Developing Strain Gauges and Instruments
STRAIN GAUGE MEASUREMENT
1-Gauge 4-Wire Strain measurement method
For strain gauge measurement, the Wheatstone bridge
circuit is employed according to the number of strain gauges
to be used and measuring purpose. In a quarter bridge
configuration, three wire method is widely used to remove the
effect of temperature on gauge leadwire resistance. However,
some measuring error occurs owing to gauge factor caused
by leadwire resistance and variation in the contact resistance
when leadwires are connected to the bridge.
The TML 1-gauge 4-wire strain measurement method does not
use the Wheastone bridge circuit, but instead a simple series
circuit with gauge resistance (R) and reference resistance
(Rs) to measure the strain. Find the strain with voltage (V)
generated in gauge resistance and voltage (Vs) generated
in reference resistance. As the path where the current runs
and the path where the voltage is measured are different, it is
possible to perform a measurement without being affected by
leadwire resistance or contact resistance (r).
R
i
r1
r2
r3
V
where
R
Rs
r1 ~ r4
i
E
V
Vs
In the conventional method, as thick and short leadwires as
possible are recommended to keep the resistance of
leadwires lower. However, as the 1-gauge 4-wire method is
not influenced at all by the leadwire resistance, it is possible
to connect thin and long leadwires to strain gauges.
Compares Quarter Bridge 3-Wire vs. 1-Gauge 4-Wire
Size of leadwire
Quarter Bridge 3-wire
(Wheatstone Bridge)
Thick
Weigt of leadwire
Not less
Less
Not available
Available
Different leadwires
Color of leadwire
1-Gauge 4-Wire
Thin
Planned
Free
Load to specimen
Not less
Less
Carrying cost
Not less
Less
Strain gauges with leadwires and modular plug
E
r4
Vs
Rs
Leadwire resistance
: Gauge resistance
: Reference resistance
: Leadwire resistance and contact resistance
: Current flowing in gauge resistance and
reference resistance
Most of our strain gauges can be supplied with the preattached 4 leadwires and modular plug (RJ12) that make up
our proprietary 1-Gauge 4-Wire system. Because a modular
plug is attached to the end of the leadwires, soldering or
screwing connections to a measuring instrument is
unnecessary, but the instrument must be of TML make. The
4-wire leadwires are covered with polypropylene resin which
does not generate noxious gas even if exposed to fire.
Single element strain gauge with 4-wire paralleled leadwire
: Excitation voltage
: Voltage of gauge resistance
: Voltage of reference resistance
Not affected by contact resistance
Conventionally, leadwire extension and connection to a
measuring instrument are done by soldering or the use of
specailly designed connectors. As the 1-gauge 4-wire method
is not affected at all by contact resistance, a modular plug can
be used. The modular plug makes leadwire extension and
connection to the instrument inexpensive and efficient while
preventing wiring mistakes are eliminated and also RoHScompliant lead free soldering is unnecessary.
Three-element rectangular rosette strain gauge
With 6-wire paralleled leadwire and modular plug
With 4-wire paralleled leadwire and modular plug
Compares Quarter Bridge 3-Wire vs. 1-Gauge 4-Wire
Plug connection
Soldering works
Wiring time
Wiring error
Quarter Bridge 3-wire
(Wheatstone Bridge)
Not available
Required
Required
Occurs
RoHS-directive
Lead inclusive
1-Gauge 4-Wire
Available
Not required
Less
None
Lead free
Using commercial interconnection
adapter, leadwire extension can be
easily done.
12