Page 139 - Mechanical Engineers Reference Book

3/22 Microprocessors, instrumentation and control

- signals
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- Trans- Telemetry Data
processing -
7 ducer - - ;
signals
1 il

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Computer/ -
controller

Figure 3.32 Typical instrumentation system
(provided time allows). However, even the average is not Ideal
necesssarily correct, because it contains the systematic error. straight-
Systematic errors can be greatly reduced by careful calibration line
under conditions resembling the operating ones as closely as response
possible. Calibration is in fact an important feature of all Hysteresis /
systems where it is required to relate the readings taken back shown by
to absolute values of the quantities concerned. The word
‘traceability’ is used to describe building up links in this chain.
Absolute values of measurands are not always important.
Sometimes it is ‘repeatability’ -the capability of giving the
same reading (even if it is the wrong one!) under the same
conditions - that matters. The ‘precision’ of a reading relates
to the smallest difference in value that can be detected.
When considering how faithfully an instrument represents
what it is supposed to be measuring, the idea of ‘influence
quantities’ should be taken into account. The resistance of a
strain gauge, for instance, varies with strain, and so is used to
measure it; but it also varies with temperature, so a one-to-one Value of measurand
correspondence with strain will only hold provided tempera-
ture is constant. Temperature is an influence quantity. Reduc- Figure 3.33 Hysteresis and non-linearity errors
ing the errors introduced by influence quantities is facilitated
by making ‘bridge measurements’. This idea is most familiar in ity, putting it on a semi-quantitative -though necesarily pro-
electrical circuits, when two components are connected so that babilistic - basis, so allowing the chances of failure to be
changes in them affect the output in opposite directions. It can calculated. While carrying out such an exercise, lessons can be
then be arranged that influence quantities affect them equally learned of features that have been identified as significant.
and so cancel, while the measurand either only affects one With large systems, the configuration in which different items
component or affects them oppositely. are connected can make an unexpected difference. The sche-
Another concept that is helpful in understanding transducer dule of servicing plays a large part in the down time that is to
performance is that of ‘gauge factor’. This is the ratio of the be expected. The consequences of any particular failure,
fractional change in output to the fractional change in measu- which may be very diverse, should be analysed. The idea of
rand. Again it is easily understood in the frequently quoted ‘independence’ may be developed; this implies that it is safer
example of the resistance strain gauge, where it is given by the for two measurements, if they are intended to corroborate one
formula another, to be made with essentially different compo-
nents - or even techniques - because they are then less likely
6RIR = 6111
to suffer from a common fault. In all considerations of
However, the concept is more generally applicable, with the instruments and their installations due weight must be placed
expectation that a transducer with a higher gauge factor will be on reliability. High accuracy has little value if it cannot be
more immune to influence factors. counted on, or perhaps it may be put that the possibility of a
‘Hysteresis’ is observed in many instruments. This is the very large error can be more damaging than the probability of
name given to the defect of indicating a different output a small one.
according to whether the quantity measured is increasing or
decreasing (see Figure 3.33). Non-linearity is also shown in 3.5.1.3 Environmental conditions
Figure 3.33. This is the situation when the relation between
input and output cannot be exactly represented by a straight In a broad way, it is always recognized that there are limits to
line. the conditions under which instruments can be used. The
working temperature range may well be specified. Users
should be aware of other significant conditions. The atmos-
3.5.1.2 Reliability phere can be harmful, from moisture or other corrosive
The reliability of any equipment is often of the utmost effects; dust in the atmosphere is an enemy to many instru-
importance, and instrumentation systems are no exception. ments. Subjection to mechanical abuse (notably excessive
Attempts have been made to develop the concept of reliabil- vibration) must also be guarded against. More sophisticated