The SolarMetrix pressure gage from Weiss Instruments, Holtsville, N.Y. (weissinstruments.com), can operate off of its solar cells even when exposed to only low levels of room light.
It operates on 5 millionths of a watt, according to the company. And unlike mechanical pressure gages, there are no gears or bearings to wear out or need lubrication. Three rotary-dip switches let users choose between four different units of pressure and four units of vacuum. It can be ordered with one of six pressure ranges, spanning from vacuum to 1,000 psi, and readings are accurate to 0.5% of full scale.