Although the SPI resembles competing weigh pan cells, it was specifically designed to have greater than normal deflection at full scale in order to provide for the addition of stops to protect the cell against compression overloads. This was necessary because the usual deflection of 0.001 inch to 0.006 inch of most load cells is much too small to allow for the accurate adjustment of an external stop to protect the load cell.
SPI cells with capacities of 3 lbf, 7.5 lbf, and 15 lbf contain their own internal compression overload stop, which is adjusted at the factory to protect the cell up to four times the rated capacity. These cells have an additional bar under the lower surface, to provide a mount for the internal compression stop screw. Capacities of 25 lbf, 50 lbf, 75 lbf, and 150 lbf can be protected by placing hard stops under the corners of a weigh pan to catch the pan before excessive deflection damages the SPI cell.
The cell mounts to the scale frame on the step at the lower left corner, while the scale pan is mounted on the upper right corner with its load centroid over the primary axis at the center of the cell.
The center bar, containing the gages, is a bending beam. It is supported by the outer frame containing four thin flexure points, two on the top and two on the bottom, to provide mechanical strength for side loads and moment loads. This construction provides the superior moment canceling capability of the SPI, which ensures a consistent weight indication anywhere within the weigh pan size limits.
The SPI is also very popular with universities and test labs for its precision and ruggedness. It is also very convenient for lab use. Fixtures and load pans can be mounted easily on the two tapped holes on the top corner.
