Pressure Gauges
Pressure Gauges, especially Bourdon pressure gauges are inexpensive and if selected and installed correctly can give years of trouble free use.
Pressure Gauge Selection
All three types of pressure gauge mentioned above, i.e bourdon tube, diaphragm element and capsule element operate on the same principle, namely: elastic deformation of a measuring element under the influence of pressure. The motion caused by the deformation of the measuring element is coupled to a pointer mechanism which moves along a scale, thereby displaying a pressure reading. The appropriate type (bourdon tube, diaphragm element or capsule element) for a particular application depends on the application, required display range and installation location.
Bourdon tube pressure gauges are the most common type of industrial pressure gauge. They are used to measure the pressure of liquids and gasses from relatively low pressures to high pressures, typically they cover measuring spans from 600 mbar to 4000 bar.
A bourdon tube measuring element is a curved tube which can either be a C-shape, circular, spiral or coiled shape. This tube moves outward when the pressure inside the tube is higher than the external pressure, and inward when its internal pressure is lower. This motion is proportional to the difference in pressures, i.e. the pressure to be measured. The movement is coupled to a pointer mechanism.
The size, shape, and material of the tube depend on the pressure range, properties of the measured fluid, and the type of gauge desired. High pressure Bourdon tubes tend to have more circular cross sections than their lower-range counterparts, which tend to have oval cross sections. The Bourdon tube most commonly used is the C-shaped metal tube that is sealed at one end and open at the other.
In 1849, Bernard Schaffer patented the Diaphragm pressure gauge therefore it is common to hear diaphragm pressure gauges referred to as Schaffer diaphragm gauges.
Like bourdon gauges, diaphragm pressure gauges are used to measure pressure in both gas and liquid applications. Their measuring range is more limited than that of boudron gauges, though they can in general measure lower pressures. Typically diaphragm pressure gauges cover measuring spans from 10 mbar to 40 bar. For mediums containing solids, where a standard Bourdon tube could be blocked, a Schäffer diaphragm gauge is preferred.
The measuring element of a diaphragm pressure gauge consists of one circular diaphragm clamped between a pair of flanges. The positive, or negative, pressure acting on these diaphragms causes deformation of the measuring element. The magnitude of the deformation is proportional to the pressure to be measured, and it is coupled to a pointer mechanism.
Capsule element pressure gauges are used to measure air and dry gases at low pressures; they are not used in liquid applications.
Typically, capsule element pressure gauges cover measuring spans from 2.5 mbar to 600 mbar.
The measuring element inside a capsule pressure gauge consists of metal diaphragms soldered together to form a cylindrical bellows chamber. This capsule element expands when the pressure inside the element is higher than the external pressure, and it contracts when the internal pressure is lower. This motion is proportional to the pressure to be measured, and it is coupled to a pointer mechanism.
Differential Pressure Gauges
Bourdon tubes, diaphragm elements and capsule elements may all be used to measure differential, or absolute pressure in addition to gauge pressure. (See our page on Pressure Measurement Fundamentals for more discussion on diferrence between gauge pressure and absolute pressure.) All that is required for these other functionalities is to subject the other side of each pressure-sensing element to either another applied pressure (in the case of differential measurement) or to a vacuum chamber (in the case of absolute pressure measurement).
Standard Ranges for Pressure Gauges
When deciding on the range of pressure gauge for a particular application, try to choose from the "standard" ranges that most pressure gauge suppliers offer. Typically these ranges, in bar, are:
Pressure Gauge Installation
- Before fitting the gauge to a pressure source check that the maximum scale value of the gauge is higher than the pressure applied. The applied pressure should be 75% of the max scale value for steady pressures or 65% of max scale value for fluctuating pressures
- Before fitting the gauge to a pressure source check that the wetted parts are compatible with the fluid being used, and that the pressure connection correctly matches that of the pipework. When fitting the gauge to the pipework, use a correct sealing method
- Do not use the gauge case to tighten the gauge to the pipework, use a correct size spanner on the neck of hexagon
- Pressure Gauges with blow-out release at the back must be mounted at least 20 mm away from a panel or wall to ensure safe dissipation of the confined pressure should the tube fracture
- Gauges marked "Hydraulic" must not be used on compressed gases
- Gauges must have "Oxygen" or "Acetylene" marked on the dial if used on these gases
Use of Pressure Gauges
- Do not use glycerine filled gauges for any fluid which has strong oxidising agents, e.g. chlorine, hydrogen, peroxide etc.
- The ambient and process temperature acting on the gauge should be within -70oF to + 180oF and protected from higher fluid temperature by means of a syphon tube filled with condensate before use
- The fluid in the pressure chamber should not be allowed to freeze or crystallise as this will lead to rupture of the sensing element
- Should the pointer of any pressure gauge not return to zero, when the pressure is removed, it is an indication that damage to the gauge has occurred and the gauge should be replaced immediately.
No comments:
Post a Comment