Page 130 - MEMS Mechanical Sensors

6.2 Physics of Pressure Sensing 119

F.S.

Sensor
output

Midscale
hysteresis

Pressure
50% 100%
Figure 6.8 Hysteresis.

loss of energy to entropy. This is more commonly a problem associated with tradi-
tional metal sensor elements rather than single crystal materials such as silicon. Sin-
gle crystal materials exhibit negligible hysteresis effects.

6.2.1.4 Sensitivity
This is the ratio of the sensor output to the applied pressure, and the units by which
it is expressed vary depending upon the manufacturers preferred units and the trans-
duction mechanism employed in the sensor.

6.2.1.5 Long-Term Drift
This is a measure of the change in sensor output over a specified period of time. Sen-
sor output at zero or full scale may be used. Drift over time is commonly associated
with the effects of temperature and pressure cycling on the sensor and its mounting.
The relaxation of adhesives, for example, is a common cause of drift.

6.2.1.6 Temperature Effects
The specified operating temperature range of the sensor can have many negative
effects on the sensor performance. Span temperature hysteresis is the difference in
span readings after application of minimum and maximum operating temperatures.
It is expressed as a percentage of full scale. Temperature coefficient of zero relates
sensor output at zero pressure over the specified operating temperature range. This
is commonly specified to fall within a percentage of full scale anywhere within the
temperature range. Temperature hysteresis of zero provides a measure of the repeat-
ability of the zero pressure reading after temperature cycling. Again this is specified
as a percentage of full scale.