sc-2043-sg: True three-wire strain gauge possible?

[Bife]

Hello,

Could anyone please give me some advice?

I can't understand the instructions on page 3-9 of the manual of the SC-2043-SG (http://www.ni.com/pdf/manuals/371201a.pdf) for the connections for a three-wire strain gage setup.

Surely, if I "connect the two wires sharing the same end of the strain gauge at the CHn+ screw terminal, and connect the single wire end to the +EXn screw terminal" then this is not the same as the three-wire set-up as shown in Fig 8(b) of the NI 'Measuring Strain with Strain Gauging' guide (http://zone.ni.com/devzone/cda/tut/p/id/3642)?

In fact, this is just doubling up one of the strain gauge wires, isn't it?

Is it actually possible to connect the SC-2043-SG for true three-wire strain gauging, as in Fig 8(b) of the NI 'Measuring Strain with Strain Gauging' guide?

TIA

[Guillaume_H]

Hi,

Thank you for posting on National Instruments Forums.

In fact, the operation you describe (from the SC-2043-SG manual) corresponds to the figure 8.b of the guide in the developer zone.

Indeed, when connecting a gauge to a quarter bridge, you will need one wire to connect your gauge to the +EXn –this will flow the excitation current to the gauge – and one wire to the CHn+ - this will create the potential difference. This is for a two wire configuration (see figure 8.a of the guide – RL (line resistors) are the resistance of your wire and RG is your strain gauge). For a 3 wire strain gauge, you will need to connect the third wire somewhere. Especially, you will need to connect it to the quarter bridge completion resistor (that, by the way, you have to install yourself, with a SC-2043-SG). As this resistor (called R3 on figure 8.b) is at the same potential (CHn+) as the pin used to create the potential difference, you have to connect it to the same spot.

It might be puzzling indeed, and it seems like you are just doubling the wires, but it really is the right way to connect your three wire gauge. As each of your wire might have a different resistance value, they have been represented on figure 8.b by the names RL1, 2 and 3.

Finally the figure 3-3 on the SC-2043-SG manual shows the connection to a quarter bridge, but with a two wire gauge, so it might be a little puzzling.

I hope this information will help you.

Best regards,