Page 117 - Mechanical Engineers' Handbook (Volume 2)

106 Bridge Transducers

self-generating response. For example, pressure applied to the diaphragm of a pressure trans-
ducer with bridge electrical power supplied modifies the impedance of the strain gage circuit
and results in a millivolt output (non-self-generating). Self-generating responses are those
attributable to various measurands applied to bridge transducers without electrical power
supplied. Examples of these responses include thermoelectric-, photoelectric-, pyroelectric-,
and magnetoelectric-induced voltages within the bridge circuit. Thus, there exist four
environment–response combinations in bridge transducers. The non-self-generating response
to the desired environment is defined as signal. The non-self-generating response to the
undesired environment, as well as the self-generating response to both the desired and un-
desired environment, is noise. Figure 28 illustrates the paths associated with these four com-
binations with path 4 being signal and paths 1, 2, and 3 being noise.
The quantifications of paths 1–4 can be accomplished by switching. If at some time
during the test bridge power is switched off, Fig. 28 indicates that only paths 1 and 3 will
exist. Since these paths are both noise, the bridge transducer response ideally should ap-
proach zero. Similarly, if the desired environment can be switched off for some time period,
only paths 1 and 2 remain. If path 1 was verified as being noise free when bridge power
was removed, path 2 also becomes quantified. If paths 1, 2, and 3 are all shown to provide
negligible signal level, transducer output becomes attributable to path 4, which has been
defined as signal. In summary:
Remove bridge power: Document paths 1 and 3.
Reapply power and remove desired environment: Document paths 1 and 2.
If the documented signal paths are of sufficiently small magnitude to be considered incon-
sequential during test, the non-self-generating response to the desired environment (signal)
is recorded.
Some question may arise as to how to implement these procedures, particularly in tran-
sient measurement situations. For example, assume a bridge accelerometer is to be used to
measure a transient acceleration event. Three accelerometers can be fielded in close physical

Figure 28 Bridge transducer model for noise hunting and documentation. (Adapted with permission
from ‘‘Information as a ‘Noise Suppression’ Method,’’ by Peter Stein, Stein Engineering Services, Inc.,
Phoenix, AZ, LR/MSE Publ. 66, 1975.)