he majority of force or torque systems we calibrate each year consist
of load or torque cells, an indicator or readout, cables, adapters and
some sort of shipping or carrying case. Around 90 % of these systems
come in with an indicator only capable of supplying an excitation to the
bridge and measuring a signal coming back from the transducer. This
is known as a 4-wire system.
We are going to discuss the difference between true 6 wire systems.
If you are using a 4 wire system, we hope to convince you the benefits
of switching to a 6-wire system far outweigh the disadvantages of
continuing to use a 4-wire system.
4 WIRE SYSTEM CABLE AND DIAGRAM
In understanding the errors associated with a 4-wire cable, we must
first understand why this error exists. In general, cable resistance is
a function of temperature. The temperature change on a cable affects
the thermal span characteristics of the load cell/cable system. On a
4-wire cable, this will affect thermal span performance. Simply put, as
the temperature changes, the resistance of the cable changes and can
cause a voltage drop over the cable length. A 4-wire setup simply
cannot compensate for variations in lead resistance.
Substituting a cable of a different gauge or a different length will
produce additional errors. A known example of this involves changing a
28-gauge or 22-gauge cable. On a 28-gauge cable, there will be a loss
of sensitivity of approximately 0.37% per 10 feet of 28-gauge cable. On
a 22-gauge cable, there will be a loss of sensitivity of around 0.09%
per 10 feet of 22-gauge cable.
What you need to know about 4 wire systems.
1. If you damage or replace your cable, the system may need to be calibrated immediately following replacement or repair.
Operating at different temperatures will change the resistance, which
will cause a voltage drop, resulting in a change of measured output.
3. Cable substitution will result in additional error and should be avoided.
Cables used for 4-wire systems should have a S/N, or a way to make
sure the same cable stays with the system, it was calibrated with. -
This would be a Good Measurement Practice Technique Morehouse highly
6 Wire System and Cable Diagram
To eliminate the errors associated with a 4-wire system requires a
6-wire cable, which is run to the end of the load cell cable or
connector, and is used with an indicator that has sense lead capability.
With a 6-wire setup, the sense lines are separate from the excitation
lines, thereby eliminating effects due to variations in lead resistance.
This allows long cable runs in outdoor environments with extreme
temperatures. Wiring a 6-wire cable for sense is as easy as running two lines from the
load cell’s positive excitation pin and two wires from the load cell’s
negative excitation pin; the remaining 2 wires are run to positive and
negative sense. The 6 wires then feed into the meter with positive
excitation and positive sense running to the meter; negative excitation
and sense are run to the appropriate meter connections, as well as
positive and negative signal.
Pictured Above: Morehouse HADI 6 Wire System
The testing we conducted at Morehouse
|Test Morehouse conducted back in early 2000 showing what happens when you vary the cable length on 4 wire systems.|
The graph above demonstrates the difference that cable length can make
on output. It should be clear that a 4-wire cannot be interchanged
without requiring a recalibration of the entire system. A 6-wire cable
should be the desired choice if you intend on interchanging cables or
are operating in an uncontrolled environment. In the video we posted (https://youtu.be/0I2-MVNqmlA
we observed a difference of 0.106% between using two different length
but same gauge cables. The graph above shows a difference in output of
around 0.05% by reducing the 4-wire cable by about 40 inches.
written by Henry Zumbrun