Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 10

Fig. 1.3 A typical spring balance—a deflection type weight measuring instrument

Fig. 1.4 Dead weight pressure gage

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