Scientific and technical instruments have been defined as devices used in observing, measuring, controlling, computing or communicating. Additionally the same volume* states that: ‘Instruments and instrument systems refine, extend or supplement human facilities and abilities to sense, perceive, communicate, remember, calculate or reason’.
The majority of the instruments described in Chapter 1, while giving a continuous indication of the measurand, have required the presence of an operator to observe variations in reading magnitude. This limitation is overcome in some of the graphical recording instruments, in particular those designed to record permanently variations in the level of a quantity, and with the ever-increasing emphasis on automation, continuously recording instruments are finding many applications, temperature recorders being but one example.
The methods described in this chapter could, as an alternative, have been termed ‘null methods’; the measurement process being to reduce the difference between a known and an unknown quantity to zero, that is, a null would be indicated. Such methods have inherently a greater precision than direct measurements, for example, by such methods it is possible to compare two voltages of the order of 1 V using a detector that has a resolution in terms of microvolts. The accuracy of such methods must, however, depend on the limits of error that apply to the particular ‘standard’ quantity.
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: (a) impurities in individual components and their effects on the measuring system, and (b) injection of unwanted signals from unrelated electrical circuits and fields into the measuring system.
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 effect the value indicated for the measurand.
Digital instruments display the measured quantity in discrete numerals thereby eliminating the parallax error, and reducing the human errors associated with analog pointer instruments. In general digital instruments have superior accuracy to analog pointer instruments, and many incorporate automatic polarity and range indication which reduces operator training, measurement error, and possible instrument damage through overload. In addition to these features many digital instruments have an output facility enabling permanent records of measurements to be made automatically.
No measurement is perfect. However, the imperfections of a measurement must be established and they are normally quoted as the tolerance or errors of measurement in relation to a standard or absolute value. Thus a system of comparisons must be established whereby any measurement made can be related to the standard value. Figure 7.1 gives a possible chain of relationships by which everyday measurements in electrical quantities may be evaluated in terms of the accepted standard values.
A transducer is a device or element that may be used to convert an input signal into an output signal of a different form6. It may therefore be a device that converts a mechanical variable into an electrical one, for example a tachogenerator, or it may be capable of converting an electrical signal into a mechanical one, for example a galvanometer. Commercial usage of the term is generally accepted as meaning a device that converts a physical phenomenon into an electrical signal.
Consider a researcher investigating a piece of equipment such that readings of temperature, pressure, movement, voltage, current, etc., are required to be recorded at regular intervals over a period of time to establish consistency of operation. Until the advent of the data recorder this type of situation required attendants to take the measurements. This used to be done somewhat laboriously and at times with dubious accuracy. However, now, when some form of data recorder is used, readings may be made with known limits of error either as a continuous record or in a predetermined sequence at controlled time intervals over an extended period.
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 ‘scope in the department!
