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B and B Scales Inc.

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How do load cells work, anway?

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To start off the new year, we thought we’d uncover a little of the mystery surrounding how four 3-pound blocks of steel, strategically placed under a solid steel platform or a 2000lb chute can detect less than pound of weight and send that data to your indicator.

We’ll keep it simple, but here goes:

Digital Load Cells are specialized transducers that convert mechanical force into an electric signal. They do this by using strain gauges that expand and contract in response to force, and measuring the change in the electrical current.

Figure 1: A typical Strain gauge, at rest.

Most load cells, and for our purposes, all of the load cells that we use, have four strain gauges strategically placed inside of them and wired together in what’s called a Wheatstone Bridge. When a force, such as an animal or a pallet exerts force on a load cell, the load cell bends. In the case of a typical 2500lb shear beam load cell, the load cell can bend as much as 1/20,000th of an inch. When the load cell bends, the strain gauges deform.

Figure 2: A Strain Gauge under stress

When the strain gauge deforms, the amount of electrical resistance increases, as the current path narrows. Conceptually, you can imagine that when you pull on a rubberband, it gets narrower. This is exactly what’s happening inside the load cell.

However, since we’re sending an electric current to the strain gauge, we can measure the change in the signal that is caused by the increase in electrical resistance. This analog signal is then amplified and then converted to a digital signal, which can be measured.

This is a vast over-simpflication, to be sure, so if you’re an electrical engineer, please don’t send us hate mail. However, even this basic understanding should shed some light on why we’re always telling our customers to NEVER EVER cut the load cell cables!!Why?Because if you cut the cables that are attached to these strain gauges, you CHANGE the amount of resistance that is exerted on the signal. And, if you’re using FOUR load cells, with FOUR cut cables, you now have all kinds of mixed signals coming back through the junction box and to the indicator!