# Calibration of DC Ammeter and DC Voltmeter

Objectives:

• To compare the readings of a given ammeter and voltmeter with the corresponding calculated values of current and voltage from the observations of a dc potentiometer.
• To calculate the percentage error for different observed values of current and voltage.

Theory:

Instrument calibration is one of the primary processes used to maintain instrument accuracy. Calibration is the process of configuring an instrument to provide a result for a sample within an acceptable range.  Eliminating or minimizing factors that cause inaccurate measurements is a fundamental aspect of instrumentation design.

The instruments must be calibrated periodically to ascertain their claimed accuracy. The best method of calibration is to measure the true value of current or voltage in the circuit and compare it with the value measured by the given ammeter or voltmeter. The true value of current or voltage in the circuit can be measured up to a fair grade of accuracy by using a potentiometer. The percentage error of the instrument can be calculated as follows:

percentageoferror=(measuredvaluetruevalue)/truevalue100

A potentiometer is an instrument for measuring an unknown emf or potential difference by balancing it, wholly or in part, by a known potential difference. Potentiometers are, therefore, used for measuring emfs. Generally, the maximum voltage that can be measured by a potentiometer is 0-1.8V. If a higher voltage range is to be measured, then a volt-ratio box is used. A volt-ratio box is actually a potential divider. For instance, a dc voltage connected at the high end terminal of the volt-ratio box is 300V gives an output of 1.8V at the low end terminal.

For calibration of voltmeter a dc source is required. A potential divider is connected to the supply. Output of the potential divider is connected between the common terminal and the high voltage post of the volt-ratio box. A voltmeter is also connected at this point. Output of the volt ratio box is connected with the potentiometer. If the reading of the potentiometer is 0.975V, then the value of the unknown voltage is 0.975×300/1.8=162.5V.

For calibration of ammeter, a standard resistor of suitable value and sufficient current carrying capacity is placed in series with the ammeter. The voltage across the standard resistor is measured with the help of potentiometer and the current through the standard resistor can be computed as

I=Vs/S ———————————————- (1)

Where, VS=voltage across the standard resistor as indicated by the potentiometer,

S=resistance of the standard resistor.

Circuit Diagrams:

Apparatus used:

Procedure:

Calibration of Voltmeter

• The connections are made as shown in the figure 1.
• The potentiometer is first standardized.
• The variable dc supply source is adjusted at any voltage value so that the pointer coincides exactly with a major division of the voltmeter.
• The corresponding dc voltage is measured by the potentiometer.
• From the actual voltage measured by the potentiometer, the error is calculated.

Calibration of Ammeter

• The connections are made as shown in the figure 2.
• The potentiometer is first standardized.
• The variable dc supply source is adjusted at any current value so that the pointer coincides exactly with a major division of the ammeter.
• The voltage drop across the standard resistance is measured by a potentiometer.
• From that, by using equation 1, the actual current measured by the potentiometer and the error are calculated.

Experimental Data Table:

Conclusion:

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