Calculate lead wire resistance

Experimental calculation of lead wire resistance is available for all bridge channel types on the legacy eDAQ layers (i.e., for EBRG/ELBRG bridge channels and EHLS/ELHLS SMSTRB bridge channels). This is also available on the eDAQXR EXRBRG bridge channels.

For the EXRBRG bridge modes that use the sense lines, lead wire resistance is generally not a factor since the sense lines automatically do the compensation. However, this is not the case when the Internal shunt resistor scaling mode is used. As such, lead wire resistance calculations are available only when the Internal shunt resistor scaling mode is used with the bridge modes that use the sense lines.

The system can accurately calculate the lead wire resistance if and only if the bridge resistances are all equal and accurately known, and the resistances of all of the relevant lead wires are equal. The lead wire that runs to the +Sig bridge node is not relevant to lead wire resistance compensation; however, all of the others (i.e., the lead wires that run to the +Ex, -Ex, and -Sig bridge connections) are relevant.

To use this method, the Bridge type and Bridge resistance fields must be defined correctly. Note that it is recommended that the Excitation be set as high as possible to produce the most accurate results; however, this is not a critical factor. Based on experiments using 350 Ohm bridges, the differences in the calculated resistances differ by less than 0.05 Ohms depending on the use of 5V or 10V excitation.

The user only needs to ensure that the transducer is stable (e.g., unloaded) then click Lead Wire Resistance in the Setup task pane or the Lead Wire Resistance button in the Edit channel dialog. The system will then run both Up scale and Down scale shunt experiments using the smallest shunt resistor available (i.e., the nominal 50K Ohm resistor for all legacy eDAQ layer channels) to optimize the calculation accuracy.

This is somewhat of a time consuming process. The user interface will report the calculated lead wire resistance values resulting from both the Up scale and Down scale shunt experiments. Normally, these two values should be very nearly equal (i.e., having a difference of ±0.1 Ohm or less). If this is not the case, then one of the requirements for using this tool has most likely been violated.