Bourdon tube pressure gauges are used for the measurement of relative pressures from 0.6 … 7,000 bar. They are classified as mechanical pressure measuring instruments, and thus operate without any electrical power.
Bourdon tubes are radially formed tubes with an oval cross-section. The pressure of the measuring medium acts on the inside of the tube and produces a motion in the non-clamped end of the tube. This motion is the measure of the pressure and is indicated via the movement.
The C-shaped Bourdon tubes, formed into an angle of approx. 250°, can be used for pressures up to 60 bar. For higher pressures, Bourdon tubes with several superimposed windings of the same angular diameter (helical tubes) or with a spiral coil in the one plane (spiral tubes) are used.
Parts of bourdon tube
The case holds all the moving parts of the pressure gauge in one place and protects the components from damage. Cases can be made out of a variety of materials, depending on the demands of the application. For industrial applications, the cases have to be made of more rugged material, such as polypropylene or stainless steel, while commercial applications may only require steel or aluminum cases.
The Bourdon tube is one of the most important parts of a pressure gauge, as it is the means by which a gauge can detect pressure. Also known as the sensing element, the Bourdon tube flexes when it is subjected to pressure. The motion that results is then transmitted to the pointer. The materials that make up the Bourdon tube are dependent on the pressure medium that it receives. A bronze or brass Bourdon tube is satisfactory for pressure gauges that receive air, gas, steam, water and other noncorrosive media. For pressure gauges designed to handle corrosive media or media that can easily solidify or can deposit solids, a diaphragm seal is included to protect the Bourdon tube.
As its name suggests, the linkage connects the Bourdon tube to the gear mechanism. As the Bourdon tube flexes on application of pressure, the linkage transmits the movement of the Bourdon tube to the gear mechanism to activate the gears.
The gear mechanism is the recipient of the movement from the Bourdon tube and the linkage, and it is the part that controls the movement of the pointer. Each gear in a pressure gauge gear mechanism is precision-built in order to move the pointer accurately. The required ANSI accuracy rating for a particular pressure gauge ultimately determines how the gear mechanism is designed. Pressure gauges that read finer increments of pressure require more-complex gear mechanisms.
Pointer, Dial Face, Window and Ring
The pointer is the visible part of the pressure gauge you see moving when pressure is being measured. It “reads” measurements as it points to value gradations or increments on the dial face. This part is directly connected to and controlled by the gear mechanism to indicate pressure within the ANSI accuracy rating of the gauge. The dial face contains the different pressure measurements. Dial faces can have gradations in kilogram force per square centimeter format, pound force per square inch, standard atmosphere, torr (mmHg) and other units of pressure. The window is made out of clear plastic or glass. It serves as a protection for the pointer and the dial face, and it provides a means by which you can view pressure readings.
Applications: Sensing elements for the measurements of medium to high pressures using atmo-spheric reference. For pressure gauges and switches using these elements.
Design Pressure: Up to 100,000 PSIG (690 MPa)
Design Temperature: Up to 600 ° F (316 ° C) with stainless steel Bourdon tube; lower with others; ± 1% zeroshift per 50 ° F (28 ° C) ambient variation
Materials of Construction: Phosphor bronze, alloy steel, 316 and 403 stainless steel, beryllium copper, Monel,Ni-Span C
Range: Minimum span 15 PSI (100 kPa); maximum span 100,000 PSI (690 MPa)
Price: $50 to $300 for indicators and switches, $500 to $1,200 for direct recorders, and $750 to $1,800 for direct controllers and transmitters. Quartz helix units range for$2,500 for an electronic transmitter to $18,000 for an IEEE and RS-232 programmable, stable-alone station.
Bourdon pressure gauge advantage and disadvantage
- Bourdon tube is simple in construction.
- It is available in wide range.
- It has long life.
- It cost is low.
- Time tested application.
- Accuracy is high.
- Susceptibility to hysteresis.
- Low spring gradient (below 50 psig)
- Susceptibility to shock and vibrations.