The difference between Bourdon tube pressure gauge and capsule pressure gauge
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In various industrial production, environmental monitoring and scientific research control systems, the role of pressure gauges is irreplaceable. It can provide real-time information about the pressure state of gas or liquid and is an important component to ensure the safe operation of equipment. There are many types of pressure gauges on the market, among which Bourdon tube pressure gauges and capsule pressure gauges are the two most widely used types. This article will provide an in-depth analysis of the working principles, structural differences and usage scenarios of these two pressure gauges, and thoroughly explain their differences from three levels: "what, why, and what are the results."
Different pressure sensing structures
The core difference between the Bourdon tube pressure gauge and the capsule pressure gauge is that they use completely different "pressure-sensing elements" to receive pressure and perform mechanical conversion. The Bourdon tube uses a metal spring tube bent into a C-shape or spiral shape. This tube is hollow inside, with one end closed and the other end connected to the measured medium. When liquid or gas pressure enters this tube, the wall of the original oval cross-section will expand into a circle. Due to the change in shape, the entire spring tube will elongate or shorten, causing a slight displacement of the free end. This tiny displacement is amplified through the connecting rod and gear mechanism, and ultimately drives the pointer on the dial to rotate, showing the pressure value.
The diaphragm pressure gauge uses a "diaphragm" welded by two corrugated thin metal sheets as the pressure sensing structure. After pressure enters the diaphragm box, the distance between the two diaphragms will change, and the diaphragms will bulge outward or shrink inward. This tiny deformation is transmitted through the lever mechanism and drives the pointer to rotate, thus reflecting the amount of pressure.
Therefore, from a structural point of view, the deformation of the Bourdon tube is the effect of the "linear elastic element", while the bellows is the deformation of the "planar elastic element". This fundamental difference in structure directly determines their differences in pressure measurement range, sensitivity, and durability.
Different Pressure Measuring Ranges
Because the metal spring tube of the Bourdon tube is thick and has strong pressure resistance, the Bourdon tube can perform accurate measurements even if there are pressures of hundreds of kilopascals or even megapascals in the system. This ability makes it very suitable for use in high-pressure scenarios such as hydraulic equipment, steam boilers, and high-pressure gas cylinders.
In contrast, the metal sheet of the diaphragm is very thin, and slightly higher pressure may cause it to permanently deform or even break. Therefore, the membrane box can only be used for measurement in the micro-pressure range, such as between tens of pascals and several thousand pascals. It is more suitable for monitoring air circulation and air pressure balance in purification systems, or used in clean rooms, gas furnaces, and small vacuum equipment.
The level of sensitivity depends on the ability of the pressure-sensitive element to respond to small pressure changes. The membrane structure of the membrane box will deform immediately under extremely small pressure changes, so it can reflect extremely small pressure fluctuations. For example, in air flow rate detection and gas control, even if there is only a slight fluctuation in air flow, the diaphragm will respond immediately to help the system fine-tune.
Since the Bourdon tube is a metal spring tube, although it can deform, it requires a certain pressure difference to cause displacement, so it is not sensitive enough to subtle changes. In systems that require high-precision low-voltage control, it may behave sluggishly, causing adjustment delays or reduced control accuracy.
Installation space restrictions vary
In small equipment with limited space, wall-mounted control panels, cabinet-integrated systems or portable devices, the arrangement of components must be very compact to meet the integration requirements of the overall structure and function. In this context, the overall dimensions of the pressure gauge-especially its thickness and installation depth-become the key to its use.
The diaphragm pressure gauge has a great advantage at this point. Its pressure-sensitive element - the diaphragm box - is welded from two metal films. The overall shape is flat and does not need to occupy a deep position inside the device. At the same time, since the pressure-sensing part is close to the inside of the watch case, and the driving mechanism also adopts a miniaturized gear and lever structure, the thickness of the entire pressure gauge can be controlled to be very thin, even within 20 mm for some models. This flat design allows the capsule pressure gauge to be easily integrated on the equipment panel or embedded in a compact instrument cavity, enabling installation with visibility from the front panel and no protrusions from the back.
In contrast, the pressure-sensing structure of the Bourdon tube pressure gauge is a metal spring tube bent along an arc, with an oval or circular cross-section. In order to ensure that it has sufficient deformation range, the tube itself occupies a large space in the undeformed state. And when the Bourdon tube deforms due to pressure, its free end will move slightly. In order to convert this small mechanical displacement into a clear rotation of the dial pointer, a complete set of transmission mechanisms must be set up inside, including connecting rods, gear sets and hairsprings (for reset). These parts are not laid flat on a plane, but are arranged one after another in the housing of the pressure gauge according to mechanical principles. That is to say, they need to extend a certain depth "from front to back" in structure. Therefore, the entire pressure gauge is not only larger in diameter, but also significantly thicker. A depth of more than 50 mm or more must be reserved during installation.
