How the capsule pressure gauge works The measuring element of the capsule pressure gauge consists of two corrugated diaphragms that are welded together from the sides to form the capsule. Measurable pressure enters the capsule through an inlet in the center of the first diaphragm. The center of the second diaphragm is attached to the moving part so that the deflection of the measuring element can be transferred to the hand. When the pressure inside the capsule rises, both diaphragms deform slightly. Using two apertures, the total contraction of the measuring element is twice as high .
As you can see in the pictures below, there are two types of capsules: convex and nested. A convex capsule is formed by connecting two convex diaphragms facing each other. A nested capsule consists of a convex and a concave diaphragm, which are also connected along their edges. If process pressure is applied along the outside of the capsule, hollow capsules have the advantage of over pressure. They have a higher resistance limit.
Capsule pressure gauge To measure a very small pressure difference, the deformation of a single capsule may be too small. Therefore, several capsules can be placed on top of each other to achieve sufficient displacement to move the hand on a full scale. This set can be made with convex or nested diaphragms.
Capsule pressure gauge feature
Using flat diaphragms, very small deviations can be achieved from rapid elastic limit changes. Therefore, diaphragms are usually wavy. These ways are more likely to deviate from the diaphragm before reaching the elastic limit. A capsule 50 mm (2 inches) in diameter, consisting of two nested diaphragms welded together, can be up to 1 mm thick. Reach 5.5 mm (0.06 inches). The size of the deviation also depends on the diameter of the aperture. The fact is that the deformation of a diaphragm is proportional to the fourth power of its diameter. As a result, as the diameter of the diaphragm increases, the deflection speed increases. Doubling the diameter means that the deviation increases by 16 times. For flat apertures, which are closed in a circle around them, the deviation can be calculated as a function of radius using the following equation. This is a simplified formula that is only valid for small aperture deviations, in other words, where the ratio between the aperture deviation and the pressure applied to the aperture is linear. The term equation, which represents nonlinear deviation, is ignored.
δ = Diaphragm deflection rate
r = radial position
R = aperture radius
D = hardness of diaphragm material
P = pressure applied to the diaphragm
δc = center of aperture deflection
Where: Hardness of diaphragm material (D) can be obtained using the following formula:
E = New elastic modulus
t = Diaphragm thickness
1 = ratio
For nested apertures, the equation will be much more complicated because you need to consider the number and shape of the nest as well, and you will probably also have a flat area in the middle of the aperture. A nested range ensures that the tension The radius in the diaphragm is reduced by a factor of 1,000 to 10,000, reducing the possibility of reducing the effect of temperature fluctuations by at least a factor of 120 and reducing the effect of welding stresses along the diaphragm margin. Because the capsule consists of two diaphragms. Each has its own curvature, this sensitivity will be twice the diaphragm pressure gauge. This sensitivity also depends on the diameter of the capsule. This can vary between 25 mm (1 inch) and 150 mm (6 inches). The larger the diameter, the greater the sensitivity. Deformation of the diaphragm can transmit large forces. As a result, command of the transmission mechanism is possible with a large ratio. Therefore, it is quite possible to move the hand in full scale, despite the small deviation of a capsule. Even a small pressure acting on a large diameter diaphragm can produce enough force to guide the transmission mechanism.
Applications with capsular pressure gauge
Because the pressure chamber of a capsule does not empty on its own, they cannot be used for liquids. Therefore, the capsules are used only to measure gas pressures. The possibility of measuring low pressure with diaphragms, together with the high sensitivity of the capsule, ensures that the capsules are ideal sensors for use in pressure gauges.
Capsules can be stacked on top of each other to create more deflection and sensitivity. The pressure gauge usually consists of 2 to 5 capsules depending on the quality of the device that are placed on top of each other. Multiple capsules are required because the atmospheric pressure changes slightly, ie only between 920 and 1050 mbar. A slight difference between the minimum and maximum air pressure is sufficient for the set of capsules to guide the hand. The image below shows the inside of a barometer with two capsules connected. Each capsule is emptied and sealed. Air pressure operates outside the capsules. To prevent the capsules from collapsing under increasing pressure, a spring is attached to a base that attaches to the upper center of the capsule.
Capsule pressure gauge measuring range
Capsule pressure gauges are suitable for measuring very low negative pressures from 0-0.5 mbar to 0-1000 with an accuracy class of 0.1 to 2.5. They can measure relative and absolute pressure and pressure difference.
Absolute pressure sensors
In absolute pressure sensors, the capsule pressure chamber is emptied and sealed. The capsule is closed in a closed chamber. The process pressure then enters the closed chamber by connecting the process and is applied to the outside of the capsule. Since the process pressure is compared to an almost complete vacuum inside the capsule, we will measure the absolute pressure. The connection, which is fixed to the capsule, is transmitted to the plate through the chamber wall using ring rings almost without friction. Loop strikes ensure that the pressure in the chamber will not leak during the connection. The capsules are designed to be stacked on top of each other to prevent excessive pressure.
Relative pressure sensors
To measure relative pressures, process pressure is applied to the capsule. Therefore, the capsule has a process connection in the middle of the diaphragm through which the process pressure enters. The reference pressure, as atmospheric pressure, is located on the outside of the capsule. Relative pressure measurement can be done with one or more capsules.
Pressure difference sensors
If the above relative pressure is measured by a second connection and the chamber is sealed at the connection with a ring, two different pressures can be compared. The resulting pressure, measured by the capsules, is the pressure difference between the two process pressures
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