Have you remembered the common steps for calibrating instruments!

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On-site calibration refers to the calibration of instruments installed at the location. A large number of instruments are installed on-site, and on-site calibration of these instruments is carried out frequently. On-site calibration of instruments falls within the scope of routine maintenance work for such instruments; generally speaking, on-site calibration merely serves to confirm the error in the indicated values. According to the definition of calibration, although it can involve verifying other metrological properties of the instrument, in most cases it is merely a verification of the indication error. I. On-site calibration of differential pressure transmitters Differential pressure transmitters are divided into two main categories: pneumatic and electric. They are widely used in industries such as oil refining, chemicals, and metallurgy; most of them are used in conjunction with throttling devices to measure flow rates, while others are used to measure liquid levels or other parameters. A large number of differential pressure transmitters are in use at production sites, and calibration is usually carried out on-site. 01(1) Tools and instruments – On-site calibration of differential pressure transmitters generally does not require removing the transmitter. First, close the positive and negative pressure valves of the pressure lead pipe, open the balance valve, and remove the plugs from the positive and negative pressure exhaust ports. The air pressure signal can enter from the positive pressure side of the transmitter via the calibration port, while the negative pressure side is connected to the atmosphere. No special requirements exist for the calibration tools; standard wrenches of 150mm and 200mm (6 inches, 8 inches) along with tools used by fitters are sufficient. For standard instruments used for calibration, their error limit should be 1/3 or 1/10 of the error limit of the instrument being calibrated. The equipment required for calibrating differential pressure transmitters is as follows: Name, specifications, model, unit, quantity. Digital pressure gauge – range determined by the range of the instrument to be calibrated; 1 unit. Precision ammeter – 0～30mA; 1 unit. Air source pressure regulator – 1 unit. Pneumatic constant value regulator – 1 unit. Air supply pipe tee, Φ6 (Φ8) – 1 unit. Rubber or steel tube, Φ6 (Φ8) – in meters. Electrical wires – several meters. Instrument connection fitting. Note: On-site conditions usually do not provide an air source; therefore, a manual pump is generally used to replace the air source pressure regulator and the pneumatic constant value regulator. 01(2) Wiring: The instrument calibration wiring provided here is the wiring used when the instrument is removed from operation. When calibrating the instrument on-site without removing it, it can be connected according to the actual conditions; for example, a pneumatic gauge does not require a separate air supply, and an electric gauge does not need a separate power source. The wiring schematic for calibrating the electric differential pressure transmitter is shown in Figure 1-1. For differential pressure transmitters with high differential pressures, the input signal can be provided by a piston manometer. Use the on-site power supply directly when calibrating the meter on site. Figure 1-1: Schematic diagram of the calibration wiring for electric differential pressure transmitters 1 – Air supply isolation valve ； 2 – Pressure relief valve ； 3 – Pneumatic setpoint regulator ； 4－Calibrated meter ； 5 – Precision ammeter ； 6 – Digital pressure gauge ； 7 – Power supply ； x – input ； s – Output 03. Procedure for calibrating electric differential pressure transmitters: ① Basic error calibration a. Close the positive and negative pressure valves of the pressure lead pipes, and open the balance valve. b. Connect the calibration circuit as shown in Figure 1-1. c. Remove the exhaust plugs on the positive and negative sides. d. Use air to blow out the residual liquid in the positive and negative pressure chambers through the exhaust plug and the vent plug. e. Connect the power supply after checking and confirming. f. Send a signal to the positive pressure side exhaust port through the gauge connection nipple. Select the transmitter’s measurement range, or use the 5 points of 0%, 25%, 50%, 75%, and 100% of the output signal as standard values for calibration. g. Output the differential pressure signal steadily, and read the corresponding measured values at each point. h. Raise the output signal to 105% of the upper limit and maintain it for 1 minute, then gradually reduce the output signal to its minimum value, and read the corresponding measured values at each point. i. Calculate the basic error: The forward travel error δz = (Az – Ao)/16 × 100%, while the reverse travel error δz = (Af + Ao)/16 × 100%. Here, δz represents the basic error during forward travel, in %; δz also represents the basic error during reverse travel, in %. Az is the measured output value during forward travel, in mA; Af is the measured output value during reverse travel, in mA; Ao is the nominal value of the output signal, in mA; and 16 is the difference between the upper and lower limits of the output signal, in mA. The allowable basic error for an electric differential pressure transmitter must not exceed the precision grade specified for that transmitter. ②The calibration of the return error is the absolute value of the difference between the measured values during the forward and reverse strokes at the same point; this value represents the hysteresis error of the electric differential pressure transmitter. Calculation of the return error: AH = |AZ – AF|. Here, AH represents the return error of the electric differential pressure transmitter, in mA; AZ is the measured value of the output signal during forward movement, in mA; AF is the measured value of the output signal during reverse movement, in mA. The return error of the electric differential pressure transmitter must not exceed the absolute value of the allowable error specified for the transmitter. ③Fill in the calibration record; the calibration form for electric differential pressure transmitters is as follows. Unit, Name of Instrument, Specification Model, Accuracy Class, Measurement Range, Manufacturer, Serial Number, Nominal Value of Output Signal/kPa, Error of Measured Output Value/kPa, Backlash Error/kPa%, Positive, Negative, Reverse, Forward: 0255075100. Allowable basic error: Maximum basic error; Allowable backlash error: Maximum backlash error. Person who performed calibration: Person who reviewed: Year Month Day. II. On-site Calibration of Pressure Transmitters Pressure transmitters are instruments that convert pressure into a pneumatic signal of 20–100 kPa or into a current signal of 4–20 mA. They are divided into pneumatic and electric types, and are widely used in industries such as petroleum refining, chemicals, metallurgy, and pharmaceuticals. On-site calibration of pressure transmitters is also carried out frequently. 01 Tool and instrument on-site calibration: Pressure transmitters do not need to be removed, nor are any special tools required; standard wrenches of 200mm and 250mm (8″, 10″) along with the tools used by instrumentation technicians are sufficient. The error limit of the calibration instrument is 1/3 to 1/10 of the error limit of the instrument being calibrated. The instruments required for calibrating pressure transmitters are as follows: Name, Specification, Model, Unit, Quantity, Remarks. Piston pressure gauge: YS-60 or YS-600, units: 1; selected according to the range of the instrument to be calibrated. Digital pressure gauge: 1 unit, selected according to the range of the instrument to be calibrated. Precision ammeter: 0–30 mA, 1 unit. Rubber hose or steel pipe: 1 meter. Wires: several meters. For the wiring of electric pressure transmitters, refer to Figure 1-3 for the wiring schematic. Figures 1-3: Schematic diagram of the wiring for calibrating electric pressure transmitters 1–Piston pressure gauge ； 2-Table to be calibrated ； 3-Precision ammeter ； 4-24VDC power supply 01(3) Operation steps: Calibration procedures for electric pressure transmitters. ① Calibration of basic error: a. Close the valve that leads the pressure tube to the transmitter, disconnect the original pressure tube connection, connect a piston manometer, and set up the calibration circuit as shown in Figure 1-3. b. Power on after verification that everything is correct. c. Select 5 points—0%, 25%, 50%, 75%, and 100% of the pressure transmitter’s measurement range or output signal—as standard values for calibration. d. Gradually increase the pressure to the desired level using a piston manometer, and record the corresponding measured values at each point. e. Raise the output signal to 105% of the upper limit and maintain it for 2 minutes, then gradually reduce the output signal to the minimum value, and record the corresponding measured values at each point. f. Calculate the basic error: The error during the forward stroke is δz = (Az – Ao)/16 × 100%, while the error during the reverse stroke is δz = (Af + Ao)/16 × 100%. Here, δz represents the basic error during the forward stroke in %; δz also represents the basic error during the reverse stroke in %. Az is the measured output value during the forward stroke, in meters; Af is the measured output value during the reverse stroke, in meters; Ao is the nominal value of the output signal, in mA; and 16 is the difference between the upper and lower limits of the output signal, in mA. The allowable basic error for electric pressure transmitters must not exceed the precision grade specified for that transmitter. ②The calibration of the return error is the absolute value of the difference between the measured values during the forward and reverse strokes at the same point; this value represents the hysteresis error of the electric pressure transmitter. Calculation of the return error: AH = |AZ – AF|. Here, AH represents the return error of the electric differential pressure transmitter, in mA; AZ is the measured value of the output signal during forward movement, in mA; AF is the measured value of the output signal during reverse movement, in mA. The return error of an electric pressure transmitter must not exceed the allowable basic error specified for that transmitter. ③Fill in the calibration record; the calibration form for electric pressure transmitters is as follows. Unit, Name of Instrument, Specification Model, Accuracy Class, Measurement Range, Manufacturer, Serial Number, Nominal Value of Output Signal in kPa, Error of Measured Output Value in kPa, Backlash Error in kPa%, Positive, Negative, Forward, Reverse: 0255075100. Allowable basic error: Maximum basic error; Allowable backlash error: Maximum backlash error; Person who performed calibration: Person who reviewed: Year Month Day. III. On-site Calibration of Control Valves (with Valve Positioners) Control valves serve as actuators in control systems; their operation is controlled by regulators, and their actions also directly affect process parameters. Therefore, aside from control valves equipped with bypass lines, which can be taken out of service via those bypass lines for calibration, all other control valves can only be calibrated when they are not in use. To improve control performance, control valves are often equipped with valve positioners, and under normal circumstances, control valves are calibrated together with their valve positioners. Valve positioners are divided into pneumatic and electric types. 01 Tools and instruments: Control valves come with valve positioners and other accessories; their mechanical structure is relatively complex and they contain many components, so a comprehensive set of tools is required. A socket wrench, hex key wrenches (200–375 mm or 8–15 in), adjustable wrenches of various sizes, as well as the tools commonly used by fitters are required; if necessary, a 0.5 t winch should also be available. The instruments used are as follows: 2 digital pressure gauges with a range of 0–160 kPa, 1 pneumatic setpoint regulator, 1 precision ammeter with a range of 0–30 mA, and 1 current signal generator. 02 Wiring and calibration steps: (a) Calibration of control valves equipped with pneumatic valve positioners. ① Connect the pipelines according to Figure 1-4. Connect the air supply and adjust the setpoint so that the output (digital pressure gauge 1) is 20 kPa; check whether the valve stroke is at the starting position (maximum stroke position). Adjust the setter output to 100 kPa, and observe whether the valve stroke reaches its maximum (starting position). The digital pressure gauge 2 in the figure is used to monitor the output of the locator. Figure 1-4 Schematic diagram of the calibration principle for a control valve with a pneumatic valve positioner. Step a: Select input signal pressures of 20 kPa, 40 kPa, 60 kPa, 80 kPa, and 100 kPa to perform calibration at these five points. b. The corresponding valve position indications should be 0, 25%, 50%, 75%, 100%. c. Calibration is performed in both forward and reverse directions. The valve position indication can be obtained by multiplying the full stroke in millimeters by the percentage on the scale, thereby giving the value in millimeters. (b) Calibration of the control valve with an electric valve positioner ① Wiring: Connect the pipelines according to Figure 1-5. The digital pressure gauge in the figure is used to monitor the output of the locator. Figures 1-5: Schematic diagram of the calibration process for a control valve equipped with an electrical valve positioner. First, an input signal of 4 mA is applied; then it is checked whether the digital pressure gauge shows 20 kPa and whether the valve is at its starting position (the full stroke position). Adjust the input signal to 20 mA, and check whether the digital pressure gauge reads 100 k kPa and whether the valve has reached its maximum travel (starting position). ②Step a. Select the input signals of 4 mA, 8 mA, 12 mA, 16 mA, and 20 mA for calibration at 5 points. b. The corresponding valve indications should be 0%, 25%, 50%, 75%, 100%. c. Calibration is performed in both forward and reverse directions. The valve position indication can be obtained by multiplying the full stroke in millimeters by the percentage on the scale, thereby giving the value in millimeters. During the calibration of control valves, since it is performed on-site with the valves already installed in their operating positions, certain tests such as airtightness tests cannot be carried out. During the joint calibration of the positioner and control valve, if it is found that the positioner is not functioning properly, it should be removed for separate calibration.