Mechanical Connectors in Strain Gauge Circuits

Mechanical Connectors in Strain Gauge Circuits
Introduction: Mechanical Connectors
Mechanical Connectors in Strain Gage
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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Introduction: Mechanical Connectors
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A Measurements Group
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Mechanical Connectors in Strain Gage
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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 (1 of 2) [2/22/2001 2:03:10 PM]
Page 2: Mechanical Connectors
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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Mechanical Connectors in Strain Gage
(... continued)
There is another connector-related source of zero-shift which can sometimes be
mistaken for contact resistance variation. The connector alloy, in contact with the
leadwire, acts as a thermocouple, and can introduce a dc potential in the gage
circuit. With dc instrumentation, the thermoelectric potential appears as zero-shift.
The thermocouple effect occurs, of course, whenever two junctions of dissimilar
metals in the same circuit are at different temperatures, and the potential is
developed whether the junctions are soldered or mechanically connected. The
sensitivity of a connection to thermoelectrically caused zero-shift can be tested by
warming the connection with a heat gun, or by cooling it with a compressed gas
such as Freon or carbon dioxide.
The prescription for minimizing contact resistance problems with mechanical
connectors can be summarized as follows:
Select only, high-quality connectors -- gold-plated when practical.
Use solid, robust leadwire terminations.
Firmly clamp leadwires in connectors, always at the same point on the
lead wire.
"Exercise" connectors periodically by moving and re-tightening wire
clamps, and by several insertion/ removal cycles of plug-and-socket
Monitor and maintain the connections for resistance stability and
In performing the previous step, be certain to distinguish between
contact resistance changes and thermoelectric potentials.
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