Null or Deflection Type Instruments
About the null or deflection type of
The pressure gauge is a good example of
a deflection-type instrument, where the value of the quantity being
measured is displayed in terms of the amount of movement of a pointer.
An alternative type of pressure gauge is the dead-weight gauge which
is a null-type instrument. Here, weights are put on top of the piston
until the downward force balances the fluid pressure. Weights are
added until the piston reaches a datum level, known as the null point.
Pressure measurement is made in terms of the value of the weights
needed to reach this null position.
The accuracy of these two instruments depends on different things. For
the first one it depends on the linearity and calibration of the
spring, whilst for the second it relies on the calibration of the
weights. As calibration of weights is much easier than careful choice
and calibration of a linear-characteristic spring, this means that the
second type of instrument will normally be the more accurate. This is
in accordance with the general rule that null-type instruments are
more accurate than deflection types.
In terms of usage, the deflection-type instrument is clearly more
convenient. It is far simpler to read the position of a pointer
against a scale than to add and subtract weights until a null point is
reached. A deflection-type instrument is therefore the one that would
normally be used in the workplace. However, for calibration duties,
the null-type instrument is preferable because of its superior
accuracy. The extra effort required to use such an instrument is
perfectly acceptable in this case because of the infrequent nature of
More Instrument Classification