Mechanical Pressure Sensor Basics I

Mechanical pressure-sensing elements include bellow type, bourdon & helical type, diaphragm or capsule type and differential pressure instruments. Each of these devices converts fluid pressure into a force. If unrestrained, the natural elastic properties of the element will produce a motion proportional to the applied pressure act as per force balance principle.

Bellows Type Pressure Sensing Element

Bellows resemble an accordion constructed from metal instead of fabric. A bellow unit elongates if we increase the applied pressure. Bellows are formed from seamless tubes that are hydraulically or mechanically roll-formed.

Applications: Bellows-type sensing elements are for low to intermediate pressures and vacuums with atmospheric or absolute reference. Also used in differential pressure applications.

Materials of Construction: Brass, stainless steel, beryllium copper, Monel, bronze, copper, Inconel

The main advantages of bellows (relative to diaphragm capsules) are their ability to provide longer strokes and to handle higher forces. Therefore, they are well suited as direct input elements into recorders and indicators. Their sensitivity increases with their diameter and noted for their good linearity

Their limitations include their sensitivity to ambient temperature effects, work hardening, drift, friction, and elastic hysteresis. In most cases, the elastic action of the bellows is insufficient for accurate measurement, and spring needs to be added to precisely characterize the relationship between force and movement.

Bourdon Type Pressure Sensing Element

The C-Bourdon element is made by winding a tubular element circularly to form a segment of a circle, the spiral is formed by winding two or three turns in a spiral around the same axis and the helical element is made by winding the tube several turns into a helix.

In each of these cases, the circular cross-section of the tube is flattened. Their cross-section tends to become more circular when these elements are pressurized,  which tends to straighten their shape. Helical and spiral elements are used when it is desired to reduce the stress in the tube and to increase the tip movement.

Bourdon tubes are circular in shape, made of spring-like metal alloys. A bourdon tube “tries” to straighten out into its original shape before being bent at the time of manufacture under the influence of internal pressure.

In the C-type bourdon sensor, the tube is the sensing element, while other parts are the linkage, lever, and gear assembly. A spiral bourdon sensor, designed to produce a wider range of motion than a C-tube bourdon sensor. The helical bourdon sensor produces an even greater motion of the free end than the spiral sensor.

Most pressure gauges use a bourdon tube as their pressure-sensing for greater motion (and therefore greater gauge resolution).

Applications: Sensing elements for the measurements of medium to high pressures using atmospheric pressure reference.

Materials of Construction: Phosphor bronze, alloy steel, 316 and 403 stainless steel, beryllium copper, Monel, Ni-Span C

List of Prominent Manufacturers: ABB, Ametek, Anderson Instrument, Barton Instruments, Dresser Industries, Dwyer Instruments, The Foxboro Co., Honeywell, Mid-West Instrument, Moeller Instrument, Moore Products, Nuova-Fima, OCI Instruments Inc., Rosemount Inc., Wika, Yokogawa

For other mechanical pressure sensors please follow the link: Instrumentation Basics: Mechanical Pressure Sensor Basics II (instrumentbasics.blogspot.com)

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