In order to calibrate the AC/DC voltage, AC/DC current and resistance of the 5500A, 5502A, 5520A, 5522A and other models, the five main functions, the standard equipment used must meet the requirements of calibration uncertainty ratio, ie instrument The ratio of uncertainty to the uncertainty of the calibration standard is at least 4:1 or 3:1.
This article discusses the requirements for the measurement uncertainty of these five calibrations and analyzes the calibration uncertainty and uncertainty ratio when using different standard equipment. The analysis shows that to complete the calibration of these five functions, the 8508A high precision is needed. Digital multimeter, A40B precision shunt, 5790A AC voltage measurement standard or other similar standard instruments; calibrating Guardbanding technology or using other measuring instruments at several calibration adjustment points of DC voltage and resistance.
I. Introduction
In the DC low-frequency electrical calibration laboratory, multi-product calibrators play an important role. Among them, Fluke's 55xx series multi-product calibrators are the most widely used calibrators. Metrology personnel use them to determine various types of calibrators. meter , such as digital multimeters, power meters, ohmmeters, switchboard meters, recorders, clamp meters, oscilloscopes, thermal tools, etc. Using a single calibrator can take most of a small calibration laboratory. Calibration load. In order to ensure the traceability of the value and ensure the calibration quality, these calibrators themselves also need to calibrate the traceability regularly. These calibrators have many functions, wide range, strong drive capability, and high calibration traceability requirements. How to calibrate reliably? These calibrators ensure reliable traceability, which is a very technical job. At present, some calibration laboratories in China have some problems in calibrating these calibrators. Sometimes, their calibration uncertainty It does not meet the technical requirements. This article analyzes these issues and proposes solutions.
Second, there are problems
When calibrating the instrument, there is a basic requirement that the uncertainty of the instrument being calibrated is much greater than the uncertainty of the calibration standard. The ratio between them is called the uncertainty ratio TUR. General requirements TUR is at least greater than 4:1 Or 3:1. When the requirements are met, calibration or verification can be performed. If the requirements are not met, comparisons between instruments can only be performed.
ISO 17025 clearly requires calibration results to report measurement (calibration) uncertainty, and calibration laboratories or test laboratories should have and use a fixed procedure to evaluate measurement uncertainty when doing all calibration measurements. China National Laboratory Accreditation Committee CNAS -CL07:2011 "Requirements for Measurement Uncertainty" requires the effective implementation of ISO 17025. The calibration uncertainty is reported from point to point in the calibration results. The reporting uncertainty is to specify the calibration ability of the laboratory in each calibration work. Whether to meet the requirements, to ensure the quality of calibration.
Fluke 55XX series calibrators, including 5500A, 5502A, 5520A, 5522A and other multi-calibrators, have nearly 2,000 units in China, serving all levels of measurement and calibration laboratories and research institutes. For multi-product calibrators themselves Traceability has always been a difficult problem. Since our country has not yet issued calibration procedures or calibration specifications for these calibrators, calibration laboratories in China use different standard instruments when calibrating these calibrators. The level of calibration is also very different.
For example, the following is part of a calibration certificate for a calibration laboratory calibration 5502A:
Standard Name: High-accuracy voltage, current, resistance measurement standard device
As can be seen from the above extract of the certificate, this "high precision voltage, current, resistance measurement standard device" is actually a Fluke high-precision digital voltmeter 8508A. The extended uncertainty listed is the total uncertainty of the year of 8508A, The confidence level is 95%. However, in this calibration certificate, each measurement function only gives the expanded uncertainty of one test point, which is also the optimal uncertainty of the measurement function. The user cannot know this measurement standard. At other measurement point uncertainties, it is impossible to know whether or not the requirements of the uncertainty ratio are satisfied at all other test points. It can be said that this calibration uncertainty reporting method does not meet the national regulations and does not meet the requirements of users. The correct reporting method should fully meet relevant regulations and report calibration uncertainty by test points.
At present, China National Institute of Metrology and other national standards and measurement agencies have begun to report calibration uncertainty point by point in the calibration certificate, and this work has been carried out for several years. In the future, the state has more stringent requirements for the entire measurement activities. Grade requirements will be higher and more rigorous.
Third, calibration 55XX series calibrator
The 55XX series calibrators include the new models 5080A, 5502A, 5522A, and past older models 5502A and 5522A. Their main functions are the following:
· DC voltage ±(0~1020) V
· AC voltage 1.0 mV~1020 V (1Hz~1MHz)
DC current ±(0~20.5) A
· AC current 29 mA~20.5 A, (1Hz~1MHz)
· Resistance 0 W~1100 MW
In the past, many laboratories calibrated these multi-product calibrators by using the 8508A eight-position semi-precision DMM for direct measurement. Can an 8508A really complete the calibration of the 55XX Series calibrators? We have to work on different models. The specific analysis is done. In the maintenance manuals of 5500A, 5502A, 5520A, 5522A, the calibration requirements and calibration methods of these instruments are described in detail. In particular, calibration adjustment points for each function are proposed. Only those calibration adjustment points are guaranteed. Accuracy can only ensure the full range of performance of the instrument's various functions. When checking the performance of the instrument, these calibration adjustment points should be included as much as possible. If the accuracy of the calibrator is found to be degraded, the accuracy of the instrument must be restored by adjusting these points. Here, we Take the 5502A with a lower accuracy index as an example. According to the various calibration adjustment points suggested in the 5502A specification, the measurement uncertainty of each calibration point is actually calculated. What kind of standard equipment can be used to analyze the 5502A can be calibrated. We will find that there are many Functionality, especially the current measurement function, the 8508A does not meet the accuracy requirements for calibrating multi-product calibrators.
1. DC voltage calibration
According to the 5502A Maintenance Manual, 5502A DC voltage is divided by zero, there are eight calibration adjustment points. At each calibration adjustment point, we calculate the total uncertainty of the 5502A, the total uncertainty of the DC voltage measurement function of the 8508A, calculation The results are shown in Table 1. From Table 1, it can be seen that the performance of the DC voltage of the 8508A is very good. At the 5502A DC voltage calibration adjustment point, the total uncertainty of the 5502A DC voltage and the total uncertainty of the 8508A TUR are both Above 7, the basic requirements for calibration can be met. Therefore, the 5502A DC voltage output function can be directly calibrated using the 8508A DC voltage measurement function.
Before calibration, you should first prepare the calibration. First of all, all instruments should be warmed up to a sufficient time after starting. Then, the 8508A and 5502A should do zero calibration of the instrument and clear the influence of the zero offset on the measurement results. , Can carry on the calibration work of each test point. When the 5502A exports the small voltage below 3V, should use the low thermoelectricity potential test lead, eliminates the measurement error caused by the thermoelectric potential of the wire to the measurement result, guarantee the measurement accuracy.
Although the 5520A and 5522A DC voltage output uncertainty is much better than that of the 5502A, in the domestic commonly calibrated 0.1V, 1V, 10V, 100V and 1000V test points, it and the 8508A's calibration uncertainty ratio TUR are still greater than 3 However, at the manufacturer's specified calibration adjustment points: 3V, -3V, and 30V TUR are only 2.2, 2.7, and 2.7. These test points do not match the DC voltage range of the 8508A, which is the 20V range of the 8508A or 200V range. Measured, the measured value is only 15% of the range, making the 8508A measurement uncertainty greatly increased. If you need to adjust these points, you need to use a voltage divider to convert the measured value to measure at full scale, you can solve this problem. The Guardbanding technique can be applied to more strictly control the offset of the calibrator to meet the calibration requirements.
2. Resistance function calibration
5502A calibrator can output continuous adjustable resistance 0-1100MΩ. According to 5502A specification, 5502A AC voltage has 16 calibration adjustment points. You can use the 8508A to directly calibrate each 5502A calibration adjustment point, which can meet the requirements of measurement at each calibration adjustment point. Certainty requirements, calibration uncertainty ratios are greater than 5. Can be directly calibrated with the 8508A resistor function to meet the measurement uncertainty requirements. Table 2 lists the calibration resistance of the 5502A for each calibration calibration point at 110 kW, 1.1 MW , 11 MW, 110 MW For four calibration points, the 8508A's measurement current needs to be set to LoI low current mode to fit the 5502A's operating current range. In low current mode, the 8508A's measurement uncertainty will increase slightly, but The calibration uncertainty ratio is still greater than 4 to meet the measurement uncertainty requirements.
Generally, when measuring a small resistance below 100kΩ, you should try to use a four-wire resistance measurement method for calibration. This is due to the additional error caused by the test lead resistance and contact resistance when measuring two-wire resistance. When measuring small resistance, it often rang. Larger. Using the four-wire resistance measurement method, separate the voltage drop loop on the current excitation loop and the measuring resistor. The measured voltage is not affected by the voltage drop in the current loop. It is the true voltage across the measured resistance. Eliminates measurement errors caused by test leads and contact resistance. When using high-resistance calibration, shielded wires should be used to reduce interference and reduce the impact of environmental factors on measurement readings.
During calibration, care should be taken to use the 8508A's conventional resistance measurement function. Do not use the HiVΩ high-voltage resistance mode. In this mode, the excitation current when measuring the resistance is 10 times that of the conventional resistance measurement mode, which may result in compliant voltages up to 240V. , May damage the 55xxA series calibrator resistor function.
In addition, when calibrating resistance function of 5502A, it is better to use the manual range of the 8508A resistance measurement function according to the value of the test point, to avoid setting the autoranging, the measuring instrument is looking for the appropriate range, and the calibrator is also looking for the appropriate working current. The two instruments can not be selected as soon as possible to the appropriate range and the appropriate current range, and can not be measured normally, and the correct measurement results will not be obtained for a long time.
When calibrating the 5520A and 5522A resistance functions, the two calibration test points at 36 MΩ and 110 MΩ have an uncertainty of 1.7 and 2.7. The calibration requirements cannot be met. The calibrating boundary (Guardbanding) can be applied. When calibrating, more strictly Control the offset of the calibrator to meet the calibration requirements.
3. AC voltage calibration
According to the 5502A specification, the 5502A AC voltage has 13 calibration adjustment points. At each calibration adjustment point, we calculate the total uncertainty of the 5502A, and the total uncertainty of the AC voltage measurement function of the 8508A. The results are shown in Table 3. From Table 3, it can be seen that the performance of the 8508A AC voltage can not meet the measurement uncertainty requirements for calibrating the 5502A AC voltage at most points. For example: 5502A output 30V, 100Hz AC voltage year indicator: 0.032%, The 8508A annual index is 0.013%, and the TUR ratio is only 2.3, which cannot satisfy the requirement that the uncertainty ratio is more than 3 times.
The reason for this situation is also related to the mismatch between the two instruments. The 5502A AC voltage calibration adjustment point is mostly a multiple of 3, such as: 30mV, 300mV, -3V, -30V, 300V; and the 8508A has a range of 2 Multiples, such as: 200 mV, 2V, 20V, 200V. When using the calibration, only 15% of the range of the 8508A can be used as the test point, which greatly increases the measurement uncertainty of the 8508A.
How to solve this problem? The answer is that the use of the more accurate 5790A. 5790A is a high accuracy AC voltage measurement standard, it has both excellent accuracy of the thermoelectric transfer standard, and the digital multimeter is easy to use. Features, is a standard equipment used to calibrate various AC calibrators. Using the 5790A not only can calibrate the 5502A and 5522A, but also can calibrate 5700A and 5720A and other high-precision AC measurement instruments.
Here, we analyze the TUR when calibrating a 5502A multi-product calibrator with the 5790A. At the 5502A AC voltage calibration adjustment point, we calculated the total uncertainty of the 5502A, the total uncertainty of the 5790A AC voltage measurement function, The calculation results are shown in Table 4. From Table 4, it can be seen that except for a test point of 300V70kHz, the uncertainty ratio is 5.3. The uncertainty ratio of all other test points is greater than 13.
When calibrating the AC voltage, the test leads can use ordinary wires. The test leads should be as short as possible, especially at higher frequencies, the shorter the better, and use shielded wires to reduce signal attenuation and avoid picking up interfering signals.
When using the 5790A to calibrate the 5520A and 5522A AC voltage functions, the uncertainty ratio TUR is greater than 3 at all calibration adjustment points, which can meet the calibration requirements.
4. Current function calibration
The calibrator current traceability problem has always been a difficult task. Some people like to use the high-accuracy digital multimeter's current measurement function to calibrate the current output function of a common calibrator. However, the accuracy of the digital multimeter's current measurement function can meet the accuracy. What is the degree of demand? The answer is no. The digital multimeter measures current using a built-in shunt. When the measured current flows through the shunt, a voltage proportional to shunt resistance is generated across the shunt, measuring both ends of the shunt. The current can be measured by the voltage. When the current is large, considerable power is dissipated in the shunt. If the heat dissipating effect of the shunt is poor, the shunt temperature will increase, causing the shunt resistor value to change. The error will be very large. The high-precision digital multimeter has a limited volume. The built-in current diverter used in the current measurement function is generally relatively small, and the space in the digital meter is limited, the heat dissipation is not good enough, and the error at high current is large. It can be said that The digital multimeter that uses the built-in shunt generally has low accuracy of current measurement.
If you directly use the current range of the 8508A DMM to calibrate the current function of the multi-product calibrator, you will find that the 8508A does not match the current range of the 5502A, 5522A, and the 5502A and 5522A calibrators have a higher accuracy of the current output. Although the 8508A is the best in the DMM, the measurement accuracy still cannot meet the calibration requirements. At many calibration adjustment points, the uncertainty ratio is less than 3. Even with the dc current function of the 5502A, the 8508A is difficult to perform. 4 lists the 5502A current output uncertainty, the 8508A current measurement uncertainty, and their uncertainty ratio TUR. It can be seen that the 8508A directly calibrates the 5502A DC current at each calibration adjustment point. At the measurement point, that is, all calibration adjustment points above 30 mA, the uncertainty ratio is less than 2.
The solution to current traceability is to use the Fluke A40B. The A40B is a new high performance standard shunt. Using the A40B with the 8508A can greatly improve current measurement accuracy. Use the 8508A+A40B as the current measurement standard. The DC current and AC current of the 5502A can achieve satisfactory results. At each calibration adjustment point of the 5502A DC current, we calculated the uncertainty of 5502A, the square root of the uncertainty of 8508A and the uncertainty of A40B. , And use this as the total uncertainty, the results are listed in Table 5. From Table 5, we can see, according to the 5502A current output uncertainty of each calibration adjustment point and the 8508A + A40B total uncertainty calculated uncertainty The degree ratio TUR is at least greater than 5.3.
A40B with 5790A AC voltage measurement standard can achieve better performance, not only can calibrate 5502A, 5522A AC current, but also can be calibrated 5700A, 5720A and other high-precision AC measurement instrument AC. 5500A, 5502A, 5520A, 5522A AC current calibration There are many adjustment points. Table 6 lists the uncertainty of the main calibration adjustment point for the 5502A AC current, the square root of the uncertainty of the A40B measurement and the AC voltage of the 5790A, and uses this as the total uncertainty. Degrees, the calculation results are shown in Table 6. It can be seen that the TUR achieved by the calibration is very good, and the TUR of all calibration adjustment points is at least 8.6.
Since the 5502A and 5520A, 5522A's AC/DC current functional output range and performance specifications are exactly the same, the analysis of the 5502A multi-product calibrator AC/DC current function calibration is fully applicable to the 5520A, 5522A multi-product calibrator.
Fourth, the conclusion
Any standard calibration task that satisfies the requirement of measurement uncertainty is the most basic requirement. It characterizes the quality of this measurement and also represents the credibility of the measurement result. Can the measurement be satisfied in any calibration work? The requirement of certainty degree is an important index to examine the capabilities of a calibration laboratory. ISO 17025 has a clear requirement for reporting measurement uncertainty. In order to carry out calibration work for the specification of multi-product calibrators such as 5500A, 5502A, 5520A, 5522A, etc., it is necessary to use Multiple calibrators are used to accomplish this task. This article only discusses the calibration of the AC/DC voltage, AC/DC current, and resistance of the multi-product calibrator. To complete these five functions, an 8508A high-precision digital multimeter is required. , A40B precision shunt and 5790A AC voltage measurement standard; In DC voltage and resistance several calibration adjustment points, also need DC voltage divider or apply calibration Guardbanding technology. If you want to calibrate multi-product calibrator frequency accuracy In order to calibrate the temperature function of a multi-product calibrator, a standard thermometer and suitable Grooves or the sink (oil cup or cups). That is, whether a calibration, must meet the requirement of uncertainty.
