A four-wire load cell carries excitation and signal on two pairs. A six-wire cell adds a sense pair that measures the excitation voltage actually arriving at the bridge, so the instrument can correct for the drop along the cable. The difference is not cosmetic: it is the difference between a stable span and a span that drifts with temperature as the copper resistance changes.
When four wires are fine
Short cable runs, a stable ambient temperature, and a cell whose cable resistance is a small fraction of the bridge impedance. A 350 ohm bridge with 2 m of 0.2 mm² cable sees a series resistance of well under 0.1 ohm, which is less than 0.03% of the bridge and well inside a C3 tolerance. In that case the extra pair buys nothing.
When you need the sense pair
Long runs in outdoor installations, junction boxes where several cells are summed, and any application where the cable sees a wide temperature swing. Copper resistance rises about 0.4% per kelvin, so 30 m of cable on a 350 ohm bridge can move the span by several tenths of a percent across a 40 K swing. With sense leads the instrument holds the bridge voltage constant and the error disappears.
Wiring discipline
Keep the sense pair connected at the cell end, never at the instrument end, otherwise it measures nothing useful. Where a six-wire cell feeds a four-wire indicator, join sense to excitation at the terminal block; the cell then behaves as a four-wire device and the compensation is lost, which is acceptable only for short cables.
Kevin Huang