The most basic form of the manometer is the U-tube manometer:
Pressure is read on the scale as the difference in height (h) between the two liquid columns. The measured pressure is the difference between the height of the two columns of liquid. Its accuracy is limited by the visibility of the scale because, in this, there are no moving parts and no friction or inertia. If the fluid used is water, the manometer may be filled and emptied at will and even rolled up for storage if the tubes are made of flexible plastic, the reason it is very inexpensive.
Cistern-style manometers are the norm for calibration bench work and are typically constructed from metal cisterns and glass tubes.
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Instrumentation Basics 