Calibration of Meters

Calibration of Meters Points : Calibration of Meters, Calibration of Ammeters, D.C. Calibration, A.C. Calibration Calibration of Ammeters Main types of calibration are:
1. D.C. Calibration
2. A.C. Calibration 1. D.C. Calibration There are two methods of D.C. calibration.
(i) Comparison with standard ammeter.
(ii) By d.c. potentiometer. (i) Comparison with Standard Ammeter Method: The test ammeter, standard ammeter, a rheostat, and a d.c. source are connected in series. The current is adjusted by the rheostat to various suitable values and the readings of the test ammeter are checked directly against hose of the standard ammeter readings. (ii) By d.c. Potentiometer: Method: The test instrument is connected in series with a rheostat and a standard shunt to a d.c. source. The potential difference across the shunt is measured by a d.c. potentiometer, and true current values are thus obtained. The ultimate standard against which the instrument is checked is therefore, the standard cell used in standardizing the potentiometer. This is most satisfactory method of calibration. 2. A.C. Calibration: Calibration of a.c. instruments by the use of alternating current is rendered difficult by the lack of a convenient reference standard corresponding to the d.c. standard cell. Accuracy therefore, depends on the use of a suitable transfer instrument which can bc checked on d.c. supply, using a d.c. potentiometer and a standard cell, and which can then be relied on to give true rms readings on an a.c. supply. Electrodynamic instruments are frequently used for this purpose, but it is also possible to use thermal instruments or electrostatic voltmeters. Following methods are used for a.c. calibration.
(i) Comparison with standard ammeter.
(ii) By ac. potentiometer.
(iii) By electrostatic voltmeter
(iv) By d.c. calibration (i) Comparison with standard ammeter This is similar to the d.c. method already described. The standard ammeter itself will have been calibrated by means of one of the methods now to be described. (ii) By a.c. potentiometer This is similar to the d.c. potentiometer method for d.c. instrument. The tandardizing of the a.c. potentiometer involves the use of a suitable transfer instrument. (iii) By electrostatic voltmeter The ammeter is connected in series with a rheostat and a suitable standard resistance to an alternating voltage source, and the p.d. across the resistance is measured by an electrostatic voltmeter; this voltmeter is used as a transfer instrument. (iv) By d.c. calibration If the instrument to be calibrated is itself of a type suitable for use as a transfer instrument it may be calibrated using direct current; the calibration should then hold for alternating current.