The digital multimeter has hundreds of modes. In order to pick out a suitable digital multimeter is a nerve-racking thing. The problem is how to find a digital multimeter with the appropriate price and performance.
Before choosing, first determine what the measured value is. Are you doing precision measurement work really using the 7th or 8th digit for measurement? If yes, you should purchase a high-precision mesa digital multimeter because of its good stability. Selectable 6 1/2 f bit PDM and VXI compatible DMMs for use in custom systems Generally speaking, many DMMs have high accuracy and resolution but require unequal measurements. Have such high accuracy and resolution. Although the resolution is a design of accurate performance, high accuracy measurements rely on the merits of measurement technology. For example, measuring with an 8 1/2-digit digital multimeter must be very careful and experienced, or else a high-end digital multimeter may not measure good results. On the other hand, there are many low-end digital multimeters that may also be the appropriate choice. If only the measurement accuracy is 5% and there are not many measurement functions, then buy a low-end digital multimeter. Digital multimeter can measure the basic amount: AC, DC voltage, current and resistance. Some people's measurement needs are between the two extremes. What kind of digital multimeter to buy is difficult to decide. Because they demand higher measurement accuracy and resolution, they also want cheaper prices. To make the best choice, it is advisable that they read the instructions and related footnotes carefully. Reading Small Body Words There are many high-precision desk top DMMs and low-end hand-held DMMs. However, the technical indicator expressions of various digital multimeters lack consistency. For example, the accuracy of the DC voltage is expressed as % of reading plus % of range. Some expressed as a percentage of the reading plus several counts, and others expressed as a percentage of the reading plus a few volts. For a particular DMM, if you know how many counts its full value corresponds to, you can convert one expression to another. The above three expressions are often seen.
But under what kind of test conditions can the accuracy be achieved? As can be seen from the description, the specified test conditions are: Accuracy is valid within one year after calibration of the DMM, and the ambient temperature is 23 °C. Earth 5 °C. There are other 90-day accuracy and 24-hour accuracy time limits. DMMs have a variety of accuracy and cannot be directly compared unless they are compared within the same effective time period. In addition, accuracy requirements can only be achieved after preheating for a certain period after power up.
Some digital multimeters have an integrated reference standard source with self-calibration capability, which means that this DMM can guarantee its measurement accuracy even when used outside the 23 °C and 5 °C environment. For example, National Instruments' PXI-4070 digital multimeter ensures its measurement accuracy even after self-calibration, even in a 0 °C to 50 °C environment. If a DMM is used outside the calibration temperature range without calibration, its measurement accuracy must be reduced. For example taking into account its temperature coefficient: ± (5ppm of reading + 1ppm of range) / 0C, it has an additional error of measuring at 50°C instead of 28°C (50°C is the upper limit of 23°C ± 5°C): ± (11Oppm of reading + 22ppm of range). For some digital multimeters, the additional error may be much greater than the accuracy it specifies in the specification. The small font footnote in the description is a supplement to the specified accuracy limits. But giving 6 or even 10 or 12 footnotes is not enough to clarify the problem.
When measuring AC voltage and current, its frequency will also affect the measurement results. A manufacturer segments frequency in one way to give its influence, and another manufacturer segments frequency in another way. For example, a company includes 60Hz (or 50Hz) in the 10Hz-3kHz segment, and another company includes it in the 45Hz-50OHz band. They all use many footnotes to describe what they all want to tell the user.
The same problem exists with the crest factor. The crest factor is defined as the peak divided by its effective value. In general, the signal with a large crest factor only occasionally deviates significantly from its average value. The large peak-emission factor corresponds to a large measurement error. A footnote limits the accuracy of an AC signal. It must also limit the crest factor of the signal under test within the allowable range to ensure the accuracy of the measurement. In addition to the influence of the signal shape on the measurement results, the signal size and frequency must also be limited within a certain range to ensure the accuracy of the measurement results. Otherwise, the input circuit of the Meter will be damaged. For example, the Digital Multimeter in the Fluke 170 Series limits the product of the voltage of the input signal to its frequency, V -Hz, to be less than 1 × 10↑7. Many DMMs have a 1000Vac range, allowing signal frequencies below 10 kHz. The reading rate of the measurement is directly related to the resolution. For example, Signametrics SMX2044 is a 6 1/2 digit DMM. The corresponding reading rate at this resolution is 30 readings per second. To increase the reading rate, you must reduce the resolution (ie, decrease the number of digits). For example, an optional digital multimeter with a reading rate of 1000 readings per second and a resolution of 4 1/2 bits. Similar constraints apply to high-resolution digital multimeters from other manufacturers. In the footnote, the relationship between reading rate and resolution is explained.
Many digital multimeters have different full-scale ranges and are difficult to compare with each other. For example, there are two different DMMs that have a voltage range and are multiplied by a factor of 10 next to them. However, the starting range of a digital multimeter is 4V, and the other starts. The range is 1V. Use them all to measure 1OV. One is measured with 1V × 1O = 1OV, and the other is measured with 4V X 1O = 40V. The former happens to be 10V with a full-scale measurement, while the latter measures 10V with 40V, and 1OV is only 45% of the full-scale value of 25%. Since there are 4 factors between two DMMs, their full-scale resolution is different. However, 10V measurements are made with their actual resolution. This means that there is annoying noise on the lowest bit of the 4OV table that is 4 times greater than the noise measured at 1OV with a 1OV meter. There is no limit to the overall accuracy of low-cost DMMs. The relevant factory representatives were asked about the matter. They said that the accuracy of the measuring instruments is closely related to their use skills. They suggested that the digital multimeter should be calibrated once a year. Although this is an ambiguous answer, calibrating the DMM in the manner it proposes enhances the user's confidence in measurement accuracy.
Measuring resistance is another matter. The details of the description vary greatly. Most methods use current concentrated through the resistor under test, and the resistance can be calculated by measuring the voltage drop across the two terminals. If the open circuit voltage appearing on the resistor under test is too high, there is a problem. Mr. Tee.Sheffer, CEO of Signamertrics, said: “I have seen some digital multimeters at the exhibition. The voltage applied to the resistance to be measured during the performance is more than 10V. This voltage becomes a source and can excite oscillations. Causes a large measurement error.†It is reported that the voltage applied to the measured resistance must be a low volt value. For example, the Fluke Model 179 Digital Multimeter has an open circuit voltage of less than 1.5V across all resistance ranges. Not all current flows through the measured resistance when measuring the resistance, and a small portion is always bypassed, which can cause measurement errors. Some models of DMM eliminate this error voltage and make ohmic readings more accurate. There are also many types of resistors, which themselves have a small voltage source connected to them. For example, in a measurement circuit, different metal phases can generate microvolt-level error voltages at different temperatures. The electrodes of the electrocardiograph (EKG) are attached to the patient's skin. Since the skin naturally has salt and moisture, a large error voltage is inevitable.
The so-called method of eliminating the ohmic error voltage is to use two measurement methods, the first time using a current source to measure the voltage value, and the second time to turn off the current source and then measure the error voltage on the resistance. The first voltage value minus the second voltage value (error voltage) is then calculated and the measured resistance value is calculated. Another method can also eliminate the error voltage and the error voltage due to continuous measurement of the measured resistance itself generating heat. The method is to make the current source equal in magnitude and opposite in polarity to the measured resistance to measure two resistance values, which is different from the aforementioned switch current source measurement method. This method ensures that the magnitude of the current used for the two measurements is constant. If two consecutive measurements are taken within a few milliseconds, the ambient temperature and resistance values ​​are still constant. This measurement shows that the noise contained in the calculation of the resistance value is 50% less than that of the simple error correction method.
Measuring resistance is also more accurate than the two-wire method, namely the 4-wire method and the 6-wire method. In the two-wire method, the resistance of the continuous resistance wire is also connected to the digital multimeter. The resistance of the connection wire is also counted in the measured resistance value, and they cannot be separated. The 4-wire method or kelvin method measures the resistance, using a pair of wires to connect the current source, and the other pair of wires (the sensing wire) introduces the measured voltage drop across the resistor to a digital multimeter for measurement. Since the current flowing through the sensing line is small, the measured resistance value is closer to the true value. Especially when measuring low-value resistance, the resistance value of the connecting wire can be of the same magnitude as the measured resistance value, and the error is large.
If you use the 4-wire method to measure the low value resistance, you can eliminate the lead resistance and make the measurement accuracy better. When measuring the large-value resistance, bypass current will be present anyway. Using the protective measures for the measured resistance can solve the error caused by the bypass current. However, protective measures are usually used when measuring large values ​​of resistance with two wires. With a digital multimeter, it is also possible to make 6-wire measurement resistors, and 4 lines plus two additional protection lines. The 6-wire method is usually used to measure a resistor connected to a resistor network. Other resistors in the network that are connected to the measured resistance are connected to the protection terminal. This effectively isolates the measured resistance from it. The voltage at each end of the resistance to be measured and the voltage at the protection end are generated by the power supplies that are separated from each other. This can also eliminate the error caused by the sense line resistance. As a typical example, high-resolution instruments have 6-wire measurement capability. The 4-wire method is commonly used for 4-bit to 5-digit digital multimeters. These measurement methods are illustrated by footnotes. If there is no footnote, only two-wire resistance can be used.
Before choosing, first determine what the measured value is. Are you doing precision measurement work really using the 7th or 8th digit for measurement? If yes, you should purchase a high-precision mesa digital multimeter because of its good stability. Selectable 6 1/2 f bit PDM and VXI compatible DMMs for use in custom systems Generally speaking, many DMMs have high accuracy and resolution but require unequal measurements. Have such high accuracy and resolution. Although the resolution is a design of accurate performance, high accuracy measurements rely on the merits of measurement technology. For example, measuring with an 8 1/2-digit digital multimeter must be very careful and experienced, or else a high-end digital multimeter may not measure good results. On the other hand, there are many low-end digital multimeters that may also be the appropriate choice. If only the measurement accuracy is 5% and there are not many measurement functions, then buy a low-end digital multimeter. Digital multimeter can measure the basic amount: AC, DC voltage, current and resistance. Some people's measurement needs are between the two extremes. What kind of digital multimeter to buy is difficult to decide. Because they demand higher measurement accuracy and resolution, they also want cheaper prices. To make the best choice, it is advisable that they read the instructions and related footnotes carefully. Reading Small Body Words There are many high-precision desk top DMMs and low-end hand-held DMMs. However, the technical indicator expressions of various digital multimeters lack consistency. For example, the accuracy of the DC voltage is expressed as % of reading plus % of range. Some expressed as a percentage of the reading plus several counts, and others expressed as a percentage of the reading plus a few volts. For a particular DMM, if you know how many counts its full value corresponds to, you can convert one expression to another. The above three expressions are often seen.
But under what kind of test conditions can the accuracy be achieved? As can be seen from the description, the specified test conditions are: Accuracy is valid within one year after calibration of the DMM, and the ambient temperature is 23 °C. Earth 5 °C. There are other 90-day accuracy and 24-hour accuracy time limits. DMMs have a variety of accuracy and cannot be directly compared unless they are compared within the same effective time period. In addition, accuracy requirements can only be achieved after preheating for a certain period after power up.
Some digital multimeters have an integrated reference standard source with self-calibration capability, which means that this DMM can guarantee its measurement accuracy even when used outside the 23 °C and 5 °C environment. For example, National Instruments' PXI-4070 digital multimeter ensures its measurement accuracy even after self-calibration, even in a 0 °C to 50 °C environment. If a DMM is used outside the calibration temperature range without calibration, its measurement accuracy must be reduced. For example taking into account its temperature coefficient: ± (5ppm of reading + 1ppm of range) / 0C, it has an additional error of measuring at 50°C instead of 28°C (50°C is the upper limit of 23°C ± 5°C): ± (11Oppm of reading + 22ppm of range). For some digital multimeters, the additional error may be much greater than the accuracy it specifies in the specification. The small font footnote in the description is a supplement to the specified accuracy limits. But giving 6 or even 10 or 12 footnotes is not enough to clarify the problem.
When measuring AC voltage and current, its frequency will also affect the measurement results. A manufacturer segments frequency in one way to give its influence, and another manufacturer segments frequency in another way. For example, a company includes 60Hz (or 50Hz) in the 10Hz-3kHz segment, and another company includes it in the 45Hz-50OHz band. They all use many footnotes to describe what they all want to tell the user.
The same problem exists with the crest factor. The crest factor is defined as the peak divided by its effective value. In general, the signal with a large crest factor only occasionally deviates significantly from its average value. The large peak-emission factor corresponds to a large measurement error. A footnote limits the accuracy of an AC signal. It must also limit the crest factor of the signal under test within the allowable range to ensure the accuracy of the measurement. In addition to the influence of the signal shape on the measurement results, the signal size and frequency must also be limited within a certain range to ensure the accuracy of the measurement results. Otherwise, the input circuit of the Meter will be damaged. For example, the Digital Multimeter in the Fluke 170 Series limits the product of the voltage of the input signal to its frequency, V -Hz, to be less than 1 × 10↑7. Many DMMs have a 1000Vac range, allowing signal frequencies below 10 kHz. The reading rate of the measurement is directly related to the resolution. For example, Signametrics SMX2044 is a 6 1/2 digit DMM. The corresponding reading rate at this resolution is 30 readings per second. To increase the reading rate, you must reduce the resolution (ie, decrease the number of digits). For example, an optional digital multimeter with a reading rate of 1000 readings per second and a resolution of 4 1/2 bits. Similar constraints apply to high-resolution digital multimeters from other manufacturers. In the footnote, the relationship between reading rate and resolution is explained.
Many digital multimeters have different full-scale ranges and are difficult to compare with each other. For example, there are two different DMMs that have a voltage range and are multiplied by a factor of 10 next to them. However, the starting range of a digital multimeter is 4V, and the other starts. The range is 1V. Use them all to measure 1OV. One is measured with 1V × 1O = 1OV, and the other is measured with 4V X 1O = 40V. The former happens to be 10V with a full-scale measurement, while the latter measures 10V with 40V, and 1OV is only 45% of the full-scale value of 25%. Since there are 4 factors between two DMMs, their full-scale resolution is different. However, 10V measurements are made with their actual resolution. This means that there is annoying noise on the lowest bit of the 4OV table that is 4 times greater than the noise measured at 1OV with a 1OV meter. There is no limit to the overall accuracy of low-cost DMMs. The relevant factory representatives were asked about the matter. They said that the accuracy of the measuring instruments is closely related to their use skills. They suggested that the digital multimeter should be calibrated once a year. Although this is an ambiguous answer, calibrating the DMM in the manner it proposes enhances the user's confidence in measurement accuracy.
Measuring resistance is another matter. The details of the description vary greatly. Most methods use current concentrated through the resistor under test, and the resistance can be calculated by measuring the voltage drop across the two terminals. If the open circuit voltage appearing on the resistor under test is too high, there is a problem. Mr. Tee.Sheffer, CEO of Signamertrics, said: “I have seen some digital multimeters at the exhibition. The voltage applied to the resistance to be measured during the performance is more than 10V. This voltage becomes a source and can excite oscillations. Causes a large measurement error.†It is reported that the voltage applied to the measured resistance must be a low volt value. For example, the Fluke Model 179 Digital Multimeter has an open circuit voltage of less than 1.5V across all resistance ranges. Not all current flows through the measured resistance when measuring the resistance, and a small portion is always bypassed, which can cause measurement errors. Some models of DMM eliminate this error voltage and make ohmic readings more accurate. There are also many types of resistors, which themselves have a small voltage source connected to them. For example, in a measurement circuit, different metal phases can generate microvolt-level error voltages at different temperatures. The electrodes of the electrocardiograph (EKG) are attached to the patient's skin. Since the skin naturally has salt and moisture, a large error voltage is inevitable.
The so-called method of eliminating the ohmic error voltage is to use two measurement methods, the first time using a current source to measure the voltage value, and the second time to turn off the current source and then measure the error voltage on the resistance. The first voltage value minus the second voltage value (error voltage) is then calculated and the measured resistance value is calculated. Another method can also eliminate the error voltage and the error voltage due to continuous measurement of the measured resistance itself generating heat. The method is to make the current source equal in magnitude and opposite in polarity to the measured resistance to measure two resistance values, which is different from the aforementioned switch current source measurement method. This method ensures that the magnitude of the current used for the two measurements is constant. If two consecutive measurements are taken within a few milliseconds, the ambient temperature and resistance values ​​are still constant. This measurement shows that the noise contained in the calculation of the resistance value is 50% less than that of the simple error correction method.
Measuring resistance is also more accurate than the two-wire method, namely the 4-wire method and the 6-wire method. In the two-wire method, the resistance of the continuous resistance wire is also connected to the digital multimeter. The resistance of the connection wire is also counted in the measured resistance value, and they cannot be separated. The 4-wire method or kelvin method measures the resistance, using a pair of wires to connect the current source, and the other pair of wires (the sensing wire) introduces the measured voltage drop across the resistor to a digital multimeter for measurement. Since the current flowing through the sensing line is small, the measured resistance value is closer to the true value. Especially when measuring low-value resistance, the resistance value of the connecting wire can be of the same magnitude as the measured resistance value, and the error is large.
If you use the 4-wire method to measure the low value resistance, you can eliminate the lead resistance and make the measurement accuracy better. When measuring the large-value resistance, bypass current will be present anyway. Using the protective measures for the measured resistance can solve the error caused by the bypass current. However, protective measures are usually used when measuring large values ​​of resistance with two wires. With a digital multimeter, it is also possible to make 6-wire measurement resistors, and 4 lines plus two additional protection lines. The 6-wire method is usually used to measure a resistor connected to a resistor network. Other resistors in the network that are connected to the measured resistance are connected to the protection terminal. This effectively isolates the measured resistance from it. The voltage at each end of the resistance to be measured and the voltage at the protection end are generated by the power supplies that are separated from each other. This can also eliminate the error caused by the sense line resistance. As a typical example, high-resolution instruments have 6-wire measurement capability. The 4-wire method is commonly used for 4-bit to 5-digit digital multimeters. These measurement methods are illustrated by footnotes. If there is no footnote, only two-wire resistance can be used.