I know I'm gonna' regret this, but I'll bite....I'm assuming a gauge intended for ammonia is made from different materials than one intended for "air", but are you hinting that they are calibrated differently??
Just how much difference is there between a "0" reading on an ammonia gauge, vs. one intended for air? Is it enough to render the test ineffective, or merely enough to raise the hackles on a lonely engineer?
Inquiring minds after all....
"Forget pounds and ounces, I'm figuring displacement!"
If we accept that: MBG(+)FGSF(=)HBG(F1) And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1) Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT? PB answer: It depends.
Is it enough to render the test ineffective, or merely enough to raise the hackles on a lonely engineer? 
