What are Pressure Gauges?
There are many different types of pressures such as barometric, differential, vacuum (negative gauge) and positive gauge. The difference among these pressures lie in the reference point against which they are measured. Owing to different reference points, many pressure gauges are being used in industries all over the world to measure these pressures.
Usually the gauges available are equipped with analog indicators i.e. pointer needle and scale. These gauges are made with Bourdon tube, diaphragm or capsule. Inside these gauges, the pressure mechanically pushes the needle across the scale to indicate the pressure applied through pressure connectors which are usually parallel or taper pipe threads. Finer calibration of the scale and larger diameter of the gauge help increase the accuracy of these gauges .
What is a Bourdon Tube Gauge?
This gauge is used to measure both gas and liquid pressures up to 7000 bar. They are mechanical instruments and thus do not require electrical supply. There is a flexible metal tube inside it and when the pressure is applied to the open and fixed end of this tube, the free end of the tube is displaced. This displacement is measured to find a reading for pressure applied. The tube used in such gauges are usually C shaped but spiral and helical shaped tubes are also used when higher sensitivity is required. All Bourdon Tube Pressure gauges are made following strict European Engineering Standard EN 837-1 
Why there is a need to maintain/clean Pressure Gauges?
Since such gauges are used in multiple industries therefore they require regular and accurate calibration. There is always a likelihood that any contaminant such as dust, water, oils, fuels make their way into the gauge therefore it is vital to clean the gauge before calibration . If the gauge is used to measure oxygen pressure then the need of cleaning is far greater because if there is any grease inside the gauge, it can cause explosion, fire or injury .
What is the cleaning procedure of Bourdon Tube with external bleeder?
There are two types of Bourdon tubes. One is equipped with an external bleeder while another type also known as the Dead-End Configuration C Bourdon tube does not have an external bleeder.
Cleaning procedure of Bourdon Tube with an external bleeder involves the following steps.
- Remove any oxygen gauge or piping systems connected to the gauge.
- Remove the back case or just the plug on the back.
- Securely attach a Teflon or similarly built tube to the bleeder port at the back of the gauge ensuring that no cleaning solvent is leaked during the cleaning. The other end of this tube is placed in a receiving vessel. At times gauge has a flexible coiled bleeder capillaries which need to be sheared off from the tip in order to clear passage for flushing the solvent when attached with the Teflon tube.
- Introduce air pressure, from the gauge’s main connection, of 5 to 10 psi which would push approximately 700 ml of cleaning solvent. It is made sure that the cleaning solvent is environment friendly as well as non-toxic. This would remove any oil/grease from the gauge.
- The gauge is flushed both ways. Once by applying pressure from the main connection and then from back plug. An electronic power pump is used for flushing purpose.
- After flushing using long wave ultra violet light, check for any residuals. If in a 100 ml cleaning solvent which has come out of the gauge has residual value of equal to or less than 1.0 mg then the gauge can be considered clean otherwise the entire procedure is repeated again.
- After cleaning, the apparatus is removed. Filtered and dry nitrogen is passed through the gauge to remove any solvent present in it. A Halogen detector is used to check whether cleaning solvent has been removed from the gauge or not.
- After completion, the bleeder tube at the back is resealed using welding or plugged back. Finally, the back cover is placed back.
The gauge is pressure tested using pressure 1 to 1.5 times the normal working pressure to ensure there is no leakage .
What is the cleaning procedure of Dead-End Bourdon Tube (without external bleeder)?
Cleaning procedure of Bourdon Tube without an external bleeder involves ultra-sonic or vacuum pressurizing method. Although in such dead-end Bourdon tubes guarantee cannot be provided for 100 percent removal of contaminants. Cleaning procedures for such tubes is given below.
In ultrasonic cleaning, gauge is separated and immersed in a cleaning solvent MIL-C-81302 Type I. It is made sure that that no air is present inside the Bourdon tube. A plastic tube attached to the dead-end makes sure that air is removed from the tube before injecting solvent in. High Frequency ultrasonic is subsequently used to clean the tube. After completion of this step, step number 6 and onwards as mentioned in the cleaning procedure of Bourdon tubes with external bleeders is followed which involves checking solvent for residuals, passing of nitrogen and leakage test.
Vacuum pressurizing method of cleaning Dead-End tubes involve creating vacuum in tube to approximately 28 in-Hg and breaking the vacuum with a cleaning solvent . The gauge is rotated when creating vacuum and also when introducing solvent in order to ensure that both evacuation and filling is complete and has reached all sections. At this stage once again stage number 6 and onwards is followed as in the previous cases .
What Precision, Fabricating & Cleaning (PFC) can offer?
PFC enjoys having on-board technical human resource having years of experience in myriad industrial cleaning applications. We are well aware of different methods and solvents which are in use and will provide best solution to your specific needs. We make sure that best industrial practices and highest international standards are followed while performing our services to make sure your products are safe and comply with all relevant regulations. In addition to this PFC also provides a wide range of other industrial services which can be seen here.
Sources https://blog.beamex.com/how-to-calibrate-pressure-gauges  https://en.wika.com/landingpage_bourdon_tube_pressure_gauge_en_co.WIKA  https://www.noshok.com/support/support-tools-faq/faqs/  https://www.sciencedirect.com/topics/engineering/bourdon-tube  https://support.flukecal.com/hc/en-us/articles/210083293-Cleaning-Pressure-Gauges-Contamination-Prevention  https://kingnutronics.com/pdfs/Model_3646_Portable_Instrument_Cleaning_System.pdf  https://books.google.com.pk/books?id=8SEr2p9RkCAC&pg=RA14-PA19&lpg=RA14-PA19&dq=cleaning+bourdon+tube&source=bl&ots=74rBHgHUpt&sig=ACfU3U2J-k_ZAkphBi9-iwqQF-7kU5grxQ&hl=en&sa=X&ved=2ahUKEwi15v74haTrAhWQZxUIHafNDB44ChDoATAHegQICBAB#v=onepage&q=cleaning%20bourdon%20tube&f=false