The majority of force or torque systems we calibrate each year consist of load or torque cells, an indicator or readout, load cell 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. A difference in load cell cable length in a 4-wire system will have large measurement errors.
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, and the 6-wire setup can eliminate almost all the errors associated with load cell cable length.
4-Wire Systems
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 varying the load cell cable length will produce additional errors. A known example of this involves changing a 28-gauge or 22-gauge cable. The errors associated with load cell cable length on a 28-gauge cable will have 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.
2. 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 an additional error and should be avoided.
4. 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 recommends.
6-wire systems
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 signals.
Test Morehouse conducted back in early 2000 showing what happens when you vary the load cell cable length on 4 wire systems.
The graph above demonstrates the difference that load cell 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 lengths but the same gauge cables. The graph above shows a difference in the output of around 0.05% by reducing the 4-wire load cell cable length by about 40 inches.
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