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01 April 2003

Pressure needles

The concept of pressure involves a number of tricky physical precepts.

Technically, pressure (P) equals force (F) divided by area (A), which intuitively makes sense when one thinks of a weight or force bearing down on a square foot of real estate and one considers that the units of P are the familiar pounds per square inch (psi).

The truth is that those pounds are pounds-force (lb_f), not pounds-mass (lb_m), and acceleration and gravity can get involved in the actual calculations. Quite frankly, it's a pretty messy computation for a physical parameter that we're all fairly comfortable with.

Pressure is indeed a force applied to an area, and it is what forces myriad fluids to and through pipes, valves, and process plants.

If the pressure at point A in a pipe is higher than the pressure at point B in that same pipe, then the fluid moves from point A toward point B.

As well, if the pressures at points A and B in the pipe are equal, the fluid does not move at all.

The physical manifestation of these simple statements resides in the movement of needles and pointers in the measurement of pressure and process fluid flow.

See the beam balance bellows sensor. An increase in process pressure causes the measuring bellows to extend, which results in an increase of readout through the motion balance mechanism.

When bellows are the pressure sensing element, it is best to add a spring for ranging and accurate characterization. A single bellows design has the inside of the bellows open to the atmosphere, which represents the pressure reference.

The spring-loaded metal bellows compresses due to process pressure, forcing the lower end of the bellows upward against the opposing force of the spring. The movement transmits through linkage to a pointer.

The spiral element Bourdon tube tends to uncoil when process pressure applies. This action produces movement of the free end, which is efficient and needs no linkage or levers to the needle indicator.

The spiral materials are several and include bronze, steel, stainless steel, beryllium copper, Monel, and Ni-Span C. This style can measure up to 100,000 psi.

Ring balance manometers see a good deal of use in Europe. A tubing ring in the vertical plane pivots at its central axis, and a pressure differential rotates this ring. The opposing torque comes from a weight attached to the ring.

It's a differential pressure, manometer-type device. Depending on the design and the filling fluid used, differential spans of 5–500 inches of water (0.18–18 psi) can register to this unit.

When remote readouts are required, or when the process material is hazardous or is at high pressure, float manometers are used.

In these units, the variations in pressure differential cause the level of filling liquid to change and the float to move with the changing level.

The float motion transfers out of the chamber by a lever rotating in a pressure seal bearing. IT

Nicholas Sheble (nsheble@isa.org) writes and edits the Control Fundamentals department. The source for this article, from which the figures also come, is Bela Liptak's Instrument Engineers Handbook, 3rd edition.