Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 14

instrument is approximately linear in that range. Sensitivity of a spring balance can
be expressed as 25 mm/kg (say), indicating additional load of 1 kg will cause
additional displacement of the spring by 25 mm.
Again sensitivity of an instrument may also vary with temperature or other
external factors. This is known as sensitivity drift. Suppose the sensitivity of the
spring balance mentioned above is 25 mm/kg at 20 C and 27 mm/kg at 30 C.
Then the sensitivity drift/ C is 0.2 (mm/kg)/ C. In order to avoid this type of
sensitivity drift, instruments are either kept at controlled temperature, or appro-
priate integral temperature compensation scheme provided inside the instrument.
1.8.1.3 Repeatability
It is defined as closeness of conformity between a numbers of repeated mea-
surements of the same variable (value) under the same operating conditions, which
is approaching in the same direction.
1.8.1.4 Range (or Span)
It defines the maximum and minimum values of the inputs or the outputs for which
the instrument is recommended to be used.
Most sensors have limited range over which a process variable can be mea-
sured, defined by the lower and upper range values. Generally, larger the range is
in result of the poorer accuracy and reproducibility.
Example: A temperature measuring Instrument input range may be 100-500 C
and the output range may be 4-20 mA. Most sensors have limited range over
which a process variable measured is define by lower and higher range value.
1.8.1.5 Precision
Precision indicates the repeatability or reproducibility of an instrument (but does
not indicate accuracy). If an instrument is used to measure the same input, but at
different instants, spread over the whole day, successive measurements may vary
randomly.
The random fluctuations of readings, is often due to random variations of
several other factors which have not been taken into account, while measuring the
variable. A precise instrument indicates that the successive reading would be very
close, or in other words, the standard deviation r e of the set of measurements
would be very small. In Simple words, it indicates how close a reading is to the
Average value.