Ammeter

Test Instruments & Measuring Devices AMMETERS Equation (14-4) is the mathematical representation of the current with the meter installed in the circuit. (14-4) I_w V R_o R_m The accuracy of the ammeter K_A is the ratio of the current when the meter is in the circuit, I_w, to the current with the meter out of the circuit, I_o. Equation (14-5) is the mathematical representation for solving for the accuracy of the ammeter (K_A). (14-5) K_A I_w I_o By substitution laws, Equation (14-6) is a mathematical representation of the accuracy using circuit resistance. (14-6) K_A � � � � � � � � V R_o R_m � � � � � � � � R_o V R_o R_o R_m The percent loading error is that percent of error due to loading effects that result from the added resistance of the meter. Equation (14-7) is a mathematical representation of the percent loading error. % loading error = (1 - K_A)(100 %) (14-7) A second error which occurs in an Figure 7 Ammeter Accuracy ammeter is calibration error. Calibration error is an error that occurs due to inaccurately marked meter faces. Typical values of calibration error in terms of full scale current are about 3 percent. Example: An ammeter, with a 10 mA full scale deflection and an internal resistance of 400 W, is placed in a circuit with a 20 V power source and a 2 KW resistor (Figure 7). Rev. 0 Page 11 ES-14