The Pressure's On

March 1, 2001

The Pressure’s On

Considerations for selecting pressure and vacuum gauges

By Fred Workley March 2001 Pressure and vacuum gages must be selected and installed so that misuse or misapplication is minimized. Remember from A & P school that direct pressure reading gauges often use Bourdon tubes, sometimes called elastic elements. As pressure tries to straighten the tube, the gear mechanism attached to the tube moves the pointer to the corresponding dial reading. Some important factors that must be considered in selecting the proper pressure or vacuum gauge are gauge failure, pressure, pulsation, vibration, fatigue, temperature, process, venting and calibration.

Hazards if the gauge fails
There are several modes of gauge failure including: a loss of accuracy, clogging of the pressure port, damage to the internal mechanism, a leak in the pressure containing parts or joints, a crack or fatigue of the Bourdon tube, rupturing of the Bourdon tube due to overpressure, or an explosion within the system due to a chemical reaction of the pressure medium with contaminants causing the Bourdon to explode.
Pressure gauges have an excellent record of safety. In most instances, the cause of failure has been misuse or misapplication. The following systems are considered potentially hazardous: compressed gas systems, oxygen systems, systems containing hydrogen or free hydrogen atoms, gas or liquid corrosive fluid systems, non-steady pressure systems, systems where a high overpressure could accidentally be applied, and pressure systems containing a flammable or explosive mixture or medium.

The range of the gauge should be approximately twice the intended operation pressure. Avoid applying pressure beyond the top of the scale. By design, the gauge should not have to continuously operate above 75 percent of its indication span. Bourdon tubes are necessarily highly stressed in ranges over 1,000 psi, and continuous operation at full scale will result in early fatigue failure and subsequent rupture. High internal stress could lead to a catastrophic failure. Pressure regulators, chemical (diaphragm) seals, pulsation dampers or snubbers, syphons and other similar devices are available for use in potentially hazardous systems. Generally the hazard increases with increases to very high operating pressures.

Excessive pressure pulsation, usually evidenced by continuous rapid pointer movement, could result in fatigue failure of the pressure element, or rapid movement-wear. A pulsation-dampening device installed between the pressure source and the gauge can eliminate cyclic pressure pulsations.
Some installations use a liquid-filled gauge. The liquid used is a viscous fluid, and these gauges have a sealed case. Some means of venting the case must be provided. In some cases, the vent is sealed to prevent the loss of fluid during shipment, and must be released prior to installation. Be sure to follow the installation instructions for proper venting of the gauge after installation. The liquid most commonly used is a mixture of glycerin and water. Be aware that when glycerin is combined with strong oxidizing agents including (but not limited to) chlorine, nitric acid, oxygen and hydrogen peroxide; potential hazards may result from the chemical reaction, such as ignition or explosion. Completely chlorinated or fluorinated fluids may be suitable for these applications calling for liquid filled gauges.

Excessive vibration could cause the movement to wear resulting in loss of accuracy. Vibration may also cause early failure of the Bourdon tube. Vibration concerns can be addressed with liquid-filled gauges. In addition, the gauge might be isolated by remotely mounting it on a non-vibrating surface and connecting with flexible tubing.

The Bourdon tube will fail after extended use, releasing the pressurized medium. The larger the numbers of applied pressure cycles, and the greater the extent of the pressure cycle, the earlier the premature failure may occur. Fatigue failure may be explosive. Precautions must be taken to direct or contain the release of the pressurized medium, such as having a box with a window over the gauge, or installing a restrictor. A restrictor will not reduce the immediate effect of failure, but it will help control the flow of escaping fluid following a rupture.

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.

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.

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.