I know the Dwyer manual talks a lot about it being essential to use the magnehelic vertically, to take the strain off the diaphragm.
But I have to tell you, while I have my gas bench magnehelic mounted vertically, I've been using a horizontal magnehelic for four years now, because it's easy to focus the overhead camera on the gauge while I'm demonstrating second stage tuning.
So I checked that both gauges were zero'ed in their respective orientations, and hooked up my little suction pump on a union, so both gauges were exposed to the same vacuum while I photographed.
After four years of being in the "wrong" position, my horizontal magnehelic is 0.05" less than my properly mounted bench magnehelic.
Jes' sayin'...
