Pressure and Force Sensors
Selection Considerations
104
Honeywell 1MICRO SWITCH Sensing and Control 11-800-537-6945 USA 1F1-815-235-6847 International 11-800-737-3360 Canada
REFERENCE AND APPLICATION DATA
COMPATIBILITY
It is very important to insure compatibility
between the pressure or force sensor and
the application in which it is used. The
following should be considered before a
sensor selection is made: (1) material; (2)
chemicals; (3) concentration; (4) temper-
ature; (5) exposure time; (6) type of expo-
sure; (7) criteria for failure; and (8) general
information such as application environ-
ment, protection of the device, and other
foreign substances in the area.
In all cases, the customer is ultimately
responsible for assuring that the de-
vice/material is suitable for the applica-
tion.
ERRORS THAT AFFECT
SENSOR PERFORMANCE
When calculating the total error of a pres-
sure or force sensor, the following de-
fined errors should be used. To deter-
mine the degree of specific errors for the
pressure sensor you have selected, refer
to that sensor’s specification page in the
catalog.
NOTE: In specific customer applications
some of the published specifications can
be reduced or eliminated. For example, if
a sensor is used over half the specified
temperature range, then the specified
temperature error can be reduced by half.
If an auto zeroing technique is used, the
null offset and null shift errors can be
eliminated.
Null offset is the electrical output present
when the pressure or force on both sides
of the diaphragm is equal.
Span is the algebraic difference be-
tween the output end points. Normally
the end points are null and full scale.
Null temperature shift is the change in
null resulting from a change in temper-
ature. Null shift is not a predictable error
because it can shift up or down from unit
to unit. Change in temperature will cause
the entire output curve to shift up or down
along the voltage axis (Figure 1).
Sensitivity temperature shift is the
change in sensitivity due to change in
temperature. Change in temperature will
cause a change in the slope of the sensor
output curve (Figure 2).
Linearity error is the deviation of the sen-
sor output curve from a specified straight
line over a desired pressure range. One
method of computing linearity error is
least squares, which mathematically pro-
vides a best fit straight line (B.F.S.L.) to
the data points (Figure 3).
Figure 1
Null Shift Error
Figure 2
Sensitivity Shift Error
Figure 3
Best Fit Straight Line Linearity
Figure 4
Terminal Base Linearity
Another method is terminal base linearity
(T.B.L.) or end point linearity. T.B.L. is
determined by drawing a straight line (L1)
between the end data points on the out-
put curve. Next draw a perpendicular line
from line L1 to a data point on the output
curve. The data point is chosen to achieve
the maximum length of the perpendicular
line. The length of the perpendicular line
represents terminal base linearity error
(Figure 4).
Pressure and Force Sensors
Selection Considerations
Honeywell 1MICRO SWITCH Sensing and Control 11-800-537-6945 USA 1F1-815-235-6847 International 11-800-737-3360 Canada
105
REFERENCE AND APPLICATION DATA
Repeatability error is the deviation in
output readings for successive applica-
tions of any given input pressure or force
with other conditions remaining constant
(Figure 5).
Hysteresis error is usually expressed as
a combination of mechanical hysteresis
and temperature hysteresis. MICRO
SWITCH expresses hysteresis as a com-
bination of the two effects (Figure 6).
Mechanical hysteresis is the output de-
viation at a certain input pressure or force
when that input is approached first with
increasing pressure or force and then
with decreasing pressure or force.
Temperature hysteresis is the output
deviation at a certain input, before and
after a temperature cycle.
Ratiometricity implies the sensor output
is proportional to the supply voltage with
other conditions remaining constant. Ra-
tiometricity error is the change in this pro-
portion and is usually expressed as a per-
cent of Span.
CALCULATING ACCURACY OR
TOTAL ERROR
When choosing a pressure or force sen-
sor, the total error contribution is impor-
tant. The following methods take into ac-
count the individual errors and the unit-to-
unit interchangeability errors.
Two methods for calculating total error
are:
Root Sum Square (R.S.S.) using maxi-
mum values, and worst case error. R.S.S.
method gives the most realistic value for
accuracy. With the worst case error meth-
od, the chances of one sensor having all
errors at the maximum are very remote.
Example
An application requires 0-15 psig, 5°to 50°temperature range, 7VDC supply. A
142PC15G will be used for the example (see 142PC15G specifications on page 37).
1. Determine error values
Parameter Max. (% Span)
Null offset .05V x 100% J1.0%
5V
Span error .05V x 100% J1.0%
5V
Linearity 0.75%
Combined null/span shift
(Calculate at max. and min.
application temperature. Use
higher of the two numbers)
50°C – 25°Cx 1% shift J0.70%
63°C – 25°C
25°C – 5°Cx 1% shift J0.50%
25°C – (−18°C)
Repeatability & Hysteresis 0.30%
Stability for 1 year 1.0%
Ratiometricity error 1.0%
2. Calculate total error
The R.S.S. method: take the square root of the sum of the squares of the errors
determined in Step 1
R.S.S. max. J£1.02+1.02+0.752+0.72+0.32+1.02+1.02
R.S.S. max. error J2.3% span max.
Worst Case Error J1.0 +1.0 +0.75 +0.7 +0.3 +1.- +1.0
Worst Case Error J5.75% span absolute maximum.
Reference/Index
