Sensitivity and Selection Techniques of Multimeters
Sensitivity is a technical indicator that represents the degree to which an instrument responds to weak energy.
Due to the fact that the energy driving the instrument measurement mechanism to deflect is taken from the current in the circuit under test, if the instrument pointer deflects significantly and uses less energy, its sensitivity will also be higher.
The sensitivity of a multimeter can be divided into three indicators: DC voltage sensitivity, AC voltage sensitivity, and meter head sensitivity. The DC voltage sensitivity is the main indicator. AC voltage sensitivity is generally lower than DC voltage sensitivity due to meter circuit design factors. They are labeled in ohms per volt (Ω/V) on the dial, making it clear to us at a glance. The sensitivity of the meter head indicates the full scale current value of the meter head, including the internal resistance and linearity of the meter head. It is the basis for calculating the meter circuit and also determines the voltage sensitivity of the entire multimeter; The internal resistance of the meter head refers to the sum of the resistance values of the moving coil of the meter needle and the upper and lower groups of hairspring; Linearity refers to the degree of consistency between the current intensity passing through the meter head and the deflection amplitude of the meter needle, which serves as the basis for drawing the dial scale. Here, the focus is on the DC voltage sensitivity of the multimeter.
When measuring, the Voltmeter is connected in parallel with the two points to be measured. Because of the existence of the internal resistance of the Voltmeter, it is equivalent to that a resistance is connected in parallel between the two points to reduce the total impedance between the two points to be measured; In addition, its shunt effect on the circuit causes the measured voltage value to be lower than the actual value. Therefore, when measuring voltage, it is required that the multimeter has a large internal resistance (i.e. sensitivity Ω/V should be high) to reduce this error.
For example, if the DC voltage range of the MF30 multimeter is 0-1-5-25-100-500V and the dial is marked with 20000 Ω/V, the internal resistance of the 1V range is 20k Ω xl=20k Ω; The internal resistance within the 5V range is 20k Ω x 5=100k Ω, and so on.
The sensitivity of a multimeter can be divided into two indicators: head sensitivity and voltage sensitivity (including DC voltage sensitivity and AC voltage sensitivity).
The full scale value Ig (i.e. full scale current) of the multimeter head is called the sensitivity of the meter head, and the Ig is generally 9.2-200 μ A. The smaller the Ig, the higher the sensitivity of the meter head. The full scale value of high sensitivity meters is generally less than 10 μ A. The full scale value of the medium sensitivity meter head is usually 30-100 μ A. Over 100 μ A belongs to the low sensitivity meter head.
The voltage sensitivity of a multimeter is equal to the ratio of the equivalent internal resistance of the voltage range to the full range voltage, and its unit is Ω/V or k Ω/V, abbreviated as ohms per volt. This value is generally marked on the instrument panel.
