanding the piezoresistive force sensor, we first understand the resistance strain gauge such elements. The resistance strain gauge is a sensitive device that converts the strain change on the device under test into an electrical signal. It is one of the main components of a piezoresistive strain sensor. The most widely used resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. Metal strain gauges are also available in wire strain gages and metal foil gages. Usually, the strain gauges are tightly bonded to a mechanical strain matrix through a special adhesive, and when the stress is changed in the base body, the strain gauges are also deformed together, so that the resistance of the strain gauges is changed, and thus The voltage applied to the resistor changes. These strain gauges usually have a small change in resistance when they are subjected to stress. In general, these strain gauges form a strain bridge and are amplified by a subsequent instrumentation amplifier and transmitted to a processing circuit (usually A/D conversion). And CPU) display or actuator.

10 operation methods

When selecting a pressure sensor, it is necessary to pay attention to many problems. For example, the range, accuracy of the pressure sensor, temperature characteristics of the pressure sensor, and chemical properties are all to be considered. The operation mode of the pressure sensor is also an important issue to be considered.
For example, sensors are used when measuring gas pressure and measuring fluid pressure. Gas is a compressible fluid. When it is increased, it will store a certain amount of compression energy, and when it is decompressed, it will be released by kinetic energy to apply a shock wave to the elastic membrane of the sensor. The pressure sensor is required to have a large overload capacity. The liquid is an incompressible fluid. When the pressure sensor is installed, tightening the screw and no compressible space can increase the liquid pressure beyond the pressure resistance limit of the elastic membrane, resulting in rupture of the elastic membrane. Because this kind of situation occurs repeatedly, also require the pressure sensor to have the greater overpressure ability. When the working environment of the pressure sensor is harsh, for example, there are large vibrations, shocks, and large electromagnetic interference, and more stringent requirements are imposed on the sensor. Not only does it have strong overpressure, but it also requires reliable mechanical seals, anti-looseness, and correct sensor installation. The leads, pins, and outer leads of the sensor should be electromagnetically shielded and the shield should be well grounded. In addition, the compatibility of the pressure sensor with the measured fluid medium should be considered. For example, the elastic membrane structure of the sensor should be separated from the corrosive medium. At this time, a stainless steel bellows sensor is used, and the sensor uses silicon oil as a pressure transmitting medium. When the sensor detects flammable or explosive medium pressure, use a small excitation current to prevent sparks and sparks from breaking when the elastic film breaks, and increase the pressure resistance of the pressure sensor jacket.
Only knowing the pressure sensor's operation method can better select the pressure sensor, especially the pressure sensor is developing rapidly at present, so it is very necessary to understand the operation mode of the pressure sensor.

Key role

Pressure sensors are not only widely used in production and measurement, but they are also often seen in our lives. In most of our vehicles there are pressure sensors. Most people may know that there are pressure sensors in cars. In fact, ordinary motorcycles have pressure sensors. application.

The power of the motorcycle comes from the combustion of the oil in the cylinder of the gasoline engine. Only full combustion can provide good power, good combustion must have good mixture, full compression and optimal ignition three conditions. Whether the electronic fuel injection system can correctly control the air-fuel ratio within the required range determines the pros and cons of the engine's power, economy and emissions. The control of the air-fuel ratio of the gasoline engine is achieved by adjusting the amount of fuel supply that matches the intake air amount. Therefore, the measurement accuracy of the intake air flow rate directly affects the control accuracy of the air-fuel ratio.

Internal structure

It is composed of base material, metal strain wire or strain foil, insulation protection sheet and lead wire. According to different uses, the resistance strain gauge resistance can be designed by the designer, but the value of the resistance range should be noted: the resistance is too small, the required drive current is too large, while the heat of the strain gauge caused its own temperature is too high , The use of different environments, so that the resistance of the strain gauge changes too much, the output zero drift is obvious, the zero adjustment circuit is too complex. The resistance is too high, the impedance is too high, and the resistance to external electromagnetic interference is poor. Generally, they are several tens of euros to several tens of thousands of euros.

working principle

The working principle of the metal resistance strain gauge is the phenomenon that the strain resistance changes with the mechanical deformation due to adsorption on the substrate material, which is commonly known as resistance strain effect. The resistance of a metal conductor can be expressed by the following formula:

R=ρ

In the formula: ρ——The resistivity of the metal conductor (Ω·

/m)

S - the cross-sectional area of ​​the conductor (

)

L——length of conductor (m)

We take the wire strain resistance as an example. When the wire is subjected to an external force, its length and cross-sectional area will change. It is easy to see from the above formula that the resistance value will change if the wire is affected by external force. When extended, its length increases, and the cross-sectional area decreases, the resistance value increases. When the wire is compressed by an external force, the length decreases and the cross section increases, and the resistance value decreases. As long as the change in the resistance is measured (usually the voltage across the measuring resistor), the strain of the strained wire can be obtained.

Principle application

Corrosion-resistant ceramic pressure sensors do not transfer liquid. The pressure acts directly on the front surface of the ceramic diaphragm, causing a slight deformation of the diaphragm. The thick film resistors are printed on the back of the ceramic diaphragm and are connected to a Wheatstone bridge (closed). Bridge), due to the piezoresistive effect of the piezoresistors, causes the bridge to produce a highly linear voltage proportional to the pressure and a voltage signal proportional to the excitation voltage. The standard signal is calibrated to 2.0 / 3.0 / 3.3 depending on the pressure range. mV/V, etc., compatible with strain gauge sensors. Through laser calibration, the sensor has high temperature stability and time stability, the sensor comes with temperature compensation 0 ~ 70 °C, and can be in direct contact with most of the media.

Ceramics is a well-known material with high elasticity, corrosion resistance, wear resistance, impact resistance and vibration. The thermal stability of ceramics and its thick film resistance can make its operating temperature range up to -40 ~ 135 °C, but also has high precision and high stability. Electrical insulation >2kV, strong output signal, good long-term stability. High-performance, low-cost ceramic sensors will be the development direction of pressure sensors. In Europe and the United States, there is a tendency to completely replace other types of sensors. In China, more and more users use ceramic sensors instead of diffusion silicon pressure sensors.

11 classification

Piezoelectric

The piezoelectric pressure sensor principle is based on the piezoelectric effect. The piezoelectric effect is that when some dielectrics are deformed by the action of an external force in a certain direction, a polarization phenomenon occurs in the interior of the dielectric, and positive and negative opposite charges appear on its two opposite surfaces. When the external force is removed, it will return to the uncharged state. This phenomenon is called positive piezoelectric effect. When the direction of the force changes, the polarity of the charge also changes. On the contrary, when the electric field is applied in the direction of polarization of the dielectric, these dielectrics also deform, and after the electric field is removed, the deformation of the dielectric disappears. This phenomenon is called the inverse piezoelectric effect. Piezoelectric pressure sensors of a wide range of types and types, according to the form of elastic sensor and force mechanism can be divided into two types of diaphragm and piston. The diaphragm type mainly consists of a body, a diaphragm, and a piezoelectric element. The piezoelectric element is supported on the body, and the pressure to be measured is transmitted to the piezoelectric element by the diaphragm, and then the piezoelectric element outputs the electrical signal that has a certain relationship with the measured pressure. This sensor is characterized by its small size, good dynamic characteristics, and high temperature resistance. Modern measurement technologies have increasingly high demands on the performance of sensors.

For example, a pressure sensor is used to measure the dynamometer diagram of the internal combustion engine. Water cooling is not allowed in the measurement, and the sensor is required to be resistant to high temperatures and small size. Piezoelectric materials are best suited for developing such pressure sensors. Quartz is a very good piezoelectric material, and the piezoelectric effect is found on it. A more effective method is to select a quartz crystal cutting method suitable for high temperature conditions. For example, an XYδ (+20°~+30°) cut type quartz crystal can withstand a high temperature of 350°C. The Curie point of LiNbO3 single crystal is as high as 1210°C, which is an ideal piezoelectric material for manufacturing high temperature sensors.

Diffused Silicon

The pressure of the measured medium directly acts on the diaphragm (stainless steel or ceramic) of the sensor, so that the diaphragm generates a micro-displacement proportional to the pressure of the medium, so that the resistance value of the sensor changes, and the change is detected by the electronic circuit, and The conversion outputs a standard measurement signal that corresponds to this pressure.

Sapphire

Using strain-resistance operating principles, silicon-sapphire is used as a semiconductor sensing element and has unparalleled metering characteristics.

Sapphire is composed of single-crystal insulator elements that do not suffer from hysteresis, fatigue, and creep; sapphire is stronger than silicon, has higher hardness, and is not afraid of deformation; sapphire has very good elasticity and insulation properties (within 1000 OC), and therefore, is utilized. Silicon-sapphire semiconductor sensing devices are insensitive to temperature changes and have good operating characteristics even under high temperature conditions; sapphire is extremely resistant to radiation; in addition, silicon-sapphire semiconductor sensitive devices have no pn drift. Therefore, the manufacturing process is fundamentally simplified, the repeatability is improved, and a high yield is ensured.

Pressure sensors and transmitters manufactured with silicon-sapphire semiconductor sensing elements can operate normally in the harshest operating conditions, with high reliability, accuracy, minimal temperature error, and cost-effectiveness.

Gauge pressure sensors and transmitters consist of two diaphragms: a titanium alloy diaphragm and a titanium alloy diaphragm. The sapphire sheet printed with the heteroepitaxial strain-sensitive bridge circuit was welded on the titanium alloy measurement diaphragm. The measured pressure is transmitted to the receiving diaphragm (the receiving diaphragm and the measuring diaphragm are firmly connected together by a pull rod). Under the effect of pressure, the titanium alloy receives the deformation of the diaphragm. After the deformation is perceived by the silicon-sapphire sensor, the output of the bridge will change, and the amplitude of the change is proportional to the measured pressure.

The circuit of the sensor can ensure the power supply of the strain bridge circuit and convert the unbalanced signal of the strain bridge into a uniform electrical signal output (0-5, 4-20mA or 0-5V). In the absolute pressure sensor and transmitter, the sapphire sheet is connected with the solder of the ceramic base glass and functions as an elastic element. The measured pressure is converted into strain gauge deformation to achieve the purpose of pressure measurement.

12 Faults and Detection

Common malfunctions

Pressure sensor prone to failure are the following:

The first is the pressure up, and the transmitter loses. In this case, check whether the pressure interface is leaking or blocked. If it is not, check the connection mode and check the power supply. If the power supply is normal, simply pressurize to see if the output changes, or check if the sensor zero is output. If there is no change, the sensor is damaged, which may be the damage of the instrument or other problems in the whole system;

The second is the pressure transmitter output does not change, and then pressurize the transmitter output suddenly changes, the pressure transmitter zero can not go back, most likely the pressure sensor seal problem.常见的是由于密封圈规格原因，传感器拧紧之后密封圈被压缩到传感器引压口里面堵塞传感器，加压时压力介质进不去，但在压力大时突然冲开密封圈，压力传感器受到压力而变化。排除这种故障的最佳方法是将传感器卸下，直接察看零位是否正常，若零位正常可更换密封圈再试；

第三种是变送器输出信号不稳。这种故障有可能是压力源的问题。压力源本身是一个不稳定的压力，很有可能是仪表或压力传感器抗干扰能力不强、传感器本身振动很厉害和传感器故障；第四种是变送器与指针式压力表对照偏差大。出现偏差是正常的现象，确认正常的偏差范围即可；

最后一种易出现的故障是微差压变送器安装位置对零位输出的影响。微差压变送器由于其测量范围很小，变送器中传感元件会影响到微差压变送器的输出。安装时应使变送器的压力敏感件轴向垂直于重力方向，安装固定后调整变送器零位到标准值。

零点漂移

造成压力传感器的零点漂移的主要有以下几个原因：

1.应变片胶层有气泡或者有杂质

2.应变片本身性能不稳定

3.电路中有虚焊点

4.弹性体的应力释放不完全；此外还和磁场,频率,温度等很多有关系。电漂或一些漂移都会存在，但我们可以通过一些方式缩小其范围或修正。

零点热漂移是影响压力传感器性能的重要指标，受到广泛重视。国际上认为零点热漂移仅取决于力敏电阻的不等性及其温度非线性，其实零点热漂移还与力敏电阻的反向漏电有关。在这点上，多晶硅可以吸除衬底中的重金属杂质，从而减小力敏电阻的反向漏电、改善零点热漂移，提高传感器的性能。

缩小电漂移和修正电漂移还有哪些方式呢？零点电漂移除了影响压力传感器的测量精度和降低灵敏度之外，还有哪些重要影响呢？

利用零点电漂移可以消除压力传感器的热零点漂移，所谓零点漂移，是指当放大器的输入端短路时，在输入端有不规律的、变化缓慢的电压产生的现象。产生零点漂移的主要原因是温度的变化对晶体管参数的影响以及电源电压的波动等，在多数放大器中，前级的零点漂移影响最大，级数越多和放大倍数越大，则零点漂移越严重。

漂移的大小主要在于应变材料的选用，材料的结构或是组成决定其稳定性或是热敏性。

材料选好后的加工制成也很重要，工艺不同，会生产出不同效果的应变值，关键也在于通过一些老化等调节后，电桥值的稳定或程规律的变化。

漂移的调节手段很多，大都根据厂家的条件或生产需求所决定，大多数厂家对零点漂移都控制得很好。温度调节可通过内部温度电阻和制热零敏度电阻补偿、老化等。

对于采用电路转换的变压器中，电路部份的漂移可用通过选用好的元器件和设计更合适的电路来补偿。

应变材料要选灵敏系数高、温度变化小的材料。

故障检测

检查施工现场出现的故障，绝大多数是由于压力传感器使用和安装方法不当引起的，归纳起来有几个方面。

1、一次元件（孔板、远传测量接头等）堵塞或安装形式不对，取压点不合理。

2、引压管泄漏或堵塞，充液管里有残存气体或充气管里有残存液体，变送器过程法兰中存有沉积物，形成测量死区。

3、变送器接线不正确，电源电压过高或过低，指示表头与仪表接线端子连接处接触不良。

4、没有严格按照技术要求安装，安装方式和现场环境不符合技术要求。