Process Engineering Division – Instrumentation and Automation Section – Daily Question – Question dated 2020-09-08

It mainly includes the selection of the standard gauge range and the selection of the standard gauge accuracy

I. Regarding standard instruments: First, select standard measuring devices in accordance with the metrological verification regulations; the uncertainty of these standard instruments must meet the requirements of those regulations, and it should be between 1/3 and 1/10 of the measurement uncertainty of the instrument to be calibrated. For example, the precision digital pressure gauges of the HS-YBS-WY series produced by Jiangsu Jinhu Huasheng Measurement Instrument Co., Ltd. has a precision of 0.05% and 0.1%; pressure transmitters and pressure gauges that are calibrated can achieve good calibration results with a precision within the range of 0.25% to 2.5%. Generally speaking, the standard instruments currently used to calibrate digital multimeters are mainly the 5000 series from FLUKE and the 9000 series of multi-functional calibrators from WAVE2TAKE. However, in our work we have found that failure to carry out the necessary self-calibration as indicated during startup, along with the failure to detect in a timely manner the deviations of the standard instruments within certain measurement ranges, can cause their outputs to deviate from the set values. This may lead to measurement errors and potentially result in incorrect judgments regarding whether the instruments under inspection are out of spec. The solution to this problem is: first, perform self-calibration strictly in accordance with the requirements for turning on standard instruments. Additionally, standard laboratories are equipped with both multi-functional calibrators and digital multimeters with high accuracy (usually 8.5 bits), which requires us to regularly compare these two types of standard instruments during daily work. During mutual checks, if there is a lack of consistency between any two levels or values, it is necessary to conduct a thorough examination to identify the cause; this should become a habit and be carried out regularly. II. Ensuring sufficient preheating time for instruments: The calibration procedures and/or technical specifications specify that the instruments to be calibrated as well as the standard devices must be fully preheated before testing can begin, and every calibrator is well aware of this. However, sometimes due to time pressure and the user’s urgent need, calibration is carried out before the instruments to be calibrated or the standard instruments have been properly preheated; in such cases, the measured results may exhibit measurement errors. The solution to this problem is to perform preheating in strict accordance with the requirements specified in the calibration procedures and the technical manual. The preheating time of the instrument cannot be shortened, as it is only by reaching this preheating time that the accuracy of the instrument’s measurements can be ensured. III. When calibrating the high-current range, it is important to control the timing of reading values. During calibration of this range, if the current is high for an extended period, the sampling resistors in the instrument under test may heat up, causing a change in their resistance value and thus leading to inaccurate measurement results. Solution: Start taking readings once the numbers become stable, without waiting for a long time. IV. Precautions for calibrating the resistance range. The calibration of the resistance range is divided into calibration for low-resistance values and calibration for high-resistance values. When calibrating the low-resistance range, a four-wire system should be used in order to eliminate the effects of the connecting wires. Fifthly, when calibrating the low-voltage range, it is necessary to take into account zero-point adjustment; during calibration of the low-voltage range, a relatively small voltage is applied to the input signal by the instrument itself. If this value is not removed during calibration, it is possible for the instrument being calibrated to show deviations from the specified specifications. Solution: Some digital multimeters have a null (zero) button on their front panel. When the input voltage is 0 mV (or when the instrument’s input terminals are shorted together with a wire), the meter usually does not display 0, but rather a value that deviates from 0. In such cases, it is necessary to press the null button to reset the reading to zero. Some digital multimeters require that the short-circuit calibration zero point provided with the instrument be connected before calibration; therefore, zero-point calibration must be carried out in accordance with this requirement. Do not underestimate this zeroing effect; sometimes it is just this small difference that causes the instrument under test to be misclassified as being out of range. VI. Standard instruments and the instruments to be calibrated must be properly and reliably grounded. Grounding is essential for accurate measurement readings by these instruments; the grounding wires must be installed in accordance with relevant requirements, and it is absolutely not permissible to use the grounding wires from three-phase AC systems as grounding wires for the instruments. Some digital multimeters have very high requirements regarding grounding; if the device is not grounded, or the grounding is incorrect or inadequate, the readings of the instrument under test can differ significantly from the output of the standard device in certain measurement ranges. In such cases, it is even more necessary to check the grounding wires. Handling method: The laboratory shall properly install grounding wires in accordance with the grounding requirements; whenever a measurement is taken, both the instrument being calibrated and the standard instrument must be properly grounded. VII. Pay attention to choosing the correct range properly. When calibrating a digital multimeter, especially for 1V voltages, since readings can be taken on both ranges, it is necessary to read the value according to the appropriate range based on the value being measured

When selecting a standard table, the accuracy and stability of the standard instrument must be taken into account; the maximum allowable tolerance of the standard instrument should be less than 1/3 of the maximum allowable tolerance of the instrument under inspection.

When selecting a standard table, the accuracy and stability of the standard instrument must be taken into account; the maximum allowable tolerance of the standard instrument should be less than 1/3 of the maximum allowable tolerance of the instrument under inspection

When selecting a standard table, the accuracy and stability of the standard instrument must be taken into account; the maximum allowable tolerance of the standard instrument should be less than 1/3 of the maximum allowable tolerance of the instrument under inspection.

I. Regarding standard instruments: First, select standard measuring devices in accordance with the metrological verification regulations; the uncertainty of these standard instruments must meet the requirements of those regulations, and it should be between 1/3 and 1/10 of the measurement uncertainty of the instrument to be calibrated. II. Ensuring sufficient preheating time for instruments: The calibration procedures and/or technical specifications specify that the instruments to be calibrated as well as the standard devices must be fully preheated before testing can begin, and every calibrator is well aware of this. However, sometimes due to time pressure and the user’s urgent need, calibration is carried out before the instruments to be calibrated or the standard instruments have been properly preheated; in such cases, the measured results may exhibit measurement errors. The solution to this problem is to perform preheating in strict accordance with the requirements specified in the calibration procedures and the technical manual. The preheating time of the instrument cannot be shortened, as it is only by reaching this preheating time that the accuracy of the instrument’s measurements can be ensured. III. When calibrating the high-current range, it is important to control the timing of reading values. During calibration of this range, if the current is high for an extended period, the sampling resistors in the instrument under test may heat up, causing a change in their resistance value and thus leading to inaccurate measurement results. Solution: Start taking readings once the numbers become stable, without waiting for a long time. IV. Precautions for calibrating the resistance range. The calibration of the resistance range is divided into calibration for low-resistance values and calibration for high-resistance values. When calibrating the low-resistance range, a four-wire system is required to eliminate the influence of the connection wires. V. When calibrating the low-voltage range, attention should be paid to zero-point adjustment. When calibrating the low-voltage range, a relatively small voltage is applied to the input signal by the instrument’s own input circuitry; if this value is not removed during calibration, it is possible for the instrument being calibrated to exceed its specified tolerance limits. Solution: Some digital multimeters have a null (zero) button on their front panel. When the input voltage is 0 mV (or when the instrument’s input terminals are shorted together with a wire), the meter usually does not display 0, but rather a value that deviates from 0. In such cases, it is necessary to press the null button to reset the reading to zero. Some digital multimeters require that the short-circuit calibration zero point provided with the instrument be connected before calibration; therefore, zero-point calibration must be carried out in accordance with this requirement. Do not underestimate this zeroing effect; sometimes it is just this small difference that causes the instrument under test to be misclassified as being out of range. VI. Standard instruments and the instruments to be calibrated must be properly and reliably grounded. Grounding is essential for accurate measurement readings by these instruments; the grounding wires must be installed in accordance with relevant requirements, and it is absolutely not permissible to use the grounding wires from three-phase AC systems as grounding wires for the instruments. Some digital multimeters have very high requirements regarding grounding; if the device is not grounded, or the grounding is incorrect or inadequate, the readings of the instrument under test can differ significantly from the output of the standard device in certain measurement ranges. In such cases, it is even more necessary to check the grounding wires. Handling method: The laboratory shall properly install grounding wires in accordance with the grounding requirements; whenever a measurement is taken, both the instrument being calibrated and the standard instrument must be properly grounded. VII. Pay attention to choosing the correct range. When calibrating a digital multimeter, especially for 1V voltages, since readings can be taken on both ranges, it is necessary to read the value according to the appropriate range based on the value being measured.

When selecting a standard table, the accuracy and stability of the standard instrument must be taken into account; the maximum allowable tolerance of the standard instrument should be less than 1/3 of the maximum allowable tolerance of the instrument under inspection

When selecting a standard table, the accuracy and stability of the standard instrument must be taken into account; the maximum allowable tolerance of the standard instrument should be less than 1/3 of the maximum allowable tolerance of the instrument under inspection