Long wire and small gage wire have greater resistance, which can result in gain error. The resistance in the wires that connect the excitation voltage to the bridge causes a voltage drop, which is a source of gain error. The FieldDAQ device includes remote sensing to compensate for this gain error. Connect remote sense wires to the points where the excitation voltage wires connect to the bridge circuit. Refer to the following figure for an illustration of how to connect remote sense wires to the FieldDAQ device.
The actual bridge excitation voltage is smaller than the voltage at the EX+ and EX- pins due to the voltage drop across the resistance of the leads. If you do not use remote sensing of the actual bridge voltage, the measured value will be the ratio of the ±AI voltage to the ±EX voltage and will result in a gain error of:
For example, if you connected a 350 Ω full-bridge sensor with lead wires that each had 0.35 Ω of resistance, you would observe a gain error of -(2 * 0.35)/350 = -0.2%, and all of your readings would be reduced by -0.2%.
If you connect the remote sense signals directly to the bridge resistors, the FieldDAQ device senses the actual bridge voltage and eliminates the gain errors caused by the resistance of the EX+ and EX- leads by returning a measured value that is the ratio of the ±AI voltage to the ±RS voltage. If you leave the remote sense signals disconnected (for example, in a 4-wire connection to a full bridge sensor), the FieldDAQ device automatically senses the excitation voltage at the device.
Shunt Calibration—Use shunt calibration instead of remote sense with quarter-bridge sensors. Shunt calibration can correct for gain errors from the resistance of the lead wires between the EX pins on the FieldDAQ device and the strain gage. The gain errors caused by lead wire resistance are the same for quarter bridge mode as for half bridge:
Shunt calibration involves simulating the input of strain by changing the resistance of the internal quarter-bridge completion resistor by some known amount. This is accomplished by shunting, or connecting, a large resistor of known value across the internal completion resistor, creating a known change in resistance. You then compare the change in measured value of the gage before and after shunting to the expected change. You can use the results to correct gain errors in the entire measurement path, or to simply verify general operation to gain confidence in the setup. The FieldDAQ provides an internal precision, shunt resistance, and a software control switch, as shown in the following figure. In 120 Ω mode, a 49.66 kΩ shunt is used; in 350 Ω mode, a 49.90 kΩ shunt is used.
To perform a shunt calibration with the FieldDAQ device, connect your external strain gage as shown in the figure and ensure that you have a stable signal, which is typically the unloaded state of the sensor. Acquire a measurement of the input first with the shunt calibration switch off and then again with the switch on. Subtract the first reading from the second reading, and compare this change with shunt calibration to the nominal values in the following table to get an indication of the gain errors from wiring resistance.
|Quarter-Bridge Resistance||Nominal Change in Reading with Shunt Calibration|
|120 Ω||-603.4 µV/V|
|350 Ω||-1747.4 µV/V|
You can correct future readings for the gain error caused by your lead wire resistance by multiplying future readings by this correction factor:
Correction Factor = Nominal Change (from table)/Measured Change
Use the DAQmx Perform Shunt Calibration VI/function or the DAQ Assistant to perform a shunt calibration, which sets the gain adjustment for a virtual channel. NI-DAQmx then uses this gain adjustment when scaling readings from the bridge. You must use the DAQmx Perform Bridge Offset Nulling Calibration VI/function immediately prior to using the DAQmx Perform Shunt Calibration VI/function because the Shunt Calibration uses the result of the Offset Nulling calibration as the unshunted value.