Measurement Computing CIO-EXP-GP - Manual (Page 36)

Referring back to Figure 7-6, the legs of the bridge are labeled A, B, C and D. Table 7-3 below matches
the legs of the bridge to the resistor number nomenclature that appears on the CIO-EXP-GP.
Table 7-3. Bridge Completion Resistor Identities
RX59 RX48 RX46 RX45 RX44 RX43 7
RX58 RX42 RX40 RX39 RX38 RX37 6
RX57 RX36 RX34 RX33 RX32 RX31 5
RX56 RX30 RX28 RX27 RX26 RX25 4
RX55 RX14 RX22 RX21 RX20 RX19 3
RX54 RX18 RX16 RX15 RX14 RX13 2
RX53 RX12 RX10 RX9 RX8 RX7 1
RX52 RX6 RX4 RX3 RX2 RX1 0
Arm Null Pot Bridge D Bridge C Bridge B Bridge A Channel
Some values of precision resistors are available from Measurement Computing Corp.
7.8.2 Nulling Potentiometers & Arm Resistor
Each circuit has a position for a nulling potentiometer and associated arm resistor. The purpose of the
nulling arm is to allow you to zero the reading of strain at a given strain position. There is no formula to
use to select the nulling potentiometer and arm resistor. Bridge resistor values and total gain selected for
the CIO-EXP-GP will affect adjustability for a given nulling circuit. An average value for the arm
resistor is 10k ohms. Start with that and adjust as required.
7.8.3 Strain Gauge Bridge Configuration Examples
Following are three typical strain gauge bridge configurations. They are by no means the only way to
connect a strain gauge to the CIO-EXP-GP. For example, there is no rule that says the ‘A’ leg must be
the strain gauge on a ¼ bridge implementation.
The examples below show how to translate strain to input voltage for the strain gauge
configuration used to measure simple bending strain. Other types of stress and strain: axial,
torsion, shearing , etc. are beyond the scope of this description.
These examples can be used to as a
guide for calculating the bridge voltage in your own application, and thus help you select the proper
amplifier gain and excitation voltage.
The use of quarter bridge, half bridge and full bridge strain gauge configurations are described.
The Application:
In these examples, imagine a beam extending out from a fixed point on a wall. Force is applied to deflect
the end of the beam downward. We know that the maximum strain to be measured will be 250µ
ε
(250
micro strain). Knowing the amount of force required and the size of the beam is not necessary, since
strain relates to the change in length of the surface of interest.
The Strain Gauge will be a metal foil type, 350 ohms resistance, Gauge Factor = 2. Refer to the
Appendix for information on these specifications.
The following example shows a bending strain measurement example. It can be used to calculate
the bridge voltage, and thus help the user select the proper amplifier gain and excitation voltage.
The use of one, two and four strain gauges will be examined.
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