Mechanical vacuum gauges and liquid column synchronization devices work - but within their limitations. And their limitations are inherent in the physics by which they work. It's somewhat like using a 100 pound torque wrench to tighten something that should be measured in inch-pounds. It will give you a ballpark torque but you can't get as accurate a measurement out of it. The image below is a picture of the vacuum cycles of a running engine. Each intake stroke causes a rise in engine vacuum. After each intake stroke the vacuum diminishes slowly. This pulsing is what drives mechanical and fluid vacuum gauges crazy!