Manometers
Pressure gauges (that is, manometers) are devices for measuring the pressure of liquids or gases. They differ from barometers in that the latter measure atmospheric pressure, whereas manometers show the pressure of liquid or gas in a closed space. The main types of pressure measuring instruments are as follows:
1. Liquid manometers (on mercury, water or other liquids, or on two immiscible liquids). The liquid is contained in a glass or metal tube; these manometers may have a single column, U-shaped tubes, an inclined tube or several tubes, or have the form of oscillating ring weights.
2. Metallic manometers. Similar to aneroids, they may have one or more membranes, an aneroid chamber, a Bourdon tube or a spiral metallic tube or some other pressure-sensitive element that directly moves the pointer or changes the electrical signal.
3. Piston pressure gauges. In these gauges, the pressure acts either directly or through a diaphragm on the piston, which is balanced or held in place by a spring.
4. Electrical pressure gauges based on changes in an electrical phenomenon (for example, resistance, capacitance) or using ultrasound
Source: "Explanations to the Harmonized System of Foreign Economic Activity of the Russian Federation (HS FEA Russia)" (prepared by the Customs Authority of the Russian Federation) (volume 4, sections XVI - XXI, groups 85 - 97)