Introduction: Mechanical Connectors

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A question that often arises, especially for short-term strain gage measurements, is: "Can I use mechanical connectors (screw-type, etc.) to simplify connecting and disconnecting strain gages in my instrument circuit?"

As for many other simple questions, there is no simple answer to this one. In general, however, it is preferable to use as few mechanical connections as possible in any strain gage circuit. This is because mechanical connections are potentially less stable in joint resistance than soldered connections. When such an answer is given, the next question is apt to be: "But what about my strain gage instruments? All of them are equipped with binding posts or other types of mechanical connectors." True enough. The connectors are there for the convenience of the instrument user, but these are carefully chosen for the intended purpose, and are often gold-plated or gold-flashed to minimize and stabilize the contact resistance.

It is always necessary to keep in mind the relatively small resistance changes involved in strain gage measurements. For a uniaxial working stress of, say, 30 000 psi (207 MPa) in steel, the strain level is 1000 microstrain. With a 120-ohm gage (G.F. ~ 2.0), the strain-induced resistance change is only about 250 milliohms. Under these conditions a 5-milliohm change in contact resistance will cause an indicated zero-shift of some 20 microstrain -- enough to alarm a careful practitioner. Yet contact resistance changes of this magnitude can be caused by an oxidized connector contact surface or by a poorly terminated leadwire . Even changing the insertion depth of the stripped end of the leadwire into the connector can produce a measurable zero-shift. With AWG #26 wire (0.4 mm dia.), for instance, changing the insertion depth by as little as 0.3 in (~8mm) when reconnecting the wire will offset the zero by close to 5 microstrain.

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