1.Preparation before adjustment
① Check the appearance of the instrument to be calibrated. The nameplate should be complete and clear, and indicate the model, specification, position number, etc. The high and low pressure chambers should be marked, the surface coating should be intact and free of rust, the internal parts should be intact, and there should be no defects that affect the use and measurement performance.
② Select standard instruments: The accuracy of standard instruments is above 0.1 level.
③ Inspection point selection: Select pressure inspection points evenly according to the range, and the inspection points (including high and low range points) shall not be less than five points. After determination, the corresponding output current value (ie true value) of each point can be calculated, and the actual calibration error should not exceed the following maximum allowable error value:
d=±16×α % mA (d-maximum allowable basic error), (α %-transmitter accuracy level).
④ Selection of standard gauge: When calibrating the instrument, the comprehensive error of the standard gauge should be greater than or equal to 1/4 of the allowable error of the calibrated gauge. The allowable error of the 1.5-level pressure gauge is calculated as 1.6-level. The accuracy level can be unchanged. You can choose a standard gauge of level 0.4 or above, and the range is 1~1.5 times the range of the calibrated gauge. Select the precision pressure gauge, pressure source, etc. according to the level and range of the gauge to be tested, and calculate the maximum allowable error of the gauge to be tested.
Maximum allowable error of the instrument = ± (instrument range × accuracy level)
2. Transmitter instrument calibration
①Installation: The transmitter and the standard instrument are installed on the pressure calibration table. The positive pressure side of the transmitter is under pressure. The input and output circuits of the transmitter should be connected to the standard meter. Pay attention to check the connection line. The positive and negative polarities of the instrument should be connected correctly. When installing, the center of the transmitter’s pressure film should be at the same level as the sensing point of the standard meter as much as possible, otherwise consider liquid column correction.
②Working medium for calibration: The pressure gauge with a measurement upper limit of no more than 0.25MPa, the working medium is clean air or other gases with stable performance. The working medium of the pressure gauge greater than 0.25MPa should use non-corrosive liquids such as turbine oil, and the application air marked with oil prohibition should be used for calibration.
③Sealing check: Slowly and steadily increase the pressure (or evacuate) to the upper limit of the transmitter pressure (or -80Kpa), stop and keep it sealed for 15 minutes, check the standard meter, and the pressure change in the last 5 minutes is no more than 2% (or according to the manufacturer’s requirements).
④ Zero position and range check: 15 minutes after the meter is powered on, check whether the zero position and full range of the transmitter meet the technical requirements. If they exceed the technical requirements, make three cycles of operation within the entire range to adjust (zero position and full range buttons) to meet the requirements.
⑤ Instrument migration: If the installation position of the pressure instrument is not on the same horizontal plane as the measuring point, the calibration shall be corrected as follows: P = P1 ±ρ g h (“+” is used when the measuring point is higher than the installation position)
P is the standard gauge pressure; P1 is the gauge pressure; ρ is the density of the measured medium;
g is the acceleration of gravity; h is the vertical height between the installation position of the instrument and the side point position; the site can be roughly estimated according to the following method: the drop of 10 meters of water column is 0. 1Mpa;
⑥ Calibration
Starting from the lower limit (zero position), steadily increase the pressure (or evacuate) to each pressure calibration point, read the reading of the calibrated meter and record it until the upper limit of the pressure is measured, maintain the upper limit pressure for 3 minutes, and then steadily return to each pressure calibration point until it drops to the lower limit value (full scale), read the minimum resolution value when reading, and record the reading.
3.Pressure transmitter calibration
Select a precision pressure gauge of the corresponding range, so that the range of the calibrated transmitter is no more than 2/3 of the precision pressure gauge. Install the precision pressure gauge on the left side of the pressure calibration table, and install the calibrated transmitter on the right side of the pressure calibration table. Unscrew the wiring cover of the transmitter, and connect the + and – terminals and the two terminals + and – of the integrated calibrator outputting 24VDC and the + and – terminals of the precision ammeter in series. After checking that the measurement circuit is correct, turn on the power switches of the integrated calibrator and the precision ammeter respectively to see if the integrated calibrator outputs 24VDC, pressurize the transmitter so that the pressure reaches 0%, 25%, 50%, 75%, 100% of the transmitter range in turn, and observe whether the current display of the precision ammeter is 4, 8, 12, 16, 20MA respectively. If the error is less than the allowable error, it is qualified. If it exceeds the allowable error, the potentiometer of the transmitter should be adjusted: full scale S, zero point Z. When the pressure is adjusted to 100%, adjust the S potentiometer, then reduce the pressure to zero, adjust the Z potentiometer, and adjust back and forth several times until the current output is within the allowable error range. When returning, the pressure is reduced from 100% to 75%, 50%, 25%, and 0% in turn, and the corresponding current display is observed. It must also meet the qualified standards.
4. Air pressure transmitter calibration
Install the tee of the gas source pipe on the hand pump, and connect one side of the tee with a latex tube and a desktop precision pressure gauge. Unscrew the terminal cover of the transmitter, and connect the +, – and the two terminals +, – of the precision pressure gauge and the comprehensive calibrator output 24VDC and the +, – terminals of the precision ammeter in series. After checking that the wiring circuit is correct, supply power to the comprehensive calibrator and the precision pressure gauge respectively to see if the comprehensive calibrator outputs 24VDC. Pressurize the transmitter so that the pressure reaches 0%, 25%, 50%, 75%, 100% of the transmitter range in turn, and observe whether the current display of the precision ammeter is 4, 8, 12, 16, 20MA respectively. If the error is less than the allowable error, it is qualified. If it exceeds the allowable error, the potentiometer of the transmitter should be adjusted: full scale S, zero point Z. When the pressure is adjusted to 100%, adjust the S potentiometer, then reduce the pressure to zero, adjust the Z potentiometer, and adjust back and forth several times in turn until the current output is within the allowable error range. During the return stroke, the pressure is reduced from 100% to 75%, 50%, 25%, and 0% in sequence, and the corresponding current display is observed. It must also meet the qualified standards.
5. Differential Pressure Transmitter Calibration
Use a pressure calibration bench to pressurize the positive pressure side of the transmitter, check the differential pressure value on the precision pressure gauge, select 0%, 25%, 50%, 75%, 100% of the differential pressure transmitter measurement range as the standard value for calibration, steadily increase the signal pressure, and read the measured value of the response of each output point. During the return stroke, the output signal is steadily reduced to the minimum, and the corresponding measured value of each point is read. The allowable basic error of the transmitter shall not exceed the accuracy level specified by the transmitter. If it exceeds the allowable error, adjust the potentiometer of the transmitter. The adjustment method is the same as the adjustment steps of the pressure transmitter.
① Record:
② Data rounding: Round the data according to the effective digits of the instrument’s allowable error. If the allowable error is 0.08mA, round it to 2 decimal places (divide the reading by 2, round it to 2 decimal places according to the rounding method, and then multiply it by 2).
③ Calculate the instrument error based on the readings, which should meet the requirements:
Instrument indication error ≦Instrument allowable basic error
Instrument return error ≦Instrument allowable basic error
④Instrument indication error is the difference between the output value rising (or falling) to each calibration point during calibration and the converted value of the standard table.
⑤Instrument return error is the difference between the output value rising (or falling) to each calibration point during calibration.
⑥Record filling: Fill in the contents of the record form according to the instrument requirements, and the data is true and reliable.
⑦ Check the instrument software configuration and use a dedicated intelligent communicator to check, which should meet the process and measurement requirements.
⑧ Check the instrument for other items according to the calibration procedures and inspection requirements.
⑨After the instrument is qualified, affix the certificate and keep records. If it fails, the reasons must be listed and reported to the team leader and technician.