The Pressure's On

Temperature
Excessive temperature could result in long term creep of solder joints and possible eventual failure. The maximum recommended operating (ambient) temperature is 150 degrees F. The capability of the pressure element is influenced by design, materials, and fabrication between its parts. The most common methods of joining are soft soldering, silver brazing, and welding. High temperatures or very low temperatures may cause uneven expansion or contraction of metals and thus the gage can not maintain a high degree of leakage integrity. In some cases there may be an allowable leakage rate at very high temperatures. Again the user must talk to the manufacturer for guidance.

Process
Pressure or vacuum gauges should not be reused. It is not recommended that gauges be moved from one application to another. There may be chemical incompatibility. For example, moving an oil service gauge to an oxygen service could result in explosive failure. Likewise, corrosion from the first installation may be accelerated in the second installation. Never intermix gauges used in aircraft hydraulic systems utilizing either Skydrol™ or red mineral oil (5606).
Some specific applications for pressure gauges exist where hazards are known. In many instances, requirements for design, construction, and use of gauges is specified by state or federal agencies or Underwriter Laboratories, Inc. Some examples are as follows: Acetylene gages are compatible only with commercially available acetylene and must have ACETYLENE on the dial. Oxygen is marked with a universal symbol and/or USE NO OIL in a red color.

Venting of case
Vents provided in the pressure gauge case must not be closed or restricted from providing a clear opening. There is always a possibility that the pressure medium will be admitted to the case interior as a result of a leaking joint or Bourdon tube failure. If this occurs, the pressure medium must be vented from the case so that the pressure build up does not rupture the case or break the glass in the window.

Calibration
Some gauges should be checked for accuracy and proper operation on a periodic basis. When was the last time you had the gauge on your engine compression tester calibrated? Mounting a pressure gauge in a position other than that at which it was calibrated can affect its accuracy. The normal calibration position is upright and vertical. For applications requiring mounting in other positions, contact the supplier of the gauge.

More on oxygen gauges
As we said earlier, oxygen gauges must be free of any contamination in the pressure sensing Bourdon tube. The whole oxygen system must be keep clean. Filters on the equipment must be examined periodically and cleaned or replaced. There are various levels of cleanliness specified in ANSI B40.1. Remember, never permit a sudden in-rush of oxygen into an empty system. If there is any contamination it may violently explode. The reason for this is that the in-rush of any high-pressure gas will momentarily create a very high temperature, which in the presence of oxygen could explode. Always open an oxygen supply valve very slowly so that the pressure build up is very gradual through the pressure regulator of the system. And remember to wear eye or face protection. If there is an explosion of the system due to contamination, the gauge will explode with particles being projected from the gauge. It is good practice to never leave the fill valve full open. Close the valve at least one-quarter turn so that the valve never gets stuck in the full open position. I have seen only three times when a full-open oxygen service valve could not be closed by hand. I can tell you that this is the potential for aircraft delays.
Again, misapplication and misuse are minimized if one considers the common factors associated with the selection and installation of pressure and vacuum gauges; so take the time to evaluate properly the correct gauges to Keep ’em flying.