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Typical Pressure Gage Low Pressure Electric Sensors Barometer
The simplest technology utilizes a piston pushing against a calibrated spring. The familiar tire pressure gage is
an example of this method. Very inexpensive, not particularly accurate, but suitable for the task.
The more typical pressure gauge is the round dial type unit with a screw fitting to allow the gas or liquid to enter the gage
works. There are several ASME grades of accuracy that dramatically effect the cost of the units.
Grade 4A: +/- 0.1% of the gage span, typically for laboratories, high precision
Grade 3A:
+/- 0.25% of span, typically for test gages
Grade 2A: +/-.5% of span, commonly specified for process measurements
Grade 1A:
+/- 1% of span, typically for industrial use
Grade A: +/-1% of span over the middle half of span, +/-2% over the rest, typical for industrial use when only middle of
span is most important.
Grade B: +/-2% of span, the common gage, sometimes referred to as commercial or utility gages.
To give some idea of how grade effects price here are some approximate costs for a 0 to 100 PSI bourdon tube style Gages:
Grade 4A: $1800; 3A: $250; 2A: $125; 2A; $70; A: $40; B: $25
 Lower pressure (under 5000 psi) and tame fluids generally
allow the less expensive copper “bourdon” tube style actuator. A small coiled tube inside the unit is subjected to pressure
fluctuations. As the pressure increases, the tube uncoils slightly which moves a lever or gear arrangement which is connected
to the indicating needle. This mechanism can also be used to sense vacuum, or negative pressure.
 Very low pressures are generally expressed in inches of water
rather than pounds per square inch. The bourdon tube style gage can accommodate this low pressure, but often a manometer is used. The conventional design is actually a U
-Tube with water in it. as pressure is applied to one side of the tube, the water rises in the other side and the pressure is read in inches of water
Higher pressures and unfriendly environments  generally call for a special bourdon tube
metal such as stainless steel. The option of a liquid filled gage helps the gage works from breaking down under rapid
pressure fluctuation. For particularly nasty environments such as very high temperatures or corrosive applications, a diaphragm seal is placed between the fluid to be sensed and
the gage works.
The diaphragm principle is also used with electronic sensing units.  A thin film resistor, piezoelectric resistor or strain gage is
attached to the diaphragm. As the diaphragm flexes or the mounting surface is strained, the resistance of the unit changes. Electrical or
electronic means then translate this change to an electric meter or electronic display. These are sometimes integrated into the indicating
gage or the sensors are placed at the pressure sensing location with electrical signals transmitted to remote displays. Data acquistition systems typically use
the remote type of mounting with the pressure to electric transducer for process control. Depending on the type of transducer, costs generally range between $150 and $400.
In most cases, pressure is expressed as “gage” pressure, for example psig or pounds
per square inch, gage. In practice, the “g” is normally left off of the representation. It
should be kept in mind that the true pressure generally includes the pressure of our atmosphere, which is 14.7 pounds per square inch at sea level. When readings include
the influence of atmospheric pressure, they are noted as “absolute” pressure, for example
psia for pounds per square inch absolute. When the true pressure needs to be known, the atmospheric pressure needs to be measured.
A barometer is devoted to this measurement task. Just to keep things interesting,
although this is a pressure unit, barometric pressure is expressed in inches of Mercury (Hg) or the metric unit of Millibars (mB). Barometers are available as analog units that
use the bourdon tube principle and electronic sensor based models. Costs for typical barometers found around the house are generally under $50 with the cost being driven
more by appearance and style than accuracy. Higher end units with accuracies in the 0.1 inch Hg range cost in the $150 range, while units with accuracies in the 0.03 inch Hg
range cost around $250.
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