E011 - Pressure
For particular generic types of loadcell, static or dynamic ambient pressure changes can cause error due to a nett resultant force exerted on a sensitive form area. Loadcells with diaphragm load sensing are particularly vulnerable as the loadcell forms an enclosure with only pressure relief through the cable free area. Thus unless the pressure is allowed to equalise across the diaphragm a nett force is experienced by the loadcell. Loadcell types F250, F251, F252, F255 and F256 all use diaphragms. Depending upon sealing requirements we can build pressure equalisation into a loadcell by providing a small vent hole.
Other generic types of loadcell employing cover plates that form areas normal to the pressure distribution can also suffer from pressure induced errors unless they are designed for pressure equalisation e.g. F254 pancake loadcell. Here the sensing elements are shear spokes but the enclosing cover plates may react upon the centre hub due to pressure across the loadcell. If the pressure is the same on both sides of the loadcell the nett effect is zero. We are increasingly involved with unusual pressure requirements, most commonly for underwater applications where we have to deal with high pressure such as a depth of 6km of water which exerts a pressure of approximately 600bar.