System
Disturbance Due to Measurement
About system disturbance due to
measurement
Disturbance of the measured system by
the act of measurement is one source of systematic error. If we were
to start with a beaker of hot water and wished to measure its
temperature with a mercury-in-glass thermometer, then we would take
the thermometer, which would be initially at room temperature, and
plunge it into the water. In so doing, we would be introducing a
relatively cold mass (the thermometer) into the hot water and a heat
transfer would take place between the water and the thermometer. This
heat transfer would lower the temperature of the water. Whilst in this
case the reduction in temperature would be so small as to be
undetectable by the limited measurement resolution of such a
thermometer, the effect is finite and clearly establishes the
principle that, in nearly all measurement situations, the process of
measurement disturbs the system and alters the values of the physical
quantities being measured.
Another example is that of measuring car
tire pressures with the type of pressure gauge commonly obtainable
from car accessory shops. Measurement is made by pushing one end of
the pressure gauge on to the valve of the tire and reading the
displacement of the other end of the gauge against a scale. As the
gauge is used, a quantity of air flows from the tire into the gauge.
This air does not subsequently flow back into the tire after
measurement, and so the tire has been disturbed and the air pressure
inside it has been permanently reduced.
Thus, as a general rule, the process of measurement always disturbs
the system being measured. The magnitude of the disturbance varies
from one measurement system to the next and is affected particularly
by the type of instrument used for measurement. Ways of minimizing the
disturbance of measured systems are an important consideration in
instrument design. A prerequisite for this, however, is an accurate
understanding of the mechanisms of system disturbance.
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Measurement System Errors
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