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Mechanical Connectors
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Mechanical Connectors in Strain
Gage Circuits
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The lesson to be learned from the above is that if mechanical connectors are to be employed within the bridge circuit, a number of precautions should be taken to help ensure accurate strain measurements. One obvious measure is to use high-quality connectors, gold-plated if possible. In addition, however, leadwire terminations should be carefully made. If the wire is stranded, the strands should be wound snugly together, and then uniformly and smoothly tinned. When connecting to a binding post, the wire should be inserted to about the same depth each time, and the binding post should be tightened for a firm but non-crushing grip on the wire. After the strain indicator is turned on and balanced, there should be no perceptible strain indication due to shaking the free portion of the wire. With conventional screw-down binding posts, the preferred leadwire termination is a "spade" terminal, which should be attached to the leadwire by soldering, not by crimping. The spade terminal allows repeatable insertion depth into the connector, and it is not subject to crushing if the binding post is overtightened. 
Even the foregoing steps do not guarantee complete freedom from contact resistance variations. The quality of a mechanical connection generally degrades with time; and the rate of degradation depends upon the environment in which the system operates. Thus, monitoring and maintenance of the connection may be necessary to preserve its initial performance. One way to monitor the stability of the connection resistance is to use a highly stable resistor (such as the Micro-Measurements S-Type ) in a verification channel. If the resistor is connected to identical terminals and exposed to the same environment as the active circuits, zero-shift in the verification channel may be indicative of contact resistance variations in the connectors. When contact resistance variations are identified, cleaning of the contact areas may be necessary. Leadwire terminations, if not plated, can be polished; and binding posts or other gold-plated connectors can be washed with M-LINE GC-6 isopropyl alcohol (but never abraded). ( continued ... ) 
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