6J22 Evanohm Stable Resistance Foil/ Strip/ Tape For Strain Gauge
Application
Nichrome, a non-magnetic 80/20 alloy of nickel and chromium, is the
most common resistance wire for heating purposes because it has a
high resistivity and resistance to oxidation at high temperatures.
When used as a heating element, resistance wire is usually wound
into coils. One difficulty in using nichrome wire is that common
tin-based electrical solder will not bond with it, so the
connections to the electrical power must be made using other
methods such as crimp connectors or screw terminals.
TANKII (Alloy 875/815), a family of iron-chromium-aluminium
(FeCrAl) alloys used in a wide range of high-temperature
applications.
Constantan [Cu55Ni45] has a low temperature coefficient of
resistivity and as a copper alloy, is easily soldered. Other
constant-resistance alloys include manganin [Cu86Mn12Ni2], Cupron
[Cu53Ni44Mn3]and Evanohm.
The Evanohm family of nickel-chrome alloys
[Ni72Cr20Mn4Al3Si1],[Ni73Cr20Cu2Al2Mn1Si], have high resistance,
low temperature coefficient of resistance, low electromotive force
(Galvani potential) when in contact with copper, high tensile
strength, and also are very stable with regards to heat treatment.
Balco [Ni70Fe30] and similar alloys have very high, but more
linear, temperature coefficient of resistivity, making them
suitable for sensing elements.
Many elements and alloys have been used as resistance wire for
special purposes. The table below lists the resistivity of some
common materials. The resistivity of amorphous carbon actually has
a range of 3.8 - 4.1 × 10−6 Ω m.
A strain gauge (also spelled strain gage) is a device used to
measure strain on an object. Invented by Edward E. Simmons and
Arthur C. Ruge in 1938, the most common type of strain gauge
consists of an insulating flexible backing which supports a
metallic foil pattern. The gauge is attached to the object by a
suitable adhesive, such as cyanoacrylate. As the object is
deformed, the foil is deformed, causing its electrical resistance
to change. This resistance change, usually measured using a
Wheatstone bridge, is related to the strain by the quantity known
as the gauge factor.
Strain Gauge Practice:
An excitation voltage is applied to input leads of the gauge
network, and a voltage reading is taken from the output leads.
Typical input voltages are 5 V or 12 V and typical output readings
are in millivolts.
Foil strain gauges are used in many situations. Different
applications place different requirements on the gauge. In most
cases the orientation of the strain gauge is significant.
Gauges attached to a load cell would normally be expected to remain stable over a period of
years, if not decades; while those used to measure response in a
dynamic experiment may only need to remain attached to the object
for a few days, be energized for less than an hour, and operate for
less than a second.
Strain gauges are attached to the substrate with a special glue.
The type of glue depends on the required lifetime of the
measurement system. For short term measurements (up to some weeks)
cyanoacrylate glue is appropriate, for long lasting installation
epoxy glue is required. Usually epoxy glue requires high
temperature curing (at about 80-100 °C). The preparation of the
surface where the strain gauge is to be glued is of the utmost
importance. The surface must be smoothed (e.g. with very fine sand
paper), deoiled with solvents, the solvent traces must then be
removed and the strain gauge must be glued immediately after this
to avoid oxidation or pollution of the prepared area. If these
steps are not followed the strain gauge binding to the surface may
be unreliable and unpredictable measurement errors may be
generated.
Strain gauge based technology is used commonly in the manufacture
of pressure sensors. The gauges used in pressure sensors themselves are commonly made
from silicon, polysilicon, metal film, thick film, and bonded foil.
