When more sensitivity is desired, we may construct something called a micro manometer, consisting of two large diameter vertical tubes connected by a small diameter horizontal tube. The horizontal tube contains an air bubble for the indication of the fluid movement, or the two large diameter tubes are filled with two nonmixing liquids of different colors, in which case fluid movement is indicated by interface travel. The air bubble comes to rest in the center when both wells are exposed to the same pressure. Then if one of the wells is connected to the process, the difference between that and the atmospheric reference causes fluid movement from one well to the other, developing a head differential.
Depending on the reference pressure, this device can then be used for sensing near-atmospheric, absolute, or differential pressures. Its performance as a high-vacuum sensor is a function of the quality of the vacuum reference available. The speed of response of this instrument is relatively slow because the smaller the bore of the capillary, the longer it takes for the bubble to assume its final position. At the same time, small capillaries are desired to prevent fluid leakage between the bubble and the capillary.
List of Prominent Suppliers: Barksdale, BD Sensors, Brooks Instrument, Bsria, Dwyer, Flowtech, Kane, Hansa Flex, Mayuri Instruments, Meriam, New-Flow, PS Instruments, Sauermann, Trox Technik, TSI, TTseries
Instrumentation Basics 