Thinking aloud here (please don't flame me for this).
If one had mercury in a vessel, wouldn't the resistance change with
pressure (since the molecules are being compressed together under
higher pressure). If so, could you measure that difference to provide
an accurate mmHG reading.
Just a thought.
And an interesting idea. Except that fluids in general, and mercury (a
liquid metal) are *mostly* incompressible. (OK, they can be compressed, as
Howard Hughs found out, but only under *extreme* pressure - not
barometric.)
Also, I'm sure you could use a laser reflected also to get very
accurate measurements (especially if a mirror was floated on the
surface of the mercury).
Another neat idea. Depending upon the diameter of the container the
mercury was in, and the purity, you might be able to bounce the laser off
the top of the mercury directly.
What might work better would be to make either a simple sealed vertical
tube manometer, or a U-tube manometer with one sealed end, and pass a
nichrome wire through the end of the tube, down into the mercury. As the
pressure changes, the mercury would rise and fall in the tube. This
changes the length of nichrome wire above the mercury. Since the
resistance of the wire would be proportional to the length above the
mewcury, the resistance is proportional to the pressure. Do a google
search on McLeod Gauge - used to measure vacuum.
I don't know how the industial grade Barometric sensors work so please
forgive me if I've stated the obvious.
Well, you are inventive. "Industrial" sensors can come in a variety of
mechanical designs, depending upon the range of pressures needed to be
measured, the type of signal needed, and the accuracy needed. I've seen
designs that were steel bellows where the end of the bellows formed a
capacitor. There is the bourdon-tube design - like a regular pressure
gauge, except that instead of the needle rotating on a dial face, it is the
wiper rotating around a potentiometer. Others used a sealed chamber with a
diaphram across one end. A strain gage of some sort is attached to the
diaphram. As pressure changes, the diaphram flexes, causing changes in the
strain gage. Strain gage could be replaced with a piezo-resistive element.
One of the most interesting gauges I've worked with was the Leybold-Heraeus
spinning ball viscometer. It was used as a reference to calibrate all the
other vacuum gauges. It suspended a small metal ball in a magnetic field.
The ball was spun up to an exact RPM than allowed to coast down to a lower,
set RPM. The rate at which the ball slowed down indicated the drag from
gas friction, and is proportional to the absolute pressure.
