An Introduction to Electrical Instrumentation and Measurement Systems

Scientific and technical instruments have been defined as devices used in observing, measuring, controlling, computing or communicating. Additionally the same source* states that: ‘Instruments and instrument systems refine, extend or supplement human facilities and abilities to sense, perceive, communicate, remember, calculate or reason’.

An analogue device is one in which the operation and output are continuously variable and bear a fixed relationship to the input.

The methods described in this chapter could, as an alternative, have been termed ‘null methods’-the purpose of the measurement process being to reduce the difference between a known and an unknown quantity to zero, that is so a null can be indicated. Inherently, such methods have a greater precision than direct measurements; for example, by using a detector that has a resolution of a microvolt it is possible to compare in terms of microvolts two voltages that have magnitudes of the order of, say, 1 V. The accuracy of such methods must, however, depend on the limits of error that apply to the ‘known’ or ‘standard’ quantity used.

Digital instruments sample the measurand, perform a valuation using digital electronics and normally display the measurand in discrete numerals. In general, contemporary instruments use either L.E.D. or liquid crystal seven-segment displays (see section 1.2.2), although research into, and development of, other techniques continues[1] and the use of other forms of display must not be discounted nor must the possibility of instruments without a visual output. The major advantage of a digital display is that it eliminates parallax errors and reduces the human errors associated with interpreting the position of a pointer on an analogue scale.

These devices have been defined as components that may be used to interconnect like or unlike systems and transmit energy between them. Although such a definition can be justified, a more generally acceptable definition to electrical engineers is that a transducer is a device used to convert a physical phenomenon into an electrical signal.

In many instances the values of electrical quantities (voltage, current and power) are too large or too small to be connected directly to the available instrument. It therefore becomes necessary to suitably reduce, or amplify, the magnitude of the measurand so that it has a value compatible with the measuring instrument to be used. In addition to these requirements, the effects of the instrument’s impedance must always be considered for this may affect the value indicated for the measurand, (see p. 27). The disturbance resulting from the insertion of a measuring instrument should be minimal, that is to say the device used in measuring a current should have as low an impedance as possible, while for voltage measurement the requirement is a high impedance. In both cases the ideal instrument’s performance will be independent of frequency.

In practical engineering the conduction of an electrical signal from the measurand to a measuring instrument can be affected by a number of forms of interference. These may, broadly speaking, be divided into

The considerations for selecting an instrument may be regarded as falling into two categories: either an engineer is selecting the most suitable instrument from those within a department or establishment to perform a particular measurement, or he is undertaking the purchase of a new instrument to perform a particular measurement and possibly at the same time extend the measurement capabilities of the department or establishment in which he works. Many of the criteria in selecting an instrument are the same, whether the engineer is selecting an instrument off the shelf or purchasing new equipment. In either case a major pitfall is to ‘acquire’ the newest and most sophisticated pieces of equipment in the department, or on the market, simply as a prestige exercise. This is of little value if within a week, justifiable pressure is brought to bear by one’s colleagues and the prestige instrumentation system is reduced to its minimum requirements— which could be two suspect multimeters and the oldest oscilloscope in the department!

Instrument systems refine, extend, or supplement human facilities and abilities to sense, perceive, communicate, remember, calculate or reason.[1] To relate this definition to practical terms means that any use of instruments constitutes an instrumentation system, since a suitable instrument or chain of instruments will always convert an unknown quantity into a record or display which human faculties can interpret.