What is the difference between 4-wire and 6-wire load cells?
Some load cells have a cable with 4 wires and a screen; others have a cable with 6 wires and a screen. Those with 6 wires, in addition to the +input, -input, +signal and -signal terminations, have 2 wires called +Sense and -Sense. These are sometimes called +Reference (or +Ref) and -Reference (or -Ref).
The main difference in function of these 2 types is that load cells with a 6 wire cable can compensate for variations in the actual excitation voltage they receive from the amplifier/indicator. The resistance of an electrical cable (conductor) varies according to its length and any temperature changes, resulting in variations in the excitation voltage at the load cells. With long cables, there will be a drop in voltage from the original value supplied by the amplifier/indicator and the advantage of a 6 wire load cell is that this drop in voltage can be quickly and effectively compensated without it affecting the weight measurement.
4 wire load cells
4 wire load cells are already calibrated and thermally compensated together with the permanent length of cable supplied during their manufacture. We recommend that you do not shorten the cable of a 4 wire load cell if it is too long; it is better to coil up the cable. This is because the factory calibration and compensation of a 4 wire load cell will be compromised if you shorten the cable. There are no sense wires to compensate for the new cable length.
When connecting 4 wire load cells together into a junction box before the weight indicator/transmitter, we recommend using a dedicated 6 wire load cell cable (for example the Thames Side polyurethane cable CA-PU-5.7MM-6C) to connect the junction box to the indicator/transmitter. This will compensate for any voltage drops over the length of cable between them. In any event, the cable should be well shielded and have sufficient cross section (at least 0.2 mm2) to limit the voltage drop along its length.
6 wire load cells
The above precaution about cutting the cables does not apply to load cells with a 6 wire cable. The two sense wires are capable of measuring the actual excitation voltage seen at the Wheatstone bridge inside the load cell, therefore the mV signal from the load cell can be adjusted according to the actual excitation it experiences. If an installation engineer wants to shorten the cables, they can do so without compromising the load cell performance.
The Sense (reference) wires are connected to the sense terminals of the weight indicator, so that this can measure and adjust the amplifier on the actual voltage that arrives to the load cells. The 6-wire load cells are therefore preferred over those with 4 wires, also on these load cells there are no limitations in the event the installer wants to shorten the cables.