Pressure transmitters are mainly used in the production of pressure pipelines in large industrial enterprises such as energy, chemical, and pharmaceutical industries to measure the pressure at the measured point and output a standard signal to achieve the purpose of automated production and safety monitoring.
According to the output signal, pressure transmitters can be divided into electric pressure transmitters and pneumatic pressure transmitters. The output signal of electric pressure transmitters is 0~10mA, (4~20)mA DC, or (1~5)V DC voltage; the output signal of pneumatic pressure transmitters is 20kPa~100kPa gas pressure. Therefore, pressure transmitters can be divided into absolute pressure transmitters, gauge pressure transmitters, and differential pressure transmitters according to the measured pressure.
1. Two-wire connection method of pressure transmitter.
1.1.1 Good immunity of the pressure transmitter to interference.
1.1.2 Infinite internal resistance of the pressure transmitter current source.
1.1.3 No loss of line resistance in the transmission of the pressure transmitter current.
1.1.4 The pressure transmitter can transmit directly over distances of up to several kilometers.
1.1.5 The pressure transmitter does not burn the meter due to the power connection.
1.1.6 The pressure transmitter is also easy to determine the line continuity.
1.2 Explosion-proof requirements:
1.2.1 The upper limit of the pressure transmitter is set at 20mA. At 24V DC, the sparking energy caused by a 20mA current break will not ignite combustible gases such as gas.
1.2.2 When the lower limit of the pressure transmitter is set to 4mA, it is to be able to detect disconnection, when working generally below a current of 4mA, when the transmission line is disconnected for any reason, the current drops to 0mA.
When the follow-up of the pressure transmitter in the calibration, due to the transmitter in the production of a wide range of enterprises, the number of large, and large, inconvenient transport, we can hardly do to send; and online field calibration and by the environment, and less efficient.
We should carry out timely and accurate follow-up calibration of pressure transmitters in use, by the unit being inspected to provide a workroom to meet the ambient temperature requirements in the regulations (15°C ~ 25°C), the smart pressure transmitter being inspected will be disassembled and sent to the workroom, by the calibrator to carry out the follow-up calibration of the transmitter. This form of calibration ensures the environmental requirements of the calibration protocol and improves efficiency, and can meet the needs of the medium in energy, chemical, pharmaceutical, and other industries. These industries use many pressure transmitters for the follow-up calibration of pressure transmitters in use.
2. Pre-validation preparation
2.1 Selection of standard ware
Following the JJG882-2004 “Pressure Transmitter Calibration Regulations” provisions, the standard should choose a portable, suitable anti-interference digital manometer and a digital multimeter (nowadays, the digital pressure tester is mainly used to provide pressure and current measurement and 24V output). Furthermore, the pressure source should be equipped with a pneumatic pump: to provide (-100~500) KPa pressure and a hydraulic pump: to offer more than 500 KPa pressure.
2.2 Handling of the inspected transmitter
As the object of the subsequent calibration is in use pressure transmitter, the use of the process to contact a large number of chemical substances, high-temperature steam, etc., the interface is more impurities and prone to rust, the application of fine sandpaper or steel wire balls will be cleaned across the interface end face, to make it and the pressure source interface sealing gasket close contact, in the process of pressurization does not occur leakage.
Transmitter pressure sensing chamber often accumulates impurities and residual liquid. When calibrated, it will be mixed into the pressure-making medium to cause pollution or block the pipeline to affect the pressure. Therefore, we should clean it. The interface corresponding to the side of the ventilation valve open and the interface facing down to rest for 3 minutes while the residual liquid released net, drained of residual liquid after tightening the ventilation valve.
2.3 Selection of calibration points
There are 5 points including upper and lower limits according to the regulations: 0% FS, 25% FS, 50% FS, 75% FS, 100% FS, corresponding to output currents of 4mA, 8mA, 12mA, 16mA and 20mA.
2.4 About Leapfrog and how to handle it
When we use pressure transmitters, often due to a variety of reasons for the range of the leap, if the transmitter for the value of the migration, we should first calculate the corresponding pressure upper and lower limits of the difference, we have to pass the conventional calibration and then use the HART375 hand operator to re-migrate back to the working state, to ensure that the tested transmitter calibration in line with the requirements of the regulations and can meet the actual use.
When the transmitter has reached thermal equilibrium after 1 hour of resting, we can then install the transmitter to the corresponding fast interface of the pressure source and connect the pressure standard module to the fast interface on the other side.
The wiring method follows the working principle of the two-wire pressure transmitter; firstly, the positive pole of the 24V power supply is connected to the positive pole of the digital multimeter, then the negative pole of the digital multimeter is connected to the positive pole of the pressure transmitter. Finally, we link the pressure transmitter’s negative pole to the power supply’s negative pole, and we also have to check that the wiring is correct and then connect the 24V power supply. We should pay attention to the order of wiring first and then energize as far as possible to avoid short-circuiting the power supply due to the touch of the charged connector during the wiring process. This can avert short-circuiting and burning the power supply during the wiring process.
2.6 Pre-validation adjustments
As the calibration process does not allow the zero point and range to be adjusted again, before we start the calibration, we must first determine whether the zero point and full scope of the transmitter under test are over-qualified to improve the efficiency of the calibration.
3. Inspection items
3.1 Appearance check
We must check whether the transmitter parts are intact, whether the fasteners are loose and damaged, and whether the movable parts are flexible and reliable, following the regulations. Transmitter with the display unit, whether the digital display is clear or lacks strokes.
3.2 Insulation resistance testing
Subsequent calibration of the transmitter only needs to test the insulation resistance between the transmitter terminals and the shell. The test will be the terminals and the body, respectively, with the wire connected to the positive and negative terminals of the insulation resistance table, shaking the insulation resistance table shaking handle, the measured insulation resistance between the terminals and the shell is not less than 20M that is qualified.
3.3. Basic error checking
When we control the pressure source smoothly, we also have to pressurize to each of the above calibration points, read and record the corresponding output current value to the upper limit after the digital pressure calibrator shows a stable pressure value, then smoothly release the pressure to each of the calibration points, we read and record the corresponding output current value to the lower limit (note that the upper stroke does not record the lower limit point, the lower stroke does not record the upper limit point data), according to the requirements of the regulation only two cycles can be.
4. A discussion of the data handling aspects of the calibration protocol
When the pressure transmitter is used frequently, we usually work in high temperature, high pressure, strong corrosive environment, the measurement error is generally large, we also often encounter the critical exceedance of the situation, the verification personnel in the judgment of qualified or not have the following problems:
① A block of pressure transmitter in 75% FS calibration point (corresponding to the nominal value of the output current of 16mA), the maximum measurement value is 15.9150mA, when the checker to repair the contract, we retain three decimal places for processing, the result 15.915mA, and thus determine that the transmitter failed.
②When the personnel of the inspected unit put forward a different opinion, that if we follow the retention of 2 decimal places, the result should be 15.92mA, in line with the requirements of the huge allowable error of 0.08mA for 0.5 level pressure transmitters, should be judged to be qualified.
In summary, the pressure transmitter is kept to two and three decimal places in the data processing should meet the requirements of the regulations, but in the calibration of the transmitter, due to the error value of the revision of the date caused by the conflicting results, which directly affects the calibration personnel for the qualified or unqualified judgment of the transmitter. It can be seen that the regulations on the number of decimal places only impact the data processing of pressure transmitters and the decision of the results.