Most instrumentation systems contain electric circuits; hence, as engineers, it is crucial that we know how to build, design and analyze systems containing electric components. The experimental analysis of electric circuits requires the generation and measurement of signals via special-purpose instruments. In the following, the types of electrical measuring instruments will be briefly described.
Electrical Measuring Instruments
The basic types of electrical measuring instruments are:
An ammeter is an instrument used to measure current and must be connected in series with the circuit. Since all the current in the circuit passes through the ammeter it must have very low resistance.
A voltmeter is an instrument used to measure potential differences and must be connected in parallel with the part of the circuit whose potential difference is required. To avoid a signiﬁcant current ﬂowing through it a voltmeter must have very high resistance.
An ohmmeter is an instrument for measuring resistance. Unlike the ammeter or voltmeter, the ohmmeter circuit does not receive the energy necessary for its operation from the circuit under test. In the ohmmeter, this energy is supplied by a self-contained source of voltage, such as a battery.
Instruments are manufactured that combine a moving-coil meter with several shunts and series multipliers, to provide a range of readings on a single scale graduated to read current and voltage. If a battery is incorporated then resistance can also be measured.
Such instruments are called multimeters or universal instruments or multirange instruments. An ‘Avometer’ is a typical example. A particular range may be selected either by the use of separate terminals or by a selector switch. Only one measurement can be performed at a time. Often such instruments can be used in a.c. as well as d.c. circuits when a rectiﬁer is incorporated into the instrument.
Digital multimeters (DMM) are now almost universally used, the Fluke Digital Multimeter being an industry leader in performance, accuracy, resolution, ruggedness, reliability and safety. These instruments measure d.c. currents and voltages, resistance and continuity, a.c. (r.m.s.) currents and voltages, temperature and much more.
The oscilloscope is a graph-displaying device – it draws a graph of an electrical signal. In most applications, the graph shows how signals change over time. From the graph, it is possible to:
- determine the time and voltage values of a signal
- calculate the frequency of an oscillating signal
- see the moving parts of a circuit represented by the signal
- tell if a malfunctioning component is distorting the signal
- ﬁnd out how much of a signal is d.c. or a.c.
- tell how much of the signal is noise and whether the noise is changing with time
Oscilloscopes are used by everyone from television repair technicians to physicists. They are indispensable for anyone designing or repairing electronic equipment.
A wattmeter is an instrument for the measurement of power in an electrical circuit.
The instrument has two coils:
- a current coil, which is connected in series with the load, like an ammeter
- a voltage coil, which is connected in parallel with the load, like a voltmeter.
The megger is an instrument used for the measurement of high resistance and insulation resistance. Essentially the megger insulation tester consists of a hand-driven dc generator and a direct reading true ohmmeter. Megger is also called a megohmmeter. An ordinary ohmmeter cannot be used for measuring the resistance of multimillions of ohms such as conductor insulation. Megger used to measure high resistance, of the order of one megaohm and above. The megohmmeter or megger is a portable deflection instrument widely used to check the insulation resistance of electrical cables and equipment. This instrument measures the insulation resistance of the electric circuits relative to the earth and one another.
8. Continuity tester
A continuity tester is the measurement of the resistance of a cable to discover if the cable is continuous, i.e. that it has no breaks or high-resistance joints.
A tachometer is an instrument that indicates the speed, usually in revolutions per minute, at which an engine shaft is rotating. Tachometers are needed in modern times to measure the rotational speed of numerous things. It is advantageous to know the duration of time and the speed of a moving object then the distance can be calculated and maintenance can be scheduled to prevent wear and tear.
A stroboscope is a device for viewing a rotating object at regularly recurring intervals, by means of either a rotating or vibrating shutter or a suitably designed lamp that ﬂashes periodically. If the period between successive views is exactly the same as the time of one revolution of the revolving object and the duration of the view is very short, the object will appear to be stationary.
A fluxmeter is superior to a ballistic galvanometer for some kinds of magnetic measurements. It is a special form of a ballistic galvanometer with some modifications, like, the torque of the suspension is made very small, and the electromagnetic damping is very heavy. Apart from portability, it has the advantage that, unlike a ballistic galvanometer, the flux change for a short time is not necessary. The deflection obtained for a given flux change does not depend on the time taken to make the change of flux.
13. LCR meter
The LCR (or RLC) meter, or bridge, is an electronic instrument specifically designed for the measurement of impedances having arbitrary phase angles (LCR stands for inductance, capacitance, resistance)
14. Non-contact voltage tester
A non-contact voltage tester or detector is an electrical tester that helps to detect the presence of voltage. Voltage presence is useful information to have when troubleshooting or working on a failed asset. The first tool you’ll reach for is a non-contact voltage detector.
15. Clamp meter
A clamp meter is an electrical tester that combines a voltmeter with a clamp-type current meter. Like the multimeter, the clamp meter has passed through the analog period and into the digital world of today. Originally created primarily as a single-purpose test tool for electricians, today’s models have incorporated more measurement functions, more accuracy and in some instruments, some very special measurement features. Today’s clamp meters have most of the basic functions of a digital multimeter (DMM) but with the added feature of a current transformer built into the product.
16. Power quality analyzer
Power quality analyzers are highly intuitive instruments, simple to use and have the ability to measure all power quality problems from high-speed transients to RVC, high-frequency harmonics, signaling and much more. They are smart enough to recognize the CT connected as well as the CT range and let you know they are connected properly.
17. Irradiance meter
The solar irradiance meter is a key component in estimating solar irradiation which is necessary and essential to designing sustainable energy systems such as photovoltaic (PV) systems.
18. Battery analyzer
The battery analyzer is an ideal test tool for maintenance, troubleshooting and performance testing of individual stationary batteries and battery banks used in critical battery backup applications.