Abstract: This paper introduces the difference research and precautions of pressure transmitter and pressure sensor.
In the daily industrial production process, we find that pressure transmitters and pressure sensors are very similar working measuring instruments, which are used to monitor pressure and convert pressure signals into reliable output signals according to certain rules (mainly A device or device that is an electrical signal). They are widely used in many industries such as water conservancy and hydropower, railway transportation, intelligent building, production automation, aerospace, military, petrochemical, oil well, electric power, ship, machine tool, pipeline and so on. Through the real-time monitoring of pressure, on the one hand, the production process level can be improved, the product qualification rate is guaranteed; the other is to monitor the use environment to avoid the occurrence of safety accidents. Due to the high similarity between the pressure transmitter and the pressure sensor, the pressure transmitter and the pressure sensor are found to be confused on the market. Some pressure device manufacturers incorrectly label the nameplate of the pressure sensor as a pressure transmitter. It is necessary to give the user the selection and the development of the metrological verification mechanism, so it is necessary to study the difference between the pressure transmitter and the pressure sensor.
2. 1 different structural principles
Pressure transmitter structure principle
Pressure transmitter structure principle: It consists of pressure sensing unit, signal processing and conversion unit, and part of the display unit is added. It is a meter that converts the pressure variable into a standardized output signal that can be transmitted, and there is a signal between the output signal and the pressure variable. A certain continuous function relationship (referred to as linear relationship) can be divided into: negative pressure transmitter and absolute pressure transmitter according to different principles.
Pressure sensor structure principle: It consists of pressure sensitive components and signal processing units, which convert the pressure variables into usable output signals according to certain rules (output electrical signals are not standard signals).
Pressure sensor structure principle
Difference comparison: From the structural analysis, the pressure sensor has fewer signal conversion units than the pressure transmitter, so the pressure sensor has no standard output signal, and the output signal can be various. At present, the standard output signals of most pressure transmitters on the market are (4 ~ 20) mA, (1 ~ 5) V, and the common electrical signals of pressure sensors are (0 ~ 5) V, (0 ~ 10) V, etc. The nameplate standard output signal of the pressure sensor does not participate in the calculation and determination.
During the test, if the pressure transmitter output signal is zero, it can be determined that the wiring mode is wrong, because the output signal should be within (4 ~ 20) mA, (1 ~ 5) V range when the wiring is correct and the power is on. For example, when the initial pressure range is 0 MPa, then the initial output signal should be around 4 mA. Under the guidance of technical documents, if the signal value is zero after wiring, it is impossible to judge whether it enters the working state. Therefore, it is necessary to try the wiring multiple times, and a certain pressure value should be applied after each wiring, and the signal of the multimeter is observed. Whether the value has changed to determine whether the wiring is correct. If the pressure sensor is used with a display unit, it can be considered a digital pressure gauge.
2. 2 Different levels of accuracy
Pressure sensor accuracy level: 0. 01, 0. 02, 0. 05, 0. 1, 0. 2 / 0. 25, 0. 5, 1. 0, 1. 5 , 2. 0 level / 2. 5 level, 4. 0 level.
Pressure transmitter accuracy level: 0. 05, 0. 1 level, 0. 2 level / 0. 25 level, 0. 5 level, 1. 0 level, 1. 5 level, 2. 0 level, 2. Level 5.
Difference comparison: The pressure sensor accuracy level is better than the pressure transmitter, and it is also more broadly classified. The pressure transmitter can be thought of as a pressure sensor plus a signal conversion unit. In the signal loop, the pressure transmitter can be equivalent to a variable resistor connected in series with the power supply and load resistors (including the wire resistance). By changing its own resistance, the current in the circuit changes in the range of (4 to 20) mA, and its current has some uncertainty in amplification, compensation, and signal conversion, thus reducing its own accurate performance.
2. 3 Different calculation methods
The pressure transmitter verification project involves appearance, sealing, insulation resistance, dielectric strength, basic error, hysteresis, and static pressure (only for differential pressure transmitters).
Pressure sensor verification items relate to appearance, sealability, insulation resistance, insulation strength, basic error limits, repeatability, hysteresis, linearity, zero drift, cycle stability, static pressure effects (for differential pressure sensors only), and curve compliance.
Digital pressure gauge block diagram
Comparison of differences:
(1) The pressure transmitter verification is based on whether the maximum tolerance is satisfied. The maximum tolerance is expressed as a percentage of the output range, that is, the output range is multiplied by the pressure change level, and the calculation amount is small and the value is fixed. The pressure sensor verification is based on whether the basic error limit is satisfied. The basic error limit is also expressed as a percentage of the range output value, that is, the calibration full scale output is the algebraic difference between the maximum output and the minimum output determined by the operating characteristics, according to JJG 860 -2015. The working line of the pressure sensor can adopt the end point translation method or the least square method, and the full scale output value calculated by using different working lines is different.
(2) The basic error of the pressure transmitter verification point is the actual output value minus the theoretical output value. The pressure sensor does not have the theoretical output value. The basic error consists of the repeatability and the systematic error of the selected line. The system error is the difference between the actual output average of the verification point and the selected working line. The basic calculation is also based on different working lines. The error values are also different.
2. 4 Inconsistent adjustment methods
(1) The diffused silicon pressure transmitter has a zero (Z) and full (F or S) potentiometer, and the zero and full scale are repeatedly adjusted until the pressure transmitter indicates the specified value;
(2) Some pressure transmitters have a reset function. Find the zero button for a few seconds to return to the initial state; (3) Recalibrate through the Communicator.
Most of them cannot be repaired, and they need to be sent to the original factory for replacement and recalibration.
3 Other considerations
(1) After correct wiring, the device has no signal output. At present, many pressure standard devices integrate power supply and data acquisition, but the actual supply voltage is smaller than the nominal output voltage, so-called “virtual voltage”. In this case, the DC stabilized power supply can be replaced to solve the problem. For example, Beijing Const 273 intelligent digital pressure calibrator with 24V output, measured pressure value, reading record, temperature measurement, switch measurement and other functions, when the power is insufficient, the 24V output voltage value will drop.
(2) After wiring according to the instruction manual, the device has no signal output. Example (left and right): 4 terminals lead to 4 different color wires red, blue, yellow and white, but the actual workers in the assembly process, the color of the wires led out by the terminals is confused, in this case we directly remove the wires, Wire the measurement directly on the terminal block.
(3) At present, many smart pressure transmitters have Hart protocol, BRAIN protocol and Profibus PA. The factory scale can be modified by the manual command. When we find that the actual data does not match the nameplate range, we should contact the customer. In addition, when it is found that the measured value of multiple verification points minus the nominal value is a fixed range value, it should be considered whether the device is absolutely pressure.
(4) The signal terminals of the pressure transmitter are set in a single compartment. The current on the test terminal (TEST + - ) is the same as the current on the signal terminal ( OUT + -). The power supply is connected to the transmitter through the signal terminal. Do not connect the test terminals, otherwise the diodes of the test terminals will be burnt out.
Combined with pressure transmitter and pressure sensor verification procedures, and in connection with actual work, pressure transmitters can be considered as pressure sensors with standard output values. The structure is very similar, but the difference between them is found by the above research. Very large, we should strengthen their classification and understanding, distinguish between the correct way to carry out testing, to ensure that the data is reliable and accurate, so that they play the role of quality control and safety monitoring in industrial production.
Note that the three instruments are original, please indicate the address of this article.