Vishay Micro-Measurements System 7000 Programmer’s Reference Manual
Page 50 of 142
Shunt Calibration (strain gage sensors only)
This calibration factor is primarily used to remove any offsets due to lead-wire resistance. The calibration
factor should be near 1. It is used as a multiplier while scaling the channel’s data readings into
milliVolts/Volt or µStrain. You should calculate the calibration factor for a strain gage channel based on
a single-point reading and either the nominal resistance and shunt (or remote) calibration resistor value or
a simulated strain value.
Excitation and the bridge configuration settings should be set to the required values before the strain gage
channel is calibrated. Typically the channel is also zeroed before calibration. If you have a remote sense
circuit attached, you should leave the calibration factor at the default value of 1.
Please refer to the “System 7000 Instruction Manual” for more detailed information.
The steps in performing a shunt calibration are:
1. Configure the channel’s excitation and bridge settings.
2. Take a zero reading.
3. Enable the calibration resistor (either Shunt or Remote).
4. Take a single-point reading.
5. Disable the calibration resistor.
6. Add the zero offset value to the reading.
7. Divide by 2 to convert the reading into µStrain.
8. Divide by the simulated µStrain.
If you are using the LabVIEW instrument driver, VIs have been written that perform steps 3 through 8 for
you.
Following is a pseudo code example:
// Set the excitation to 1000mV
ConfigureStrainGageCardExcitation(card, ENABLE_EXCITATION,1000)
// Configure the channel’s bridge settings as a 120Ω, quarter bridge
ConfigureStrainGageChannelBridgeSettings(card,channel,QUARTER_BRIDGE,120)
// Take a single point reading for the zero reading
Zero_Offset = GetStaticADCReading(channel)
// Enable the shunt calibration resistor (may use the remote resistor instead)
ShuntCalEnable(ENABLE)
// Take a single point reading
Cal_Factor = ReadSingleChannel(channel)
// Disable the shunt calibration resistor
ShuntCalEnable (DISABLE)
// Add the zero offset value to the cal factor
Cal_Factor = Cal_Factor + Zero_Offset
// Convert the cal factor into units of microStrain
Cal_Factor = Cal_Factor / 2
// Calculate the simulated strain value (or use a fixed value). K is the gage
// factor. Rg is the resistance of the shunted bridge arm (nominal resistance).
// Rc is the calibration resistor value in ohms. (10**6 is notation for 10
6
)
Simulated_Strain = (Rg * 10**6) / (K * (Rc + (Rg / 2)))
// Calculate the final calibration Factor
Calibration_Factor = Cal_Factor / Simulated_Strain
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