What is Pressure Gauge
The device that is used to measure the pressure is called pressure gauge. Normally pressure gauges indicate the pressure relative to atmospheric pressure as the Zero that means when atmospheric pressure is 1.03 Kg/Cm2, gauge pressure is 0.
Gauge pressure is positive if absolute pressure is above atmospheric pressure and for absolute pressures below atmospheric pressure, gauge pressure is negative. The pressure gauge used to measure negative pressure is also known as Vacuum Gauges and the pressure gauge used to measure both positive and negative pressure together, is also called Compound Gauge.
Type of Pressure Gauge
Bourdon Tube Type
Bellow Type
Manometric Type
Piezometric Type
Let me explain about Bourdon Tube Pressure Gauges in this Post:
Bourdon Tube Pressure Gauge
History
Bourdon Tube was invented in 1849 by the French engineer Eugène Bourdon. Bourdon Tube is the fundamental component of Modern Pressure Gauge, and it was named Bourdon Tube after the name of Eugene Bourdon only, which marked a significant advancement in Pressure Measurement Technology.
The early applications of Bourdon tube pressure gauges were primarily in the field of industrial instrumentation, where they proved to be highly reliable and accurate instruments for measuring pressure. Some of the early Industries are Steam Engine, Steam Boilers, in manufacturing processes to monitor pressure in hydraulic and pneumatic systems.
With the passage of time, now a days Bourdon Tube Pressure Gauges are used almost in each and every Industry.
Introduction
Pressure Gauges with Sensing Element as Bourdon Tube are named as Bourdon Tube Pressure Gauges or Bourdon Sensing Pressure Gauges.
Bourdon is basically a normally a C-shaped or Coiled Type (Helical or spiral), fine bore hollow tube made of metal, whose one end is fixed and connected to the pressure tapping, the other end set free. For Pressure up to 100 Kg/Cm2, C-Shaped Bourdon Tubes are used and for Pressure of 100 Kg/Cm2 and above, Coiled Bourdons are used.
It’s one end which is set free is capped and sealed properly. The other end, which is open Fixed to pressure tapping is kept opened.
Bourdon tube pressure gauges is most commonly used mechanical pressure gauge among the pressure Gauges mentioned above. They are used to measure pressure range from vacuum to pressure as high as few thousand Bar as per manufacturers capabilities.
Law Applicable
Bourdon Tube Pressure Gauges operates on Principle of Elastic Deformation called as Hooks Law, that describes the relationship between the force applied to a material and the resulting deformation or strain in that material. It states that the deformation (strain) of a material is directly proportional to the force applied to it within its elastic limit.
Hooke’s Law of Elastic Deformation was formulated by the English scientist Robert Hooke in the 17th century.
Construction and Working Principle
One end of the Bourdon is kept open, which is fixed by connecting it to the pressure tapping, the other end of the Bourdon Tube is sealed and kept free.
If we discuss the law in context to Bourdon Tube Pressure Gauge, when Pressure is applied in the Bourdon Tube (from its open end fixed with pressure tapping), it exerts internal force (on the bourdon tube as it is sealed at other end) due to which, bourdon tube tries to expand or straighten up.
Due to this deformation (expansion or straighten up) in bourdon tube, the free end of the tube will have mechanical movement, depending on magnitude of pressure applied on it. A deflecting and indicating mechanism are attached to the sealed, free end of the Bourdon Tube that rotates the pointer and indicates the Pressure reading on calibrated Scale.
Important Parts of Bourdon Tube Pressure Gauge
Bourdon Tube Pressure gauge is most simple instrument though it is most complex because there are so many components that are used to manufacture the same.
Some of the key parts of Bourdon Tube Pressure Gauges are as under:
Bourdon
BOURDON is a C-shaped hollow tube, whose one end is fixed, and the other end is kept free. For Pressure 100 bar & above Bourdon is in coil shape. It is also treated as heart of Gauges.
C Shaped or Coiled shaped bourdon tubes are available in seamless design as well as in welded design. Seamless design is more preferred as it ensures more safety.
As it is already explained, bourdon expands or contracts depending upon the magnitude of pressure applied on it and if it is seamless bourdon, chances of leakage are minimal.
However, if we use welded bourdons, there are more chances of leakage from welded portion due to more stress on the welded portion of the bourdon.
Normally purchasers are neglecting this fact and are compromising on quality and prefer the price part.
Instrumentation persons also not showing much concern to the quality, when it comes to Pressure Gauges, because of low-cost Instrument. If it is for Pressure Transmitters or other pressure instruments, they give due attention.
Sealed end of Bourdon has to be kept Free for connection with movement and Open end of the bourdon has to be Welded with socket.
Different Bourdons are used for different ranges. Same Bourdon can not be used for range of 0 to 10 Bar and 0 to 25 Bar
Socket
SOCKET is the part of pressure gauge which is Fixed (welded) with open end of the Bourdon. Socket is provided with threads in such a way that it is suitable for mounting of Pressure gauge on Pressure tapping provided on the pipe or tank or vessel. Threads provided on Socket is termed as Connection of Pressure Gauge.
Bore is also provided for entry of fluid. Process fluid enters through this bore and enters the bourdon welded with it.
It is recommended to punch the material of Socket on the Socket because the sockets are available in different material. Depending upon the application, operator or Engineer can check the material of construction of socket from punching and can match its compatibility with process media.
Movement
MOVEMENT is the Gear Mechanism having Sector and Pinion arrangement which is connected to the bourdon with linkage to control the movement of pointer with respect to expansion / contraction of Bourdon due to pressure.
After Bourdon, Movement is the second most vital part because this is the part, which plays important role to convert the expansion of Bourdon into reading by rotating the pointer fixed on Spindle of the movement. It is movement only, which amplify the small amount of movement of Bourdon Tube and transfer it to Pointer on the gauge face.
Different Movements are used for different accuracy levels required. Movement used for gauge accuracy +/-1% cannot be used for gauge accuracy +/- 0.5% or 0.25%.
Dial
DIAL is the scale provided with black markings on white background to indicate the pressure applied on the Bourdon Tube. Dial marking is provided based on the magnitude of the pressure to be measured. Marking is provided on the dial within the 270 Deg. Arc. The Marking provided on the dial is called as Range of the Pressure Gauge.
Some customers require 2 Scales on the dial. In such case, one reading is provided with black colour and other marking is provided with red colour. Dial with 2 Scales are called Dual Scale.
Unit of Pressure, Sensing Element material and accuracy of the pressure gauge is permanently marked on the dial. In case of Dual Scale, both the units need to be permanently marked on the Dial.
Case
CASE is the enclosure, which is used to mount / fix the measuring mechanism (Bourdon Fixed to socket at one end and connected to the movement at the other end). After fixing the measuring mechanism, dial is provided with suitable range marking. Pointer is also fixed inside to indicate the pressure.
Depending upon the area where Pressure Gauge needs to be installed, enclosure protection of such grade is provided. Normally pressure gauge enclosure is provided with weatherproof Protection – IP65 as per IS:13947. Best available enclosure protection is IP68 till date.
Pointer
POINTER is the needle, which is fixed on spindle of movement and based on magnitude of pressure applied on the Bourdon Tube, sector & pinion arrangement of the movement rotates, which ultimately move the pointer depending upon the pressure applied.
It is recommended to select the micrometer pointer for the pressure gauge, because with micrometer pointer, once can adjust the zero of the Bourdon Sensing Pressure Gauge, without removing the pointer from the spindle of the movement.
Window
After enclosing the measuring mechanism, WINDOW is provided to measure the reading of the pointer of pressure gauge. It is advisable to opt for Shatterproof Safety Glass as window to protect the Worker / Engineer standing in front of the Bourdon Sensing pressure Gauge to note the reading.
Shatterproof Safety glass is a glass which is made of minimum 2 layers of glasses held together by an interlayer (normally of Polyvinyl Butyral or PVB).
When Shatterproof Safety glass is broken, it is held in place by an interlayer between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken.
Some manufacturers provide Toughened Glass, sheet Glass or Acrylic also as window of the pressure gauge.
Blow-Out-Disc
Though BLOW OUT DISC is very small fitment in the Bourdon Sensing Pressure Gauge, it is most important for the safety purpose.
Blow out disc is provided either on top or at back of the pressure gauges depending upon the dial size. Some Standards even talk about blow out disc based on the Pressure.
Whenever there is leakage in the Bourdon Tube of Bourdon Sensing Pressure Gauge, pressure is built up inside the case of the Gauge.
Since, pressure of breaking of glass is approx. 1 Bar, blow out disc has to be designed and tested for blow off at pressure less than 1 Bar to avoid breaking of glass for the safety of person standing in front of the gauge.
Connections available for Pressure Gauges
Connections available for Bourdon Tube pressure Gauge are
NPT Threads (National Pipe Taper are American National Standard Taper Pipe Thread). These are available in 1/8”, ¼”, 3/8”, ½”, ¾” & 1” Sizes.
BSP Threads (British Standard Pipe). Two Types of threads are available in BSP threads, which are BSPP (which is Parallel Thread). BSPT (which is Tapered Thread). If we are using the word BSP in India, it is referred to BSPP.
BSPP Type Threads are also known as G-Type Threads, whereas BSPT Type Threads are also known R-Type Threads.
These are available in 1/8”, ¼”, 3/8”, ½”, ¾” & 1” Sizes.
Metric Threads these threads are designated by the letter M followed by the value of maximum thread diameter and the pitch (e.g., M33 x 2)
Main Purposes of Pressure Gauges
Measurements
Pressure gauges are mainly used to measure the pressure of fluids or gases in various systems like pipeline, vessel, tanks, etc.
We use them for visual indication of the pressure reading.
Pressure Gauges allows operators to monitor and control the pressure.
Controls
Pressure gauges provide valuable feedback for controlling and adjusting processes.
By monitoring pressure variations, operators can make suitable adjustments to the control mechanisms to maintain desired pressure levels to ensure efficient operation.
Safety
Pressure gauges play a role in the safety of equipment and processes.
By monitoring pressure reading, we can monitor overpressure situations to protect the equipment from failure, leaks, etc.,
Troubleshooting
Pressure gauges are also used for troubleshooting issues in pneumatic or hydraulic systems.
By observing pressure reading, we can monitor overpressure situations to protect the equipment from failure, leaks, etc.,
Specifications Selection for Pressure Gauge
Specifications are selected base on the availability, your requirement and other parameters.
Case Material (Normally SS304 or SS316)
Entry of Gauge (Bottom or Back)
Mounting (Direct / Local or Surface / Surface of Panel or Panel / Flush Panel or Bracket mounting or 2” Pipe mounting)
Pressure gauge with top alignment is also available in Market.
Dial will be with white background with black markings.
Dial sizes are normally 1.5” (40mm), 2” (50mm), 2.5” (63mm), 4” (100mm), 6” (150mm) or 10” (250mm). Some manufacturers also provide 8” dial size and some 160mm dial size also.
Bourdon & Socket is available in SS316 / SS316L / SS316Ti / Monel Depending upon the application and media. Some manufacturers provide Phosphor Bronze also.
Movements are available in SS304 / SS316 / Brass.
Connection can be selected as per requirement
For refrigeration application, ask for temperature scale based on gas used.
Things to be Taken Care About the Specifications for The Quality
Compliance to latest EN-837 standard to be ensured.
Range to be selected as per EN-837 Standard.
Glass should be shatterproof safety glass for safety of person standing in front of Instrument.
Repeatability and Accuracy of the instrument to be checked as per requirement.
Accuracy possible are ±1% FSD (Normal), ± 0.5% FSD (Special)
Micrometer pointer is must for 4”, 6” & 10” dial sizes. Manufacturers are providing normal pointer only.
Weatherproof certificate should be asked. Best available in market is IP68.
Mechanical & Chemical composition certificates from NABL Accredited lab.
Advantages of Bourdon Tube Pressure Gauge
Accuracy
Bourdon Tube Pressure Gauges are available with high degree of Accuracy. They can be provided for calibration purpose also.
Durability
Bourdon Tube Pressure Gauges are robust in construction and are durable. They are able to withstand harsh operating conditions like vibration, shock and temperature fluctuations.
Wide Industry range
Due to their robust construction, they are suitable for wide range of applications and Industries including Chemical, Refinery, Oil & Gas, Power and various manufacturing Industries.
Compatible
Bourdon Tube Pressure Gauges are compatible with wide range of fluids, gases and environments and can be used by selecting the compatible material of their construction.
Long Life
Bourdon Tube Pressure Gauges have long operational Life if properly maintained. There are less chances of mechanical failures in Bourdon Tube Pressure Gauges.
User Friendly
Bourdon Tube Pressure Gauges are easy to read because of circular dial showing the complete span and one can easily read the pressure through needle provided on it, even from distance.
Cost Effective
Bourdon Tube Pressure Gauges are cost effective if we compare Bourdon Tube Pressure Gauges with other type of Gauges available in market. These are popular in market because of their cost-effectiveness.
Low Maintenance
Due to their simple and Robust Construction, Bourdon Tube Pressure Gauges require minimum maintenance. Regular calibration and occasional inspection of their functionality are sufficient to keep them in working conditions.
Wide Pressure Range
Bourdon Tube Pressure Gauges can be provided with variety of Pressure Ranges. These are available from 0 to 0.6 Kg/Cm2 range up to 0 to 2500 Kg/Cm2 range commonly. Some manufacturers can provide range of 0 to 7000 Kg/Cm2 also.
Response
Response of Bourdon Tube Pressure Gauges is almost instantaneous with change in pressure.
Only in Pulsating fluid applications, these gauges are not able to provide instantaneous readings, however, the same can be overcome by filling fluid in the Case.
No External Power Requirement
Bourdon Tube Pressure Gauges don’t require any external power source for the operations, which make them self-sufficient and reliable in remote or power-restricted areas.
Dis-Advantages of Bourdon Tube Pressure Gauge
Reading & Accuracy
Bourdon Tube Pressure Gauges reading is limited to the least count provided in the dial. Though in Digital Gauges, we can read readings in decimals also and readings shown by Digital gauges are more accurate.
Response time in Fluctuation
Bourdon tube pressure gauges may not respond quickly to rapid pressure fluctuations. This slow response time can be a disadvantage in applications where precise and fast pressure monitoring is crucial.
Limited Pressure Range
Bourdon Tube Pressure Gauges can be used only for the specific range, which is shown on the dial. It is not possible to use the gauges for other ranges.
Data Output
Bourdon Tube Pressure Gauges are suitable for visual data only and it is not possible to provide electronic output of their data to control room or otherwise, which are making them less suitable for applications where automated data logging and remote monitoring is required.
Application where Bourdon Tube Pressure Gauge can be used
Clean Air
Clean Gas
Clean Water
Clean (Non-Corrosive) Liquids
For Corrosive / Hazardous / Highly Viscous Fluids (But with Chemical Seal Unit)
For Fluids with solid particles / Sediments (But with Chemical Seal Unit)
For fluids having tendency to crystalize / Solidify / Freeze (But with Chemical Seal Unit)
For Vibrating area (But with Glycerine Filled Case)
For Pump Discharge Applications (But with Accessory – Snubber)
For Steam Applications (But with Accessory – Syphon)
For High Temperature Applications (But with Accessory – Impulse Tube or Cooling Tower)
For Applications where design pressure is outside the limit of over range protection (But with Accessory – Gauge Saver)
Conclusion
Bourdon tube pressure gauges were started 172 years ago and continue to be a reliable and widely used method for pressure measurement across various industries.
They offer a balance between accuracy, simplicity, and cost-effectiveness. However, their suitability depends on the specific application’s requirements and conditions.
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