An instrument calibration is performed by comparing its reading with that of a more accurate instrument when both instruments are measuring the same quantity. In this case, the more accurate instrument is employed as a standard for comparison purposes. To calibrate ac ammeters and ac voltmeters, ac supply and an ac standard instrument must be used. Normally the ac supply is derived from an autotransformer connected to the 60 Hz, 115 V supply as illustrated in the diagrams below. If an instrument is to be checked for accuracy at higher frequencies, the supply must be derived from an oscillator with a suitable output voltage level.
AC Ammeter Calibration
Figure 1.0 below shows two ac ammeters connected in series, so that they are both measuring same current. In this case, the first ac ammeter is the standard instrument i.e. known to be accurate and the second ac ammeter is the one to be calibrated. The current is derived from ac power supply and a current-limiting resistor is included as demonstrated in the figure however, If the power supply has a current level control, the current-limiting resistor is not necessary. The power supply is adjusted to set the measured quantity in convenient steps over the desired range. The two instrument readings are noted, and a calibration chart is prepared.
AC Voltmeter Calibration
Figure 1.1 demonstrates the technique for calibrating an AC voltmeter. An ac voltage is applied from the power supply to the parallel connected standard instrument and the instrument to be calibrated. The voltage is then adjusted in steps over the desired range. The two instrument readings are recorded and a calibration chart is prepared. The calibration chart is helpful when it comes to establishing whether or not the instrument is within its specified accuracy.
Related articles:
Comments