This will be the least technical "technical" page on the web.  Our purpose is to explain, in laymen's terms, some of the  issues relating to pressure, temperature, and flow that might be of interest to our visitors.  Please tell us how we are doing.

Basic Definitions:

Gravity:
Newton tells us that two masses attract each other depending on how large they are and how close they are to each other.  The attraction of the very large earth on everyday objects around us results in the weight of those objects, otherwise known as the force due to gravity.   Almost everything that follows is based on this concept.

Atmospheric Pressure:
There is a thin layer of atmosphere surrounding the earth which is held in place by gravity.  Although it is only 1/100 the diameter of the earth, this film increases in density as it approaches the surface of the earth.  At the surface, the weight of the mixture of mostly nitrogen (78%) and oxygen (21%) that we call "air" creates a pressure on the surface of the earth that we measure at approximately 14.7 psia (pounds per square inch absolute).  This pressure varies depending on your position and altitude and is reported by your local weather service in "Inches of Mercury" rather than psia.  The barometer (a means of measuring atmospheric pressure) will be "29.95 and falling", for instance. 
Absolute pressure = the difference between a referenced pressure and the absence of pressure.


Gauge Pressure:
Pressure gauges, and most other pressure measuring devices, generally measure gauge pressure rather than absolute pressure.    In other words, a gauge not attached to any pressure source will show the "gauge" pressure as "0" even though, as stated above, the "absolute" pressure is actually 14.7 psia.  In general, we are interested in the difference between the atmospheric pressure and what is in our compressor, so we want to know the "gauge pressure" of our tank or compressor.  Here is a new definition:
Gauge pressure = the difference between a referenced pressure and atmospheric pressure.
In PSI units it should be referred to as PSIg but the g is generally assumed and left off.  By convention, measurements in psia always include the "a".  As a rule, pressure gauges read gauge pressure.

Vacuum:
If all this is true, what is a vacuum?  A partial vacuum exists when a referenced pressure is below atmospheric pressure.  Vacuum readings increase as the referenced pressure goes from atmospheric to absolute zero, or a full vacuum.  A vacuum gauge at rest reads  zero and increases until it could read -14.7 psig or -30 (29.92) inches of mercury if it were measuring a perfect vacuum.  However, on earth, assuming an atmospheric pressure of 14.7 psia, a full vacuum can never be more than -14.7 psig or -30 inches of mercury, because that is all the atmospheric pressure there is here.  If you pulled a vacuum on Uranus however.....

Differential Pressure:
For applications like filter monitors, we are also interested in differential pressures or the difference between two measured pressures.  Differential pressure measurements are designated with the suffix -d so that in English units the pressure might be measured in PSId.  One more definition:
Differential pressure = the difference between two referenced pressures.
So gauge pressure is actually as sort of differential pressure where one of the referenced pressures is atmospheric pressure.  Clear?  I didn't think so.

Pressure Units:
There are a huge number of pressure units theoretically possible.  Any force over any area would be a pressure, so "Tons per Acre" would be valid but not very useful.  There are only a limited number that are generally used.  The Instrument Society of America as well as ISO have designated kPa as an international standard.  The world has yet to be impressed, so different scales are used in different areas for different reasons.  Here is a synopsis of the most prevalent:

  For a good source of other conversion factors at a remote site click HERE

A pressure gauge senses these pressures and displays what they are so a human can read them.  Pressure gauges can be analog or digital or a combination of both.  By far most prevalent type is the mechanical analog type that has been around for about 150 years.  Around 1849, a Frenchman, Mr. Eugene Bourdon, invented the bourdon tube pressure gauge which is used for reading pressure ranges from 0-10 psi up to about 0-60000 psi and more.  Instead of a long explanation of how a bourdon tube gauge works here, visit this remote page  for a good overview.  Come back when you are done.  Lower pressures, like 5 PSI down to 10 IWC (0.36 PSI) full scale, are generally read by diaphragm or capsule gauges.  In all these types, the linear motion associated with changes in pressure is translated to the rotary motion of a pointer by the gauge movement.

A pressure gauges is made up of the following parts:

1) The "pressure assembly".  Usually soldered, brazed, or welded together and made up of:
          a) Socket - a bored out and threaded barstock piece that accepts the sensing element
          b) Sensing Element - Bourdon Tube or Capsule
          c) Tip - connected at the moving end; closes the Bourdon tube and provides for link.

2) The "movement".  Usually a rack and pinion geared assembly that translates the linear
            motion of the element into the rotary motion of the pointer.  It consists of
          a) Link - a free moving piece that links the tip to the sector
          b) Sector - a "rack" gear and fulcrum with a pivot point between the tip and the pinion
          c) Pinion - a gear with center shaft that attaches to the pointer
          d) Plates - hold the gears together, the dial on,  and mounts to the socket

3) The "dial".  Mounts to the plates and contains the pressure units and graduations

4) The "pointer".  Points to the appropriate pressure.  We hope.

5) The "case".  Holds and protects all the other parts in the assembly

6) The "lens".  Acts as a clear cover so the operator can see the pointer and dial.

Wetted Parts:
The three pieces that make up the pressure assembly, namely the tip, tube, and socket, (TTS) are the only "wetted parts".  That means that these parts are "wetted", or touched by, the fluid that the pressure gauge is measuring.  Therefore, in the absence of a chemical seal, these parts MUST be chemically and mechanically compatible with the measured media.  Normally made of brass, stainless steel, Monel®, or steel, the wetted parts are crucial to the safe operation of the pressure gauge.  The customer must be sure that the gauge he is ordering is compatible with the media he wants to measure.  Since we offer a few fixed materials only, the customer must accept responsibility for insuring that his media is compatible with the wetted parts offered.  We cannot "recommend" a particular material for your application but we will do our best to steer you away from combinations that we suspect are not appropriate.  The responsibility of knowing your media and it's chemical compatibility's remains yours.

Accuracy:
Pressure gauge accuracy in the U.S. was governed by ANSI and now by ASME under the ASME B40.1 guidelines.  It is important that you understand how accuracy claims are made.   Linearity, repeatability, hysterisis, and temperature effects are all components of accuracy.  Bottom line, though, is that gauges are rated at plus or minus some percentage of full scale and must be within those tolerances everywhere on the dial no matter what the source of the inaccuracy.  Full scale means the highest pressure shown on the scale.  On a 0-160 PSI gauge rated at 1% of full scale, the gauge should be within 1.6 psi (1% of 160) at any point on the dial.  So if the 160 PSI gauge reads "dead on" 80 PSI, you only know that the pressure is between 78.4 and 81.6, assuming the gauge is "in calibration".  As you will see below, grade B, and grade A gauges are given a designation X%-Y%-X% of full scale.   The X% applies to the first and last 1/4 of the scale and the Y% applies to the middle half.

ASME B40.1 Grades (Except for B+, added for convenience)

[ Home ]

Send mail to terryb@hjkirbycorp.com with questions or comments about this web site.
Last modified: May 25, 2004