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This post was last edited by yunrun on 2019-6-17 09:35. Modifications such as the range and zero point of analog capacitive pressure transmitters are carried out by adjusting potentiometers. This post outlines the methods for calibrating the zero point, range, drift, linearity, and damping of analog capacitive pressure transmitters, for everyone’s reference. 1. Wiring for calibrating capacitive pressure transmitters (analog). The wiring for calibrating capacitive pressure transmitters (analog type) is shown in the figure below. When using a piston pressure gauge as the standard instrument, it serves both as the pressure source and as the standard instrument, so there is no need for the standard pressure gauge shown in the diagram. http://yunrun.com.cn/upload/201906/14/201906141801498501.png Capacitive pressure transmitters (analog type) output a current signal of 4-20mA. However, the accuracy of ammeters is limited; therefore, in calibration, digital voltmeters are used to measure the voltage across the standard resistor, thereby obtaining the measurement results indirectly. The theoretical output value of a capacitive pressure transmitter (analog type) must be calculated with an accuracy of 0.01%, and its unit can be current or voltage. At a load resistance of 250Ω, the output voltage is 1–5VDC. In the diagram, the load resistance is 500Ω, and the output voltage ranges from 2 to 10VDC, which allows for an increase in the sensitivity of the digital voltmeter’s readings. The capacitive pressure transmitter (analog type) with a precision of 0.2% has an allowable error of: (5-1)V×(±0.002)=±8mV ； (10-2)V×(±0.002)=±16mV ； (20-4)mA×(±0.002)=±32μA 2. Calibration procedures for capacitive pressure transmitters (analog): ① Adjustment of the zero point and range of capacitive pressure transmitters (analog). The zero point and range of analog capacitive pressure transmitters can be adjusted continuously, over a wide range; moreover, their range can be adjusted continuously across the entire measurement scale. Since the output of capacitive pressure transmitters (analog type) is 4-20mA, the starting current of the instrument is not “0”; therefore, adjusting the range affects the zero point value. The impact is minimal in the absence of drift, while the change in the zero point caused by range adjustment is roughly equal to the percentage change in the range setting. However, zero-point and positive/negative offset adjustments have almost no effect on the range. Therefore, before calibrating a capacitive pressure transmitter (analog type), it must first be set to the range that will be used in practice, and then the zero point should be adjusted. If necessary, the position of the positive and negative migration switches should also be changed to determine the desired direction of migration; before making adjustments, turn the damping potentiometer counterclockwise to its extreme position first. Calibration of capacitive pressure transmitters: yunrun.com.cn/tech/2541.html. Example of calibration: There is a capacitive pressure transmitter (analog type) whose original range was 6–30 kPa; now it is necessary to change this range to 30–40 kPa, that is, to shift it upward by 30 kPa, resulting in a range of 10 kPa. The operation is as follows: A. First, cancel the migration; that is, when the input pressure is 0, adjust the zero potentiometer so that its output is 4mA, at which point the range becomes 0-24kPa. B. Adjust the range to achieve the desired value. Since the range is to be reduced, the range potentiometer should be turned clockwise, so that when the input pressure to the capacitive pressure transmitter (analog type) is 0, the output will be: 4mA × (original range of 24 ÷ desired range of 10) = 9.6mA. If the range needs to be increased, for example from 10 kPa to 24 kPa, then the range potentiometer must be turned counterclockwise, and the adjustment should be made at the full-scale output; that is, when the input pressure is 10 kPa, the output of the capacitive pressure transmitter (analog type) should be set to: 20mA × (original range of 10 ÷ desired range of 24) = 8.333mA. C. Reset to zero: when no pressure is applied, the output is 4 mA; at this point, the range of the capacitive pressure transmitter (analog type) is very close to 0–10 kPa. D. Check the full-scale output value again; if necessary, fine-tune the range and zero point. For capacitive pressure transmitters (analog type), adjusting the zero point does not affect the range, but adjusting the range affects the zero point. To compensate for this effect, an overshoot of 25% can be used. For example, when the input pressure is 10 kPa, the output current is 19.900 mA. The adjustable range potentiometer results in an output of: 19.900 + (20.000 – 19.900) × 1.25 = 20.025(mA). Since the range has increased by 0.125mA, the zero current will increase by 1/5×0.125mA. The zero-adjustment potentiometer reduces the output from 20.025mA to 20.000mA. Finally, make another adjustment to the range and zero point. E. Shift the zero point positively to the desired starting point of the range, namely 30 kPa. If the amount of shift is small, the zero potentiometer can be adjusted counterclockwise to reach the desired starting point of the range; however, in this case the shift amount is large, so the power supply should be turned off and the shift switch placed in the positive shift position. Then power it on and adjust the zero point to 30 kPa. ②Methods for adjusting the positive and negative drift of analog capacitive pressure transmitters: In the production process, it is necessary to keep certain process parameters (such as pressure, flow rate, liquid level, etc.) at a specific value. When the variation range of these parameters is not too large, it is possible to improve the quality and sensitivity of process control by adjusting the range of the analog capacitive pressure transmitter. That is, the range of such transmitters does not start from zero, but from a certain value. For example, if a capacitive pressure transmitter has a range of 0–6 kPa, and the actual variation range of the parameter being measured is 3–4 kPa, then when this parameter changes by 60 Pa, the percentage change would be 60÷6000 = 1% using the original range. If the range is adjusted to 3–5 kPa, the same 60 Pa change in the parameter would result in a percentage change of 60÷2000 = 3%. Thus, the change in output from the analog capacitive pressure transmitter is three times greater in this case. Moving the start point of the range from 0 to 3 kPa is referred to as positive offset; the amount of this offset is 1.5 times the range. However, the offset cannot be increased indefinitely – it cannot exceed the maximum measurement range – nor can it be reduced below the minimum allowable range. The steps for calibrating a capacitive pressure transmitter (analog type) are as follows: A. Before calibration, set the range to the desired value; for example, if a measurement range of 3–5 kPa is required, first set the range to 0–2 kPa and then proceed with the calibration. B. If the amount of migration is small, during positive migration, without applying any input pressure, it is possible to directly adjust the zero potentiometer so that the output is less than 4 mA; thereafter, the pressure value corresponding to positive migration is applied to make the transmitter output 4 mA. C. When the amount of migration is too large, it is necessary to operate the migration switch; that is, the positive or negative migration switch is adjusted according to the required direction of migration, and then the pressure at the specified range starting point is applied, with the potentiometer being adjusted so that the output is 4mA. D. Finally, check the upper measurement limit: when the input pressure reaches this limit, the output should be 20mA. If there is a deviation, the range potentiometer can be adjusted slightly. ③Linear adjustment method for capacitive pressure transmitters (analog type): The linearity of capacitive pressure transmitters is optimized at the time of manufacture, so adjustment is generally not required. If the measurement range is changed significantly and good linearity within a specific range is required, linear adjustment should be performed. The steps are as follows: a. Enter the pressure value at the middle of the desired measurement range, and note down the theoretical and actual values of the output signal. b. Enter the pressure value for the desired range, and adjust the linear potentiometer so that the output is as follows: when the actual value is less than the theoretical value, use the plus sign ； When the actual value is greater than the theoretical value, use a minus sign. c. Re-adjust the range and zero point. ④Damping adjustment method for capacitive pressure transmitters (analog type). Damping is used to suppress rapid fluctuations in the output caused by input variations in the parameter being measured. Its time constant is 0.2–1.67 seconds; at the time of manufacture, the damper is adjusted counterclockwise to its extreme position, resulting in a damping time of 0.2 seconds. It is best to choose the shortest damping time in the application. Since the calibration of capacitive pressure transmitters is not affected by damping adjustments, it is advisable to adjust the damping time on-site according to actual conditions, by turning the damper clockwise to select the desired damping time.