A Compound Pressure Gauge can measure Vacuum Pressures and Gauge Pressures. This Pressure Gauge has a range of 30 in Hg Vacuum Pressure to 100 psi A Spiral-Wound Bourdon Tube made from stainless steel can sense and measure PV Pressures in this range.

The nearby Pressure Indicator (PI) has a range of 30 in Hg Vacuum Pressure to 100 psi which reveals this gauge is a Compound Pressure Gauge.

Note that this PI is not yet in service and is thus sensing and measuring Atmospheric Pressure (1 ATM = 0 Gauge Pressure).

A Spiral-Wound Helix Bourdon Tube made from bronze can sense and measure Vacuum Pressures from 14.25 in Hg to 0 psia. A Spiral-Wound Helix Bourdon Tube made from bronze could also be the Pressure Sensor within a Compound Pressure Gauge with a range of 0 psia to 15 psia.

Change the metal of fabrication to stainless steel and the Spiral Wound Bourdon Tube can sense and measure the low-Pressure range of 0-50 psia in a Compound Pressure Gauge. The higher-PV Pressure range version of the stainless steel Spiral Wound Bourdon Tube can sense and measure Absolute and Gauge Pressures in the range of 0-4000 psia.

Obviously, the materials of manufacture of Bourdon Tubes impacts the desired range of PV Pressure sensing and measuring.

ALL BOURDON TUBES MUST HAVE AND MAINTAIN "ELASTIC HEARTS"

All versions of Bourdon Tube Pressure Sensors must be "elastic." Otherwise stated, the Bourdon Tube must be able to deflect and contract to the same position while continuously sensing increasing and decreasing PV Pressures.

For example, a Bourdon Tube that is currently at a position that represents 45% of range can expand to 65% of range upon sensing an increase in PV Pressure.  Perfect "elasticity" means that same Bourdon Tube must contract from 65% of scale to the same position it had at 45% of scale upon sensing the same magnitude of decreased Pressure.

And the Bourdon Tube must be able to continuously expand, and contract as described above.

If the Bourdon Tube fails to sense the same PV Pressures going up scale and down scale, the movement of the Pressure Sensor is not linearly proportional to the PV Pressure that is being sensed and measured and displayed.

What can cause the Bourdon Tube Pressure Sensor to lose its elasticity?

• Springs can lose their elasticity, too. Some Pressure Sensors have Springs. Springs lose their elasticity over time.
• Metal Fatigue - the gradual formation of tiny cracks which eventually join up and cause the Bourdon Tube's metal to break.
• Corrosion - the design of the Pressure Sensor/Measurement/Indicator did not protect the Bourdon Tube from the corrosive, viscous, hot, and/or hazardous environment that is having its PV Pressure sensed and measured.
• Creep - The Bourdon Tube has been sensing maximum pressure for a prolonged duration which will cause the Bourdon Tube to elongate and permanently deform.
• Over-Pressuring - the PV Pressure that the Bourdon Tube is sensing has exceeded the calibrated high PV Pressure range during an overpressure event.  The Bourdon Tube, counter-acting spring, and PI are now useless

The Bourdon Tube used to sense/measure/display a PV Pressure is impacted by Hysteresis.

The descriptors "elastic" and "elasticity" mean that the Bourdon Tube can return to its original size, shape and position after being stretched.

Consequently, "losing elasticity" means that the Bourdon Tube does not sense and measure the same magnitude when the PV Pressure is repeatedly increased and decreased.

The ten-dollar word for the phrase "Bourdon Tube has lost its elasticity" is Hysteresis.

The metals chosen to fabricate Bourdon Tubes optimize between the metal's flexibility and ability to maintain elasticity. (aka minimize the "Hysteresis characteristic").

Assume the Y-axis is labelled Sensed/Measured/Displayed Output Pressure and has a magnitude of 0-100 psig. Assume the X-axis is labelled Percent of Pressure Measurement Range and runs from 0% on the left to 100% on the right.

The nearby graph illustrates the relationship between the Sensed/Measured/Displayed Output Pressure (Y-axis) and the Percent of Pressure Measurement Range on the X-axis. Assume the Y-axis and X-axis are labelled as was just stated.

Assume the Y-axis range is "0 to 100 psi" and the X-axis is range is "0-100%."

Ideally, this graph would show one straight, 45-degree line slanted upward from the place where the 0% scale intercepts the Y-axis.

A single 45-degree upward-slanting line would prove that the Sensed/Measured/Displayed Output Pressure on the Y-axis is the same whether the sensed Pressure is increasing or decreasing throughout the entire range of measurement (from 0-100%).

Hysteresis is evident in the nearby graph of a Bourdon Tube because there are two separate curved lines depending upon if the sensed Pressure Output is increasing (the lower, concave curved line) or decreasing (he higher, convex curved line).

Note that the PV Pressure of both curves are the same at two points, 0% of scale range and 100% of scale range.

A Process Operator or Instrument Tech would observe an accurate reading if the Bourdon Tube happened to be sensing and measuring a PV Pressure at 0% or 100% of scale.

Not good enough! Why would anyone want to observe PV Pressure at 0% and 100% of the Pressure range?

• Any other PV Pressure sensed and measured by the Bourdon Tube would be inaccurate. The sensed and measured Output Pressure would be too high when the sensed and measured Pressure was decreasing. The sensed and measured Output Pressure would be too low when the sensed PV Pressure was increasing.
• PTOA Readers and Students already learned that operating continuously at 100% of range is not feasible because the Bourdon Tube will become deformed as was already explained.
• As was stated above and reiterated here, well designed Process Plants use PIs that will be operating in the 45%-55% range during normal operations. The two curved lines infer a significantly different Pressure Sensing/Measurement Output would be displayed on the PI when at 45-55% of scale.

Maintaining Pressure Gauge accuracy can be accomplished via calibration with a Deadweight Tester,

A Deadweight Tester uses a known, accurately created Applied Pressure and compares this Applied Pressure to the output from the Pressure Gauge.

The operating principle of the Deadweight Tester is the definition of Pressure:

Pressure = (Weight Force ÷ Area)

The weights that are used for calibrating a Pressure Gauge are themselves calibrated with precision. The Surface Area of the Primary Piston of the Deadweight Tester is likewise precisely manufactured. Thus, the created Applied Pressure of the Deadweight Tester is extremely accurate.

A calibration procedure for a bench model Deadweight Tester is below. To test gauges in the field a different model tester with a hydraulic pump is used.

The Deadweight Tester calibration procedure described below applies to the Spiral-Shaped and C-Shaped Bourdon Tube which sense and measure high-range PV Pressures.  Manometers are used to calibrate low PV Pressures.

The bench model Deadweight Tester will have a connection for the Pressure Gauge of interest (labelled Pressure Gauge in the nearby schematic), a hydraulic oil reservoir (labelled Oil Reservoir in the middle) and a Primary "Pressure Piston" where the known Applied Pressure is created (on the right side of the schematic).

A Screw plays the role of a pump; as the screw is turned, it displaces hydraulic oil first into the Pressure Sensor and subsequently into the Primary Pressure Piston.

The manufacturer of the Deadweight Tester will provide instructions similar to the following:

• 

  The Primary Piston (with accurate weights and Surface Area), the oil reservoir, the Test Gauge connection, and hand pump with Screw Pump are the parts of a Deadweight Tester.

  The Pressure Gauge to be calibrated is attached so that it can sense the pressure of hydraulic fluid in the manifold that connects the Pressure Gauge, Oil Reservoir, and Primary Piston.

• The desired total test weight is placed on the surface area of the Primary Piston.
• A hand pump is probably used to quickly approach the comparison Pressure. A Screw Pump is used to achieve the final comparison Pressure with finesse.
• Turning the Screw displaces the hydraulic oil and the displaced oil first flows into the Pressure Sensor.
• Once the Sensor is filled, the hydraulic oil back pressures the Primary Piston. Eventually the weights on the Primary Piston appear to "float "which signifies the Pressure sensed by the Pressure Sensor is equivalent to the known Applied Pressure.
• Does the PI on the Pressure Gauge match the magnitude of the known Applied Pressure?

TAKE HOME MESSAGES:  Like their family members Diaphragms and Bellows, Bourdon Tubes are "Dry" Pressure Sensors.  The Force factor of the sensed Process Pressure causes the Bourdon Tube to deflect when sensing an increase in PV Pressure and contract when sensing a decrease in Pressure.

In order of increased sensing accuracy, the list of Bourdon Tubes includes:

• C-Shaped Bourdon Tubes
• Spiral-Shaped Bourdon Tubes
• Helical/Helix-Shaped Bourdon Tubes