I think mercury is now on the prohibited list with plutonium and grenades... not available in manometers to us home mechanic types and pretty hard to get at all. The cheap ones were all mercury manometers. Making water or oil filled manometers has been beat to death here but in essence you need a 30 odd foot high water manometer to avoid sucking the water out. There are oddball workarounds but I don't believe any work very well.
So you have either
1) four vacuum gauges, not a bad choice since even cheapo inaccurate gauges can be set to read almost the same at the vacuum level we're interested in but the gauges are bulky and fragile
or 2) the Morgan Carbtune which is really a flow gauge measuring airflow though a small orifice. It's compact, accurate, and almost indestructible.
