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WEEK Sections: Instruments | Permanent-Magnet | Voltmeter | Ammeter Instruments / Measuring Devices Definition. Measurement of a quantity consists either in its comparison with a unit quantity of the same kind or in its determination as a function of different kinds whose units are related to it by known physical laws.1 Usually measurements are taken to test the reliability of the equipment or the system to which the measurements are done. Thus, the purpose of test is to verify the rules, principles, and laws, and to determine the behavior of an equipment under operating conditions. There are two basic types of instruments or measuring devices. Galvanometers, the first type, are deflecting instruments which are used mainly to detect the presence of a small electrical quantity -- current, voltage or charge 00 but which are also used in some instances to measure the quantity through the magnitude of deflection. Meanwhile, Detectors are used to indicate approach to balance in bridge or potentiometer networks and usually generally responsive to small currents or voltages. Also, instruments can be Analog, electromechanical devices in which an electrical quantity is measured by conversion to a mechanical motion, or Digital type. Analog instruments can be classified a) according to the principle on which the instrument operates and b). basis of use. For the first classification, the usual types are: permanent-magnet moving-coil, moving-iron, dynamometer, and electrostatic. In terms of usage: panel, switchboard, portable, and laboratory-standard. Digital instruments are basically analog devices couple with analog-to-digital converters. Errors. All these types are subject to a number of errors. As such, two factors must be considered in dealing with measuring devices: precision and accuracy. Precision is a measure of the spread of repeated determinations of a particular quantity. Accuracy is a statement of the limits which bound the departure of a measure value from the true value of a quantity. Precision errors can be eliminated through several trials: the average of several readings is better than one. Accuracy limits of the instruments, standards, and methods used should be known, so that appropriate choice of these measuring elements may be made. Operation of an instrument, with energy applied over a prolonged period of time, may cause errors due to elastic fatigue of control springs or resistance changes in instrument elements becaus of heating under load. For best performance, meters should be operated near to rated voltage. Other errors might be due to magnetic fields, leakage paths, variations in ambient temperature, phase-defect angles, potential differences, electrostatic charges, or position influence. Magnetic fields, produced by currents or by various classes of electrical machinery or apparatus, which may combine with the fields of portable instruments to produce errors. In instruments involving high resistances and small current, leakge paths across insulating components of the measuring arrangements should be eliminated if they shut portions of the measuring circuit. Variations in ambient temperature or internal temperature rise from self-heating under load. [Remember, resistance of a conductor varies with remperature]. Phase-defect angle errors affect only ac measuring devices. Large potential differences should be avoided between the winding of an isntrument or between its winding and frame. Electrostatic charges, such as rubbing the insulating case or window of an instrument with a dry dustcloth, may result to consequent disturbance to readings. Some of the synthetic textile fibers -- such as nylon and Dacron -- are particularly strong sources of charge. Position influence, usually resulting from mechanical unbalance, may affect the reading of an analog-type indicating instrument when they are used in a position other than in which it was calibrated. Portable instruments are normally intended to be used with the axis of the moving system vertical and the calibration is generally made in this position. Sections: Instruments | Permanent-Magnet | Voltmeter | Ammeter Theory. When current is passed through the suspended coil, torque is developed. The force tending to turn the coil is proportional to the strength of the magnet, and the ampere-turns of the moving element. The torque or turning moment is proportional to both. T = 0.2 BNIRl
Prior to D'Arsonval, major contributions to electrical measurement are:
Sensitivity. Galvanometer sensitivity can be expressed in a number of ways, depending on application: a. The current constant is the current in microamperes that will produce unit deflection on the scale -- usally a deflection of 1 mm on a scale 1 meter distant from the galvanometer mirror. b. The megohm constant is the number of megohms in series with the galvanometer through which 1 Volt will produce unit deflection. It is the receiprocal of the current constant. c. The voltage constant is the number of microvolts which, in a critically damped circuit will produce unit defelection. d. The coulomb constant is the charge in microcoulombs which, at the specified damping, will produce unit ballistic throw. e. The flux-linkage constant is the product of change of induction and turns of the linking search coil which will produce unit ballistic throw. Sections: Instruments | Permanent-Magnet | Voltmeter | Ammeter Definition. An ammeter, permanent-magnet moving coil, basically a torque motor, is capable of measuring current. Parts. An ammeter consists of a permanent magnet, a moving coil, a shunt, and a calibrating resistor. How to Use. Ammeter is placed in series in a circuit with a low voltage drop. Calculation: Given a 5-Ampere ammeter with a 0.05 voltage across a 2.5 Ohms moving coil. Compute for the coil current and the resistance at shunt. Coil Current: 0.05 Volts / 2.50 Ohms = 0.02 Amperes Shunt Current: 5.00 - 0.02 = 4.98 Amperes Shunt Resistance = 0.05 Volts / 4.98 Amperes = 0.01004 Ohms. Sections: Instruments | Permanent-Magnet | Voltmeter | Ammeter Defintion. A permanent-magnet moving coil, basically a torque motor, capable of measuring electromagnetic force (volts). How to Use. Voltmeter is connected accross a circuit with comparatively high resistance thus draws very little current. Calculation: Given a 150-Volt full deflection voltmeter with a 0.05 voltage across a 2.5 Ohms moving coil. Compute for the highest resistance capabiltiy and the resistor ohmic value. Resistor is capable: 150 Volts - 0.05 Volts = 149.95 Volts. External multiplier. The ratio of the multiplier's resistance over the intrument's internal resistance is directly proportional to the ratio of the multiplier's voltage drop over the instrument's voltage drop. Thus: Calculation: A 150 Voltmeter has a total resistance between terminals of 1,500 Ohms Solution:
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Where: