'DRY' PV PRESSURE SENSORS

Brilliant PTOA Readers and Students ... meaning those who are reading the PTOA Segments in the intended, sequential order ... just learned that any PV Pressure Sensor must have a feature that is impacted by changes in the PV Pressure only ... and nothing else.

The impact of the PV Pressure upon this "feature" must then somehow be translated into a PV Pressure measurement and indication which human beings can understand.

One way to classify the "feature that is impacted by only a change in PV Pressure" is a "Wet" Pressure Sensor or a "Dry" Pressure Sensor. U-Tube Manometers and Inclined Manometers are "Wet" Pressure Sensors and were just featured in PTOA Segment #226.

This PTOA Segment focusses on the first two on the below list of common "Dry" Pressure Sensors:

• Diaphragm.
• Bellows.
• Bourdon Tubes.

Quickie Review: Any Force that Impacts a Surface Area Creates a Pressure

Fred, do you know what time it is?

Time for a quickie review of the relationship between Pressure and Force as was first featured in PTOA Segment #142.

When a Force (perhaps expressed in lb_f or Newtons) is applied over a Surface Area (perhaps expressed in square inches or square meters), the outcome is a Pressure expressed in lb_f per square inch (psi) or Pascals (1 Newton/Meter² = 1 Pascal).

In summary, Force and Surface Area are the two factors needed to create a Pressure!

 

The nearby schematic illustrates three common "Dry" PV Pressure Sensors.

Note how the "Dry" Pressure Sensors known as the Bellows (left side) and the Diaphragm (center) look a lot like the above diagram which shows how the impact of a Force (F) upon a Surface Area (A) creates the PV Pressure (not labelled).

Note also that the Force factor of the Pressure that is being sensed and measured by the "Dry" Pressure Sensors originates from an unknown source labelled "Applied Pressure."  This Force is exposed to the Pressure Sensor via the opening at the bottom of the sensor and points upward.

This Force factor is equal to the unknown Pressure multiplied by the Surface Area. Why?

If P = F/A, then F = (P * A).

In summary, the Force factor of the to-be-measured Pressure is directly related to that Force factor.

All Pressure Sensors use this relationship to sense, measure, and even display an unknow PV Pressure:

• The Force factor of the PV Pressure that is being measured pushes the top, interior of the known Surface Area of the Bellows and Diaphragm upward.
• The magnitude of the deflection is measurable and quantifies the Force factor of the unknown Pressure that is being sensed and measured.
• The Force factor is equal to the unknown Pressure multiplied by the Surface Area.
• The mechanical linkages of the Pressure Sensor translate the magnitude of the deflection into a PV Pressure that is displayed on a PI that the human being can understand. 

 

The above bullet points describe how all "Dry" PV Pressure Sensors operate to sense and measure the PV Pressure, including the Bourdon Tubes that will be featured in the next PTOA Segment.

Quickie Review of Pressure Classifications

The nearby graphic introduced in PTOA Segment #150 illustrates the classification and boundaries of Absolute Pressure (red arrows), Atmospheric Pressure (blue arrows), and "positive" and "negative" Gauge Pressures (green arrows).

A Sealed Bellows PV Pressure Sensor made from stainless steel can sense and measure Absolute PV Pressures ranging from 0-200 mm Hg and 0-60 inches Hg.

"Negative Gauge Pressures" (which are also Vacuum Pressures) can be measured in the range of 0-30 in Hg with a Capsule Diaphragm Pressure Sensor made from bronze. A wide range of "Positive" Gauge Pressures are measured with Spiral-Shaped Bourdon Tubes (range from 0-4000 psig) and C-Shaped Bourdon Tubes can sense and measure PV Pressures up to 10,000 psig.

The Single-Diaphragm Pressure Sensor logically has a single Diaphragm which is held firmly at the edges. The Diaphragm is "the feature" of the Diaphragm Pressure Sensor which is only impacted by changes in the PV Pressure to which it is exposed.

The Diaphragm is a thin, flat, and flexible disc; its outer edges are tightly secured and immobile. The Diaphragm might be fabricated from rubber or synthetic materials. The Diaphragm provides the Surface Area upon which the Force factor of the to-be-measured PV Pressure acts.

When one side of the Diaphragm is exposed to the PV Pressure of a process, the Force component of the PV Pressure will cause only the center of the Diaphragm disc to be deflected.

As the process PV Pressure fluctuates, so does the magnitude of the disc's deflection.

The High-Pressure side of the Diaphragm is the side where the Force component of the PV Pressure causes the Single-Diaphragm Pressure Sensor to deflect.

The Single-Diaphragm Pressure Sensor depicted in the nearby graphic senses changes in the PV Process Pressure on the top side of the Diaphragm. The Force factor of the Pressure causes the Diaphragm to deflect. This deflection is on the High-Pressure side of the Diaphragm.

The Low-Pressure Side of the Diaphragm is the known "Reference PV Pressure."

The "Reference PV Pressure" determines the service of the Diaphragm and Bellows Pressure Sensors.

For example, when the Reference PV Pressure is as stated below, the service of the Pressure Sensor is revealed:

• Atmospheric Pressure (1 ATM). When the Low-Pressure, Reference-Pressure side of the Diaphragm is Atmospheric Pressure, any deflection of the disc is due to a PV Pressure above Atmospheric Pressure which is also known as a (positive) Gauge Pressure (psig).
• Zero PV Pressure. When the Low-Pressure, Reference-Pressure side of the Diaphragm is evacuated and thus Zero Pressure, the Diaphragm is sensing Absolute Pressure (psia).
• A different Process PV Pressure: When the Low-Pressure, Reference-Pressure side of the Diaphragm is measuring a lower PV Process Pressure, the Diaphragm is measuring a Differential Pressure in psi (aka ΔP) sensed between two pressurized locations in the process.

 

PTOA Readers and Students were recently reminded in PTOA Segment #226 how ΔP can be used to infer PV Flowrates and PV Levels.

The below paragraphs explain how the magnitude of the disc's deflection can be translated via a linkage into movement on a Pressure Indicator (PI) that human beings can understand.

The Real-World version of the PI that uses a Capsule Diaphragm Pressure Sensor is also shown nearby. The encapsulated Diaphragms are in the housing at the bottom of the picture.

 

 

 

 

 

 

 

Form and Function of Capsule Diaphragm Pressure Sensors

A Capsule Diaphragm has two corrugated Diaphragms welded together at the edges; thus, a cavity is created between the two Diaphragms. The use of two Diaphragms magnifies the total deflection of the Capsule Diaphragm Pressure Sensor. Capsule Diaphragms can be '"stacked" to yield better accuracy in PV Pressure measurements.

As the nearby animated graphic shows, the leftmost Diaphragm does not move as much as the rightmost Diaphragm whose interior Surface Area is impacted by the Force factor of the unknown Process PV Pressure. The deflection of the rightmost Diaphragm is linked to the movement of the PI's pointer.

Capsule Diaphragm Pressure Sensors can be fabricated from bronze, stainless steel, and other alloys. Low range Bronze-derived Capsule Diaphragms can measure Vacuum Pressures in the range of 0-10 inches Hg (remember! 1 ATM = 29.92 inches Hg). The high range, stainless steel Capsule Diaphragm senses and measures PV Pressures in the range 0-100 psig range.

The inside cavity of the Capsule Diaphragm is the High-Pressure sensing side of the device and is exposed to changing, unknown PV Pressures.

The Low-Pressure Side of the Capsule Diaphragm is the intentionally created Reference PV Pressure that surrounds the capsule and is contained within the instrument's casing.

How does a Capsule Diaphragm Pressure Sensor sense and measure Gauge Pressure?

When the Low-Pressure, Reference PV Pressure is Atmospheric Pressure (1 ATM), then the Capsule Diaphragm will be deflected only when the unknown, sensed PV Pressure is greater than 1 ATM. In this service, the Capsule Diaphragm is sensing and measuring a Gauge Pressure.

How does the Capsule Diaphragm sense/measure/indicate a low Absolute Pressure (aka negative and/or Vacuum Pressure) in the range of 0 - 0.5 millibar and a sub-Atmospheric PV Pressure in the range of 0-1000 mbar with an accuracy of 0.1 to 2.5? (Remember! 1 ATM = 1013 mbar).

In this case, the Low-Pressure, Reference PV Pressure environment which surrounds the capsule is evacuated to as near a perfect vacuum as possible and thence hermetically sealed.

The unknown process PV Pressure (typically a less severe vacuum PV Pressure) is thus compared to a nearly perfect vacuum and a sensing, measurement, and indication of an Absolute Pressure in psia is the outcome.

How does a Capsule Diaphragm measure Differential Pressure aka ΔP?

When the encapsulated environment that surrounds the conjoined Diaphragms is exposed to a lower PV Process Pressure, the Capsule Diaphragm Pressure Sensor is sensing a ΔP.

The nearby schematic shows a stack of four Capsule Diaphragms, conjoined at the edges but with interiors connected. Capsule Diaphragms are stacked like this when accuracy of Pressure measurement and indication are warranted.

The Force factor of the labelled, unknown "pressure 1" causes all of the Diaphragms to expand and contract.The labelled "pressure 2" is a different and lower process PV Pressure to which the environment between the stack of capsules and the interior of the housing is exposed.

The Pressure Indicator (PI) is indicating the sensed, measured and hence indicated Difference in Pressure (aka ΔP) between "pressure 1" and "pressure 2."

 

THE BELLOWS PRESSURE SENSOR

From the outside, the Bellows Pressure Sensor looks like a dinky barrel with flexible sides but a rigid top and bottom.

Unlike the Diaphragm, the Bellows is in a sealed case and surrounded by the High-PV Pressure environment which is to be sensed, measured, and indicated.

Fluctuations in the PV Process Pressure to which the Bellows is exposed make the Force factor of the unknown PV Pressure push the Bellows upward, making it contract.

In the nearby gif of a Bellows Pressure Sensor, the Force is pushing upwards on the lower exterior Surface Area of the Bellows. A linkage between the rigid top of the Bellows and a pointer makes it possible to display the measured PV Process Pressure.

The Force that pushes the Bellows upward is counteracted by a calibrated spring.

Otherwise stated, the spring presses down on the Bellows with a force of compression that counteracts the Force factor of the Pressure. The existence and action of the counter-acting spring classifies the Bellows as a "Force-Balance" pneumatic instrument.

The tension of the spring corresponds to the desired amplification of the PV Pressure sensing and measurement.

For example, if the spring is compressed an inch by the Pressure's Force factor, the tension of the selected spring could be made to push back with a force of 10 lb_f. Ergo, if the spring is compressed 2 inches, the spring would push back with a force of 20 lb_f.

Brilliant PTOA Readers and Students who have slogged through this PTOA Segment can probably predict how the Bellows measures Gauge Pressure (psig),

Righteeo! The environment external to the Bellows is Atmospheric Pressure (1 ATM).

Bellows Pressure Sensors made from Brass can sense and measure PV Pressures in the range of 0-5 inches Water (0-0.184 psig) and in the high range of 0-90 inches Water. (0-3.2 psig)

Modifying the Bellows into a Sealed Bellows is required to sense and measure Vacuum Pressures (inches Hg) and Absolute Pressures (psia).

In that case the unknown Below-Atmospheric Pressure-to-be sensed-and-measured is piped directly into the Bellows interior ... the same architecture as a Diaphragm. The surrounding environment is a hermetically sealed "as perfect as can be Total Vacuum.

When the Sealed Bellows is made from bronze, the low range of Vacuum Pressures measured is 0-100 mm Hg (0-1.93 psia) and the higher range is 0-50 inches Hg (0-24.6 psia).

When the Sealed Bellows is made from stainless steel, the low range PV Absolute Pressure measurement ranges from 0-200 mm Hg (0-3.9 psia) and the high PV Pressure measuring range is 0-60 Hg (0-29.5 psia)

The Bellows Pressure Sensor can be further modified for sensing, measuring, and indicating a Differential Pressure (ΔP). Separate chambers housing a High-Pressure Bellows and Low-Pressure Bellows are sealed and the difference in their Force factors deflects a common medium, like a Diaphragm. The movement is linked to a pointer.

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TAKE HOME MESSAGES: The PV Pressure sensing and measuring element in "Dry" PV Pressure Sensors is not impacted by anything else except a change in the PV Pressure. Common "Dry" PV Pressure Sensors include:

• Single Diaphragms, Capsule Diaphragms, Stacked Diaphragms
• Bellows and Sealed Bellows.
• C-Shaped Bourdon Tubes, Spiral-Shaped Bourdon Tubes, and Helix-Shaped Bourdon Tubes, 

©2022 PTOA Segment 0227

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