Vince
Well, yeah, atmospheric pressure is 760mm of mercury, that means the pressure of the atmosphere supports a column of mercury (supported in a tube!) 760mm high. For water, atmospheric pressure is 10.3 meters. 13.55 times higher.
But this only applies to closed tube manometers, where the pressure of the atmosphere is pushing the column up, and not also pushing down on the top of the column of liquid. For a pressure manometer like we're discussing all you need are 2 or more essentially rigid tubes with the same volume of fluid in each tube, adjusted so the top of the liquid level is the same in all 3 tubes. When the liquid in the tube is subjected to pressures above or below atmospheric pressure, the level of the liquid will move up or down.
Mercury has the advantage that it's nice and dense and damps out fluctuations as the inlet port pressure fluctuates, providing a fairly steady reading. Water tends to bounce around a lot because it's a lot less dense and not very viscous. Adding something (like methyl cellulose) that makes water more viscous helps to stabilize it, but then it tends to start to get full of smelly bacteria. Water also evaporates too quickly.
Manometers these days tend to use a light oil because it's nice and viscous and doesn't evaporate (and doesn't produce the toxic fumes you get from mercury). But this technology is certainly not rocket science. Home made manometers can have very high precision, allowing Laverda owners to set their carburetors up to very close to the same vacuum. But they're not accurate. You can't read off from the gauge and say just exactly what the manifold pressure is. You need a very high quality instrument to calibrate them. So what, nobody cares what the pressure is exactly, as long as all the carburetors are the same.
Ken
