nominal bridge resistance setting for load cell

[IamRubber]

Labview asks for a "nominal bridge resistance" when setting up a channel for a load cell.  Is it asking for the input resistance (measured at the EXCITATION terminals) or the ouput resistance (measured at the SIGNAL terminals)?

[RavensFan]

It should be the resistance of any specific leg of the bridge.  But for a perfectly balanced bridge, that would be the resistance across the entire bridge whether reading across the input/exciation leads, or the output/signal leads.

[IamRubber]

This is a full bridge load cell, and the calibration data sheet from the manufacturer lists input and ouput resistances seperatley, and at 2 different values.

[RavensFan]

They must have extra resistances added to either the input legs or output legs.  But the nominal bridge resistance should be the resistance of a given leg and would not include these extra resistances.  If you're not sure, ask the vendor of the load cell.

[IamRubber]

Ok, waiting to hear back from the manufacturer.  The input resistance is listed at 377.40 Ohms.  The output is listed at 352.10 Ohms.  So if extra resistance is added to one leg it must be the output and I'm guessing I should use the input.  The thing is Labview asks for the number right next to where you put in the excitation voltage, which worries me that I should use the external.  I'm not sure what Labview is doing with that number and how it affects the data output.

[RavensFan]

It sounds like a 350 ohm load cell bridge to me.  There is probably an extra 2.1 ohms on the output due to lead wires.  And approximately 27.4 ohms added to one of the input leads.

[MarkMoss]

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@IamRubber wrote:

Ok, waiting to hear back from the manufacturer.  The input resistance is listed at 377.40 Ohms.  The output is listed at 352.10 Ohms.  So if extra resistance is added to one leg it must be the output and I'm guessing I should use the input.  The thing is Labview asks for the number right next to where you put in the excitation voltage, which worries me that I should use the external.  I'm not sure what Labview is doing with that number and how it affects the data output.

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Perfectly normal.  The small variations (i.e. 377 vs 350) ohms aren't going to affect your measurement.  As far as I know, the only reason to specify the bridge resistance is to ensure that the resistance of your bridge(s) is not so low that it will overload the excitation voltage source.

Mark Moss