Transmitter calibration procedure

Thread Content

Specifications for the Calibration of Pressure Transmitters These specifications apply to the calibration of pressure transmitters that are newly manufactured, in use, or after being repaired (hereinafter referred to as transmitters): I. Overview A pressure transmitter is a device that converts pressure variations into a uniform output signal that can be transmitted; there is a defined continuous functional relationship between its output signal and the pressure variation, which is usually linear. Pressure variables include positive and negative pressure, differential pressure, and absolute pressure. Pressure transmitters are divided into two main categories: electric and pneumatic. The standard output signal for electric transmitters is a direct current signal of 0-10mA, 4-20mA (or 1-5V), while the standard output signal for pneumatic transmitters is a gas pressure of 20-100kPa. Pressure transmitters can be classified into force (torque) balance type, capacitive type, inductive type, piezoelectric type, frequency type, and so on, based on different conversion principles. II. Technical Requirements 1. Appearance 1.1 The nameplate of the transmitter should be complete and clear, indicating key technical specifications such as the product name, model, specifications, and measurement range. The high- and low-pressure chambers should be clearly marked, and the manufacturer’s name or trademark, serial number, and date of manufacture should also be indicated. 1.2 The components of the feeder shall be intact and undamaged; fasteners must not be loose or damaged, and the movable parts shall be flexible and reliable. 1.3 The casing and surface coatings of newly manufactured transmitters shall be smooth, intact, free from rust and mold; there shall be no debris such as chips or residues inside. Transmitters that have been repaired or modified must not have any defects that could affect their performance in use and in terms of measurement accuracy. 2 Sealing performance: The measuring part of the transmitter must not exhibit any leakage or damage when subjected to the maximum measurement pressure (the rated operating pressure for differential pressure transmitters). 3 Basic error: The basic error of the transmitter shall not exceed the values specified in Table 1. Accuracy class, Basic error (%), Rebound error (%) – Electric, Pneumatic, Electric, Pneumatic, Electric, Pneumatic: 0.2 (0.25), 0.5, 1.0, 1.5, 2.5; 0.5, 1.0, 2.5, 2.5. ±0.2 (±0.25), ±0.5, ±1.0, ±2.5, ±2.5, ±0.5, ±1.0, ±1.5, ±2.5. For newly manufactured transmitters, the rebound error should not exceed the values specified in Table 1; for transmitters that have been serviced or repaired, the rebound error should not be greater than the absolute value of the basic error listed in Table 1. Project Accuracy Level: 0.5, 1.0, 1.5, 2.5
Indicators (%): Static Pressure Effect
Ps – Static pressure value (Mpa)
Minimum value variation:
Ps≤6.4: ±2.0, ±2.5, ±3.0, ±3.0
Ps≤6.4 (differential pressure range ≤6 kPa): ±3.0, ±3.5, ±4.0
6.4<Ps≤16: ±3.0, ±3.5, ±4.0
6.4<Ps≤16 (differential pressure range ≤6 kPa): ±4.0, ±4.5, ±5.0
16<Ps≤25: ±3.5, ±4.0, ±4.5
25<Ps≤32: ±4.5, ±5.0, 5.5, ±5.5
32<Ps≤40: ±5.0, ±6.0
*Expressed as a percentage of the output range; the meaning of the percent sign is the same in all subsequent tables. 4 Hydrostatic pressure effect 5.1 The hydrostatic pressure effect on differential pressure transmitters with force-balancing mechanisms shall not exceed the values specified in Table 2. 5.2 The influence of static pressure on pneumatic differential pressure transmitters shall not exceed the values specified in Table 3.
| Parameter | Accuracy Class | 0.5 | 1.0 | 1.5 | 2.5 | Indicator (%) | Static Pressure Influence | Ps – Static pressure value (Mpa) | Lower limit value variation | Ps≤2.5 | ±0.75 | ±1.0 | – | – | Ps≤2.5 (differential pressure range ≤2.5 kPa) | ±1.5 | ±2.0 | – | ±1.5 | 2.5<Ps≤6.4 | ±1.0 | ±1.5 | ±1.5 | – | 2.5<Ps≤6.4 (differential pressure range ≤6 kPa) | ±1.5 | ±2.0 | – | – | 6.4<Ps≤16 | ±1.5 | ±2.5 | ±2.5 | – | 16<Ps≤25 | ±2.0 | ±3.0 | ±3.0 | – | 25<Ps≤32 | ±2.5 | ±3.5 | ±3.5 | – | 32<Ps≤40 | ±4.0 | ±5.0 | ±5.0 | –
5.3 For other types of differential pressure transmitters, the influence of static pressure shall not exceed the values specified in the manufacturer’s own standards. 6 Electrical Performance of Electric Transmitters 6.1 Effect of Open Circuit at the Output After undergoing a open-circuit test, when the wiring is restored to normal, the output range of the transmitter shall not deviate beyond the values specified in Table 4, and it must still meet the requirements regarding basic error and hysteresis error. Accuracy class Output variation (%): 0.22 (0.25), 0.5, 1.0, 2.5, 2.5. Output AC component: For transmitters with an output of 0–10 mA, the RMS value of the AC voltage across a 200 Ω sampling resistor should not exceed 20 mV. For transmitters that output 4-20mA, the RMS value of the AC voltage across the 250Ω sampling resistor should not exceed 50mV (and should not exceed 150mV for force-balance type transmitters). 6.3 Insulation Resistance: At an ambient temperature of 15–35°C and a relative humidity of 45%–75%, the insulation resistance between all terminals of the transmitter should be no less than the following values: Output terminal to ground terminal (chassis): 20 MΩ; Power terminal to ground terminal (chassis): 50 MΩ; Power terminal to output terminal: 50 MΩ. 6.4 Insulation Strength: Under the same conditions of ambient temperature and humidity, a sinusoidal alternating voltage with a frequency of 50 Hz is applied between all terminals of the transmitter for 1 minute; no breakdown or arcing should occur. Output terminal to ground terminal (chassis): 500V; Power terminal to ground terminal (chassis): 1000V; Power terminal to output terminal: 1000V. It is a two-wire transmitter with a voltage of 500V. Capacitive transmitters, as well as those with special requirements specified by their manufacturers, do not need to undergo this type of calibration. 7 Effect of changes in the air supply pressure of the pneumatic transmitter: When the air supply pressure of the transmitter changes from 140 kPa to 154 kPa or from 140 kPa to 126 kPa, the change in its output does not exceed the values specified in Table 5. Accuracy class, Effect of air supply pressure changes, Effect of exceeding the range, Output variation (%), Change in output lower limit and range (%): 0.5, 1.0, 1.5, 2.5; 0.5, 0.5, 0.75, 1.25; 0.25, 0.5. For pneumatic transmitters, after undergoing a test in which the input value exceeds the range by 25%, the change in the output lower limit and range should not exceed the values specified in Table 5. 3. Verification Conditions 9. Verification Equipment: The verification apparatus, which consists of the standard instruments and associated equipment selected, shall have a total measurement uncertainty not greater than 1/4 of the allowable error of the transmitter being tested. The method for evaluating the total uncertainty measured by the testing device is given in Appendix 1. The standards and equipment required for calibration are listed in Table 6. Serial Number Name of Instrument/Equipment Technical Requirements Purpose 1 Piston Pressure Gauge The accuracy class and measurement range are determined based on calculations in Appendix 1; it serves as a pressure standard for the input of transmitters, while items 2 and 4 function as output measurement standards for pneumatic transmitters. It is also used to verify sealing performance and the effects of static pressure. 2 Liquid pressure gauge (and associated air supply) 3 Digital pressure generator 4 Digital pressure gauge (and associated pressure generator) 5 Precision pressure gauges, etc. 6 DC ammeter, with a measurement range of 0–20 mA; the accuracy class is determined as specified in Appendix 1. This device serves as a standard for measuring the output current of electric transmitters. 7 DC voltmeter, with an input resistance of over 5 MΩ; the accuracy class is determined as specified in Appendix 1. These two devices are used together as a standard for measuring the output current of electric transmitters. 8 Precision resistors, 100 Ω or 250 Ω, with an accuracy class of at least 0.05. 9 Insulation resistance meter, with an output DC voltage of 500 V and a measurement range of 0–00 MΩ; accuracy class of 10. (For capacitive transmitters, a meter with an output DC voltage of 100 V is used to measure insulation resistance.) 10 Voltage withstanding test apparatus, capable of generating an AC voltage of over 1500 V and having a power rating of at least 0.5 kV. Insulation strength test: 11. DC resistance box with accuracy of at least 0.1 grade, measurement range of not less than 0–1.5 kΩ, and current capacity exceeding 20 mA. Load resistor: 12. AC millivoltmeter with accuracy of at least 2.5 grade, input impedance greater than 100 kΩ, for measuring the AC component. Vacuum system: The requirements for mechanical pumps (and diffusion pumps) are determined according to the measurement range and accuracy level of the transmitter under consideration, in accordance with the provisions of 14.4. Pressure source for absolute pressure transmitters and negative pressure transmitters: 14. AC voltage stabilizer, 220 V, 50 Hz, with stability of over %, and power output of not less than 1 KW. AC power supply for transmitters: 15. DC voltage stabilizer, 24 V, with allowable error of 1%, ripple of less than 0.1%, and power output of not less than 30 W. DC power supply for transmitters: 16. Air supply device and regulator, with stable output pressure ； 126–154 kPa, allowable error of ±1%; no oil or dust present, with a dew point 10°C lower than that of the transmitter’s housing. The air supply for the pneumatic transmitter: 10. Environmental conditions: 10.1 Temperature at 20±5°C, with changes of no more than 1°C every 10 minutes; relative humidity between 45% and 75%. 10.2 The environment in which the transmitter is located should be free from mechanical vibrations that could affect its output stability. 10.3 Apart from the earth’s magnetic field, there should be no external magnetic fields around the electric transmitter that could affect its normal operation. 10.4 For transmitters with a maximum measurement value of not more than 0.25 MPa, the pressure-transmitting medium is air or other non-toxic, harmless gases with stable chemical properties. For transmitters with a measurement upper limit greater than 0.25 MPa, the pressure-transmitting medium is generally a liquid. IV. Inspection Items and Inspection Methods 11 Inspection Items The inspection items are shown in Table 7; “+” in the table indicates that an inspection is required, “–” indicates that an inspection is not necessary, and “/” indicates that there is no such item. 12 Inspection Items, Inspection Categories: Electric, Pneumatic, Newly Manufactured, After Repair, In Use, Newly Manufactured, After Repair, In Use. Appearance, Sealing Performance, Basic Error, Return Error, Effect of Static Pressure, Effect of Open Circuit at Output, AC Component in Output, Insulation Resistance, Insulation Strength, Effect of Changes in Air Supply Pressure, Out-of-Range Effect. + + + + + + + + + / / + + + + + + + + + + / / / + + + + — — — + — / / + + + + + / / / / + + + + + + + / / / / + + + + + + — / / / / — — Inspection is carried out by tactile feeling and visual inspection. 13 Sealing test: Gradually increase the pressure (or reduce it) until the pressure in the transmitter’s measurement chamber reaches the upper limit of the measurement range (or 90% of the local atmospheric pressure in the case of reduced pressure). Once this value is reached, cut off the pressure source and seal the chamber for 15 minutes. During the last 5 minutes, monitor the pressure using a gauge; the decrease (or increase) in pressure should not exceed 2% of the upper limit of the measurement range. During the sealing test of the differential pressure transmitter, the high and low pressure chambers are connected, and the rated operating pressure is applied simultaneously for observation. 14 Verification of basic error 14.1 According to the connection principles shown in Figure 1, place the transmitter in its specified operating position, and connect it to the pressure standard, the output load, and the detection device (for specific connection methods, see the instruction manual). Ensure that the pressure transmission tube is filled with the pressure-transmitting medium. When the pressure-transmitting medium is a liquid, the geometric center of the pressure tap on the transmitter should be at the same horizontal level as the lower surface of the piston in the piston-type pressure gauge (or the geometric center of the pressure tap on the standard instrument), with the height difference not exceeding the value calculated in Equation (1). h=∣a%∣Pm/20ρg. Here, h is the allowable height difference (in meters); a is the accuracy class index of the instrument ； Pm-transmitter input range (Pa) ； ρ - Density of the pressure-transmitting medium (g/m3) ； g-Local gravitational acceleration (m/s2) ； The output load is selected in accordance with the manufacturer’s specifications. If more than one resistance value is specified, for transmitters that output direct current, the maximum value should be used, while for those that output direct voltage, the minimum value should be chosen. The load for pneumatic transmitters consists of an air resistance created by a conduit with an inner diameter of 4 mm and a length of 8 m, followed by a gas volume of 20 cm3. 14.2 Unless otherwise specified by the manufacturer, electric transmitters generally need to be powered on and preheated for 15 minutes. 14.3 The calibration points shall include no less than 5 points, including the upper and lower limits (or within 10% of the input range near them), and these points shall be distributed fairly evenly across the entire measurement range. For transmitters with an adjustable input range, those that have been neutralized and repaired can be calibrated only at the commonly used range or the range specified by the tester; whereas newly manufactured ones must have their input range adjusted to the specified minimum and maximum values before calibration, with necessary adjustments allowed prior to calibration. 14.4 Before calibration, perform three cycles across the entire measurement range by varying the input pressure signal; during this process, the lower and upper limits of the output can be adjusted to match the theoretical lower and upper limits. The zero pressure of the absolute pressure transmitter must be reduced to 1/10–1/20 of the allowable error. 14.5 During testing, the pressure signal is gradually increased from the lower limit value to each testing point; the output values are read and recorded up to the upper limit. Then, the pressure signal is gradually decreased in the opposite direction to each testing point, and the output values are read and recorded until the lower limit value is reached. One cycle is defined as such an inspection of the up and down strokes. In case of doubts or during arbitration, 3 cycles of testing are required. During calibration, it is not allowed to adjust the zero point and range, nor is it permitted to tap or vibrate the transmitter. As the inspection point is approached, the input pressure signal should be applied slowly enough to avoid overshoot. The upper limit is checked only during the upward stroke, while the lower limit is checked only during the downward stroke. 14.6 The basic error of the transmitter is calculated using formula (2). △A = Ad – As In equation (2), △A represents the basic error value at each calibration point of the transmitter (expressed as an absolute error, in mA or kPa) ； △Actual output values (mA or kPa) at each calibration point for the upper or lower travel on the d-zao transmitter ； The theoretical output values (mA or kPa) at each calibration point for the As-zao transmitter. 15 Verification of return error: The verification of the transmitter’s return error is carried out simultaneously with the verification of its basic error. The return error can be calculated using formula (3). △h=∣Ad1-Ad2∣ (3), where △h represents the return error value (expressed as an absolute error, in mA or kPa) ； Ad1 and Ad2 represent the actual output values during the upper and lower strokes at each testing point, respectively; the arithmetic averages (in mA or kPa) are taken over 3 cycles. 16 Verification of the effect of static pressure 16.1 Connect the high and low pressure chambers of the differential pressure transmitter to the atmosphere and measure the lower limit value of the output. 16.2 A static pressure is applied, gradually changing from atmospheric pressure to the rated operating pressure; after 3 minutes of stabilization, the lower limit value of the output is measured, and the difference between this value and the lower limit value of the output at atmospheric pressure is calculated. 16.3 Transmitters with adjustable input ranges shall, unless otherwise specified, be calibrated for the effect of static pressure at the minimum range. It should be returned to its original range after calibration. 17 Electrical performance testing of electric transmitters 17.1 Testing the effect of an open output circuit 17.1.1 Apply a pressure signal at 50% of the input range, and disconnect each output terminal for 5 minutes in sequence. 17.1.2 Restore the wiring, and perform a basic error verification in one cycle using the method described in Article 14 (zero adjustment is allowed only before the verification). Calculate the basic error and return error using equations (2) and (3), and check whether the results still meet the requirements. At the same time, the range change of the output must be calculated using formula (4). △Ar=∣(Aˊdmax-Aˊdmin)-( Aˊdmax-Aˊdmin)∣ Equation (4): Here, △Ar represents the range variation caused by the open-circuit effect at the output, in mA ； Aˊdmax and Aˊdmin represent the upper and lower output values (in mA), respectively, measured after the output has recovered from an open circuit condition ； Aˊdmax and Aˊdmin represent the upper and lower limit output values obtained during basic error detection, respectively; the arithmetic average (in mA) is taken for each of the 3 cycles. 17.2 Verification of the output AC component 17.2.1 Connect a specified sampling resistor in series with the load resistance in the output circuit. 17.3.2 Input pressure signals at 10%, 50%, and 90% of the range respectively, with the load resistance set to its maximum and minimum values, and measure the effective value of the AC voltage across the sampling resistor. 17.3 Testing of insulation resistance: Disconnect the power supply to the transmitter, short-circuit the terminals of each circuit, and use an insulation resistance meter to make measurements at the locations specified in section 6.3; read the value after it has stabilized for 10 seconds. 17.4 Inspection of insulation strength: Disconnect the power supply to the transmitter, short-circuit the terminals of each circuit, and use a dielectric strength tester to make measurements at the locations specified in section 6.4. During the measurement, the test voltage should be increased gradually from zero to the specified value and held at that level for 1 minute, to check for any breakdowns or arcing. Finally, the voltage should be reduced gradually back to zero, and the power supply to the equipment should be turned off. To ensure safe operation during testing and to minimize the risk of damage to the transmitter, it is possible to set on the dielectric strength tester the leakage alarm current value specified by the manufacturer, and use whether the leakage current in the insulated circuit exceeds this specified value as the basis for determining the result of the insulation strength test. 18 Calibration for the effect of changes in air supply pressure in pneumatic transmitters: Keep the transmitter’s output at its upper limit value; when the air supply pressure changes from 140 kPa to 154 kPa, and also from 140 kPa to 126 kPa, read the output signal and calculate the amount of change. 19 Calibration of the effect of out-of-range conditions on pneumatic transmitters: Gradually increase the pressure (or reduce it), bringing the pressure in the transmitter’s measurement chamber to the specified value; maintain this pressure for 10 minutes. Then reduce the input pressure to the lower limit value, and after 5 minutes, measure the lower and upper limits of the output signal. Calculate the lower limit value of the output and the change in range; the change in range can be calculated using equation (4). 5 Handling of calibration results and calibration cycle 20 Handling of calibration results 20.1 The basic error (the maximum value of the errors over 3 measurement cycles), the return error, as well as the lower output limit and range variations caused by influencing factors must all comply with the allowable error limits specified in the regulations. When the allowable errors are expressed in terms of absolute values, they can be calculated using formula (5). △=±In equation Am c% (5), △ Jujubes represents the allowable error expressed in terms of absolute error (in mA or kPa) ； Output range specified for the Am-jujube transmitter (mA or kPa) ； C is the allowable error index specified in Tables 1 to 5. 20.2 The data obtained from the calibration must be rounded after calculation using formulas, and the rounding error resulting from this rounding should be less than 1/20 of the transmitter’s allowable error. The rounding rule is: if the leftmost digit to be discarded is less than 5, it is omitted ； When it is greater than 5 (including 5 followed by a non-zero number), carry 1, that is, increment the retained trailing digits by 1 ； When the leftmost digit of the number to be discarded is 5, and there are no digits following it or all of them are 0, if the remaining digit is odd, then 1 is carried over; if it is even, then the digit is discarded. The determination of the test result is based on the rounded data. 20.3 A calibration certificate shall be issued for transmitters that pass the calibration ； For those that fail, a test result notification is issued, indicating the defective items. 21 Calibration cycle: For transmitters that are subject to mandatory calibration, the calibration cycle is half a year. For transmitters used in other applications, the cycle can be determined flexibly based on the conditions of use, their importance, and the stability of the transmitters themselves; however, it is generally one year.