Voltmeters are used extensively in industry where the surveillance of input and/or output voltages is vital for plant operation.

EO 1.2STATE the electrical parameters measured by each of the following in-place measuring devices:

a. Voltmeter

EO 1.3EXPLAIN how the following electrical test equipment and measuring devices are connected to a circuit:

a. Voltmeter

Voltmeter

A simple DC voltmeter can be constructed by placing a resistor (Rs), called a multiplier, in series with the ammeter meter movement, and marking the meter face to read voltage (Figure 4). Voltmeters are connected in parallel with the load (R_L) being measured.

Figure 4 Simple DC Voltmeter

When constructing a voltmeter, the resistance of the multiplier must be determined to measure the desired voltage. Equation (14-1) is a mathematical representation of the voltmeter's multiplier resistance.

where

Example: A 2 mA meter movement with internal resistance of 25 ohms is to be constructed as a voltmeter.

What value must the series resistance be to measure full scale voltage of 100 volts?

Solution:

When a voltmeter is connected in a circuit, the voltmeter will draw current from that circuit. This current causes a voltage drop across the resistance of the meter, which is subtracted from the voltage being measured by the meter. This reduction in voltage is known as the loading effect and can have a serious effect on measurement accuracy, especially for low current circuits.

The accuracy of a voltmeter (K_v) is defined as the ratio of measured voltage when the meter is in the circuit (V_w) to the voltage measured with the meter out of the circuit. Equation (14-2) is a mathematical representation of the accuracy of a voltmeter, or true voltage (V_o).

Meter accuracy can also be determined by comparing the relationship between the input and

circuit resistances using Ohm's Law as described below.

where

Example: A voltmeter in the 100 volt range with a sensitivity of 40 KS2/V is to measure the voltage across terminals ab (Figure 5).

Figure 5 Measuring Circuit Voltage

Solution:

Summary

Voltmeters are summarized below.

Voltmeter Summary Measures voltage

Connected in parallel with the load being measured