A few principles for selecting a multimeter
1,Functions
Digital multi-meter 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, IEEE-488 interface or RS-232 interface and other functions, the use of the use of specific requirements to be selected.
2,Range and range
Digital multimeter has many ranges, but its basic range accuracy * high. Many digital multi-meter has automatic range function, do not have to manually adjust the range, making the measurement convenient, **, rapid. There are many digital multi-meter over-range capability, in the measurement of the value over the range but not yet reached the * large display can not be changed when the range, thus improving the accuracy and discriminating power.
3, accuracy
Digital multi-meter allows the error should not only look at its variable term error, but also look at its fixed term error. When choosing, we also need to look at the stability error and linearity error to ** how much, whether or not the resolving power to meet the requirements. General digital multi-meter such as the requirements of 0.0005 level to 0.002 level, at least 61 digital display; 0.005 level to 0.01 level, at least 51 digital display; 0.02 level to 0.05 level, at least 41 digital display; 0.1 level below, at least 31 digital display.
4, input resistance and zero current
Digital multi-meter input resistance is too low and zero current is too high will cause measurement errors, the key is to look at the measurement device allowed by the limit value is how much, that is, to see the size of the signal source of the internal resistance. When the signal source impedance is high, the instrument with high input impedance and low zero current should be selected 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 measurements, it is easy to mix into the interference signal, resulting in inaccurate readings, so it should be used in accordance with the environment to select the series, common mode rejection ratio of high instruments, especially for high-precision measurements, should be selected with the protection of the end of the G digital multi-meter, which can be very good suppression of common-mode interference.
6,Display form and power supply
Digital multi-meter display form not in digital, but also can display charts, text and symbols, in order to facilitate field observation, operation and management. According to the external dimensions of its display device can be divided into small, medium, large and large four categories.
Digital multi-meter power supply is generally 220V, and some new digital multi-meter power supply range is very wide, can be between 1100V ~ 240V. Some small digital multi-meters can be used with batteries, and some digital multi-meters can be used in three forms: alternating current, internal nickel-cadmium batteries or external batteries.
