Vacuum Gauge Tech

    An explanation as to why a vacuum gauge
    using an inches of mercury scale
    registers a lower reading at higher elevations. 

    By Michael Ballou, IMPCO, Inc.
    October 20, 2003

    Paragraphs 4.4.2 and 4.4.3 of MIL-STD 276A requires a minimum vacuum of 29 inches of mercury.  This requirement creates a dilemma for people performing MIL-STD 276A at elevations above sea level.  The reason: this scale is measuring vacuum relative to sea level and using a gauge with this scale at elevations other than sea level requires a correction.

    To understand this let's look at the operation of the gauge.

    This gauge registers a vacuum level resulting from a force that is placed internally on the gauge by a displacement of air.  As air is removed or displaced by a vacuum pump, a force is generated within the gauge deflecting a component in the gauge which causes a needle to move.  At sea level, and this is important, there is a set volume of air.  To obtain a measurement of 29 inches of mercury with this gauge, a vacuum pump must displace a certain amount of this volume.

    At higher elevations, the volume of air is naturally less resulting in the vacuum pump achieving the same level of vacuum but displacing less air because there is less air to be displaced.  Because the vacuum pump has displaced less volume, the amount of force put on the internal mechanism is less resulting in less needle deflection.  Thus the same level of vacuum registers a lower value on the gauge using this scale.

    When measuring vacuums at elevations other than sea level, a better gauge would be one using an absolute scale such as torr or millimeters of mercury.  This type of gauge sets zero as a perfect vacuum and then the scale increases in pressure from there.

    For example, 0 is a perfect vacuum, 760 torr is atmospheric pressure at sea level and 5931 torr correlates to 100 psig.  Anything below 760 is a vacuum and anything above 760 is pressure. Using this scale, 29 inches of mercury at sea level correlates to 25 torr.  Now as you go up in elevation, and it doesn't matter to what elevation, 0 is still a perfect vacuum, but atmospheric pressure say as an example may be 670.  Anything below 670 is a vacuum and anything above 670 is pressure, but 25 torr still correlates to 29 inches of mercury at sea level.  100 psig would have to be corrected per the formula below.

    So, to satisfy your requirement of 29 inches of mercury for MIL-STD 276A at elevations above sea level, set your process to obtain 25 torr.  25 torr regardless of elevation will always equate to 29 inches of mercury at sea level.

    Note:  The formula for calculating torr readings of elevated pressures is as follows:

               (gauge pressure) X 51.71486 + atmospheric pressure in torr.

               At sea level this would be 100 psig X 51.71486 + 760 = 5931.

    Another way to perform this calculation using barometric pressure is as follows:

               (gauge pressure) X 51.71486 + (barometric pressure X 25.4).

    Method for Determining  in.Hg. Vacuum at Elevations Above Sea Level

    Present Altitude (Feet) X 0.0010285 = X.

    Full Vacuum @ Sea Level = 29.921 in. Hg.

    Full Vacuum @ Present Altitude = 29.921 – X = Y

    Per Mil-Std 276A, vacuum level must not be less than 29 in. Hg.  (It is inferred to be measured at sea level.)

    29 in. Hg @ Sea Level = 25 torr.

    in. Hg corresponding to 25 torr @ present Altitude = (-(25 torr X 0.03937)) + Y.

    For example: at 3500 feet;  X = 3.6.

    Full Vacuum at 3500 feet = 29.921 – 3.6 = 26.321.

    25 torr at 3500 feet = (-(25 X 0.03937)) + 26.321 = 25.5 in Hg.

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