5 bridge resistance measurements – Campbell Scientific CR5000 Measurement and Control Module User Manual
Page 71
Section 3. CR5000 Measurement Details
3-17
3.5 Bridge Resistance Measurements
There are six bridge measurement instructions included in the standard
CR5000 software. Figure 3.5-1 shows the circuits that would typically be
measured with these instructions. In the diagrams, the resistors labeled R
s
would normally be the sensors and those labeled R
f
would normally be fixed
resistors. Circuits other than those diagrammed could be measured, provided
the excitation and type of measurements were appropriate.
All of the bridge measurements have the option (RevEx) to make one set of
measurements with the excitation as programmed and another set of
measurements with the excitation polarity reversed. The offset error in the two
measurements due to thermal emfs can then be accounted for in the processing
of the measurement instruction. The excitation channel maintains the excitation
voltage or current until the hold for the analog to digital conversion is
completed. When more than one measurement per sensor is necessary (four
wire half bridge, three wire half bridge, six wire full bridge), excitation is
applied separately for each measurement. For example, in the four wire half
bridge when the excitation is reversed, the differential measurement of the
voltage drop across the sensor is made with the excitation at both polarities and
then excitation is again applied and reversed for the measurement of the
voltage drop across the fixed resistor.
Calculating the actual resistance of a sensor which is one of the legs of a
resistive bridge usually requires additional processing following the bridge
measurement instruction. In addition to the schematics of the typical bridge
configurations, Figure 3.5-1 lists the calculations necessary to compute the
resistance of any single resistor, provided the values of the other resistors in the
bridge circuit are known.
BrHalf
X = result w/mult = 1, offset = 0
X
V
V
R
R
R
x
s
s
f
=
=
+
1
(
)
R
R
X
X
R
R
X
X
s
f
f
s
=
−
=
−
1
1
BrHalf3W
X = result w/mult = 1, offset = 0
X
V
V
V
V
R
R
X
s
f
=
−
−
=
2
2
1
1
R
R X
R
R
X
s
f
f
s
=
=
/