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Question about gas leak engineering units

  1. Jun 21, 2011 #1
    I have a somewhat mundane question that I hope somebody can help me with. I am working with several standards for performing leak testing on hermetically sealed electronics packages using helium bombing. The question I have is about the engineering units used in the leak test specifications. Both the NASA Leakage Testing Handbook (NASA CR-952) and the Nondestructive Testing Handbook - Leak Testing, 3rd Edition, published by the American Society for Nondestructive Testing indicate that the units to be used for specifying a gas leak are Pressure (P) x Volume (V) / Time (t). Pressure x Volume reduces to Energy (see, for example, the Ideal Gas Law, which is an energy balance equation: PV = nRT) and Energy / Time is Work or Power. This being the case, I would expect the units for a gas leak specification to be in terms of Power (Watts or other Power units). Instead, in every standard I have seen, the units are left in their unreduced form such as Atmosphere-Cubic Centimeters per Second, Pascal-Cubic Meters per Second, or Millibar-Liters per Second. To add to the confusion, some standards employ the abbreviation STD to indicate a leak rate at some "standard", sometimes unstated, conditions. An example of this would be STD cm^3 / sec. In this case they seem to have dropped the units of Pressure since, I am assuming, the Pressure is defined as part of the standard conditions (STD). Anyway, bottom line, I am looking for an explanation as to why unreduced engineering units are universally used in gas leak standards.
     
  2. jcsd
  3. Jun 21, 2011 #2
    the reason might be that most measurements relate to the rate of flow of gas, such as the increase in concentration over time measured by a concentration-based sensor , or calculated from the ultrasonic sound signature of a leak, so an unreduced expression is more directly 'usable' without any calculations needed... the pressure should have already been known inside the pressurized chamber, and it is unlikely that a gas leak investigator will need to know the power output as a result of the gas leak, unless the gas is being mechanically used i.e. pneumatic system leaking leading to power loss, etc
     
    Last edited: Jun 21, 2011
  4. Jun 21, 2011 #3
    Torr-liters per second is equivalent to mass flow per second. For example, since 22.41 liters of helium at 760 mm is 4 grams, 1 torr-liter per second of helium is 2.3 x 10-4 grams per second.
     
  5. Jun 22, 2011 #4
    Just saw this post and no nothing about this topic...but, I just wanted to say that sometimes units for certain quantities are given a set of units different from their reduced form just to be able to tell them apart. Or, like carmatic says, just to have them ready for their use in a rather practical manner....easy mental operation, easy look up tables, etc.

    For example...what do you get when you multiply volts and amps?

    Well, I am an electrical engineer and when it comes to generators, there are 3 popular quantities that have the same fundamental units, yet, they are given 3 different units just so that we know what we talking about...

    Apparent Power
    Real Power
    Reactive Power

    they are all powers and bascially volt-amperes or (mega volt-amperes), yet, we talked about them in units of

    MVA mega volt-amperes
    MW mega watts
    MVAR reactive MVAs

    so, sometimes it is just a convenience thing and that's it.
     
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