Question

State the purpose of calibration of measuring instruments. Explain the procedure of calibration of D.C. voltmeter by using D.C. potentiometer.

Answer 1

Purpose of calibration : Calibration is the process of evaluating and adjusting the precision and accuracy of measuring equipment. Proper calibration of an instrument allows people to have a safe working environment and produce valid data for future reference.

Precision is the degree to which repeated measurements under unchanged conditions show the same result Accuracy is the degree of closeness of measurements of a quantity to its actual true value. For calibration, reference standards with known values for selected points covering the range of interest are measured with the instrument in question. Then a functional relationship in terms of error is established between the values of the standards (true value) and the corresponding measurements.

Calibration of D.C. voltmeter by using D.C. potentiometer:

For calibration of voltmeter using DC potentiomter, a volt-ratio box is required which consists of 50Ω to 100kΩ variable resistors and the arrangement steps down the input voltage so that it can be safely measured by DC potentiometer.

• Potentiometer is standardized before measurement.

• The circuit is connected as shown in the figure. With the help of potential divider the reading of voltmeter is set to the certain value.

• Let the reading of votlmeter is the measured voltage ‘Vm’ and the voltage measured by DC potentiometer is ‘Vp’.

• The true value of the voltage ‘Vt’ is found out by multiplying the reading of potentiometer ‘Vp’ by corresponding ratio of the volt-ratio box.

• Calculate the relative error in the voltmeter for each reading.

• A calibration curve can be drawn by plotting the percentage error against the measured voltages.