How to choose the right multimeter correctly?

When choosing a suitable multimeter, usually the accuracy and range must be considered. In addition, more detailed indicators are also needed for some special measurements.

This article will introduce how to correctly choose a suitable multimeter through resolution, accuracy, test types and ranges, various indicators, etc.

Keysight digital multimeters have resolutions ranging from 3.5 to 8.5 digits, and there are 36 models of handheld, desktop, PXI and USB digital multimeters to choose from. The following will introduce several important concepts or indicators for your reference.

1. Resolution
Resolution determines the ability of the multimeter to distinguish the smallest unit. The first digit is 0 or 1, which represents a half digit, and the number displayed afterwards is composed of a 6-and-a-half-digit multimeter.

2. Accuracy

Accuracy is a description of how close the measured value is to the true value. For example, the accuracy index is generally given in the technical indicators. Take the 34465A voltage accuracy as an example: ± (% reading + % range)

When testing a 1V voltage in the 1V range, the final accuracy is 0.0035%+0.0004%=0.0039%V, which is 0.000039V. Compared with the resolution just mentioned, the multimeter can display 1.000000 in the case of 6.5 digits. That is to say, even if the resolution can display the last two digits, it cannot be simply assumed that all 6 digits are valid numbers.

3. NPLC (aperture time)
Most of Keysight's benchtop multimeters have NPLC options ranging from 0.001-100. PLC stands for Power Line Cycle and N stands for number. China's mains frequency is 50Hz (20ms), so 1PLC is an integration time of 20ms. Therefore, when PLC is set to greater than 1, it can better eliminate common-mode interference and improve accuracy, but the corresponding speed will be slower. If PLC is set to 0.02 or even lower, the resolution will be reduced and the accuracy will also be worse, but the sampling speed will be very fast. Therefore, choosing the right NPLC value is also very important for testing.

4. Test types and ranges
Generally, desktop multimeters can measure parameters including direct and alternating voltage and current, 2/4-wire resistance, capacitance, diode, temperature and frequency, and their functions are relatively rich. Handheld meters and USB multimeters generally focus on the measurement of voltage, current, resistance and diode. The ranges of different meters are also different. For example, the maximum test voltage of the desktop 34465A is 1000V, while the U2741A USB multimeter is 300V. Even the ranges of different handheld meters are different.

5. Special test conditions

Under certain test conditions, it is found that the 6.5-digit tabletop meter is not as good as the 3.5-digit handheld meter. This certainly does not directly prove that the 6.5-digit meter is damaged, but it is necessary to consider some hardware matching or limiting factors, such as

• AC current measurement: attention should be paid to input impedance and shunt capacitance value