Measuring/monitoring vacuum in an industrial vacuum cleaner

Motorola and SensorTechnics both offer small and reasonably priced pressure sensors that should be able to do the job just fine. You can either use a single port absolute pressure sensor, or a dual port differential sensor if you want to know the difference between the vacuum area and outside pressure.

Rob

Vacuum, as such, is a hypothetical state that doesn't really exist, and thus not something you will ever be measuring. What you're looking for is a _pressure_ sensor, differential or absolute, for a range somewhere between 1 and 1000 millibar (or hPa, if you're a stricty acolyte of SI units), as I doubt any "vacuum cleaner" will get below 1 hPa.

Mechanically. Eg. a some variation on the plain old barometer --- a sensor based on an elastic object that expands as pressure around it drops, and reports the expansion. The classic version is a gas-filled pill-box shaped metal enclosure. It tries to become more spherical if pressure inside is higher than outside. The change in shape or the force trying to achieve it can be measured, e.g. by elongation measurement strips.

Won't and cannot work if the volume you're measuring is saturated in fluids, but that's fine --- you don't have any vacuum left to measure then, anyway.