Sensitivity of Multimeters and Selection Skills
Since the energy that drives deflection of the meter‑movement is drawn from the current in the circuit under test, high sensitivity is achieved when the pointer deflects substantially with little energy consumption.
Multimeter sensitivity falls into three indicators: DC‑voltage sensitivity, AC‑voltage sensitivity, and meter‑movement sensitivity. DC‑voltage sensitivity serves as the primary indicator. Due to circuit‑design constraints, AC‑voltage sensitivity is generally lower than DC‑voltage sensitivity. Both values are printed on the dial in ohms‑per‑volt (Ω/V) for quick reference. Meter‑movement sensitivity denotes the full‑scale current of the movement, together with two other parameters: meter‑movement internal resistance and linearity. It provides the basis for measurement‑circuit calculation and determines the overall voltage sensitivity of the multimeter. The meter‑movement internal resistance is the sum of resistance of the moving coil and upper‑lower hairsprings. Linearity describes the consistency between current passing through the meter‑movement and pointer deflection, which is used for dial‑scale plotting. This section focuses on DC‑voltage sensitivity of multimeters.
For voltage measurement, the voltmeter is connected in parallel across the two measured points. Its internal resistance is equivalent to a resistor connected in parallel at these points, lowering the total impedance across them. Combined with its shunting effect on the circuit, the measured voltage reading will be lower than the true value. Therefore, for voltage‑measurement work, the multimeter shall have high internal resistance (i.e., high Ω/V sensitivity) to minimize such errors.
Take the MF30 multimeter as an example. Its DC‑voltage ranges are 0‑1‑5‑25‑100‑500 V, and the dial is marked 20000 Ω/V. The internal resistance for the 1 V range equals 20 kΩ × 1 = 20 kΩ; for the 5 V range it is 20 kΩ × 5 = 100 kΩ, and so on.
Multimeter sensitivity can also be classified into two categories: meter‑movement sensitivity and voltage sensitivity (including DC‑voltage sensitivity and AC‑voltage sensitivity).
The full‑scale value Ig (full‑scale deflection current) of the multimeter's meter‑movement is called meter‑movement sensitivity. Ig normally ranges from 9.2 μA to 200 μA. The smaller Ig is, the higher the meter‑movement sensitivity. A high‑sensitivity movement typically has a full‑scale value below 10 μA; medium‑sensitivity movements range from 30 μA to 100 μA; values above 100 μA correspond to low‑sensitivity meter‑movements.
