I've noticed a problem with my meter, the reading at the top end of the low scale is different from the bottom end of the high scale. eg, using the invercone I measured "9" on the low scale, but 10 on the high scale. Without the invercone I had just below "10" on the low, but 12 on the high.
My gut feeling is that the cell is starting to fail and isn't producing full output, possibly causing a linear slope error over the whole range or just giving a shoulder at the higher values.
Does anyone know if there is any adjustment possible on the Weston apart from the mechanical zero on the back? What is the mode of failure for a cell,ie do they normally fail dead or does the response change over it's lifetime? In short, is my meter heading for repair or the circular file?