9 conditions for choosing a digital multimeter
(1) function.
Digital multimeter in addition to measuring AC and DC voltage, AC and DC current, resistance and other five functions, there are digital calculations, self-test, readings to maintain, error readout, diode detection, word length selection, IEE-488 interface or RS-232 interface, etc., according to the specific requirements of the use of the selected function.
(2) Range and range.
Digital multimeter has many ranges, but its basic range of the highest accuracy. Many digital multimeters have automatic range function, no need to manually adjust the range, making the measurement convenient, safe and rapid. There are also many digital multimeter over-range capability, in the measurement value over the range but not yet reached the maximum display can not change the range, thus improving the accuracy and resolution.
(3) accuracy.
The maximum error allowed by the digital multimeter depends not only on its variable term error, but also on its fixed term error. When selecting a digital multimeter, it is also necessary to see how much stability error and linearity error are required, and whether the resolving power meets the requirements. General digital multimeter, such as the requirements of 0.000 5 level ~ 0.002 level, at least 6 ½ digits should be displayed; 0.005 level ~ 0.01 level, at least 5 ½ digits should be displayed; 0.02 level ~ 0.05 level, at least 4 ½ digits should be displayed; 0.1 level below, at least 3 ½ digits should be displayed.
(4) input resistance and zero current.
Digital multimeter input resistance is too low and zero current is too high will cause measurement errors, the key is to look at the measurement device to allow the limit value is how much, that is, to see the size of the signal source of the internal resistance. Signal source impedance should be high when you choose a high input impedance, low zero-current instrument, so that its effect can be ignored.
(5) Series mode rejection ratio and common mode rejection ratio.
In the presence of a variety of interference such as electric field, magnetic field and a variety of high-frequency noise or long-distance measurement, it is easy to mix into the interference signal, resulting in inaccurate readings, therefore, should be based on the use of environmental selection of series, common mode rejection ratio of high instruments, especially for high-precision measurements, should be selected with a protective end of the G digital multimeter, can be very good suppression of common-mode interference.
(6) Display and power supply.
The display form of digital multimeter is not only limited to digital, but also can display charts, text and symbols, in order to facilitate on-site observation, operation and management. According to the external size of its display device can be divided into small, medium, large and large four categories.
Digital multimeter power supply is generally 220 V, and some new digital multimeter power supply range is very wide, can be in 100 ~ 240 V between. Some small digital multimeter with batteries can be used, there are some digital multimeter can be used with alternating current, internal nickel-cadmium batteries or external batteries in three ways.
(7) response time, measurement speed, frequency range.
The shorter the response time, the better, but there are some meters with longer response time, and the readings can only be stabilised after a period of time. Measurement speed should be based on whether or not the system test with the joint, such as the joint, the speed is very important, and the faster the better. Frequency range, according to the need for appropriate choice.
(8) AC voltage conversion form.
AC voltage measurement is divided into average conversion, peak conversion and RMS conversion. When the waveform distortion is large, the average conversion and peak conversion is inaccurate, while the RMS conversion can not be affected by the waveform, so that the measurement results are more accurate.
(9) Resistance wiring.
There are four-wire and two-wire wiring systems for resistance measurement. Small resistance and high-precision measurements, should be selected with four-wire resistance measurement wiring.
