GAUGES – A GENERAL INTRODUCTION
The earliest known pressure gauges date back to the mid 17th century. These gauges measured liquid pressures using a tube as much as 30 feet in height which in most cases was sufficient for the systems of that day.
However, with the introduction of mechanical devices such as steam engines, the tubular system was completely inadequate, unable to cope not only with the higher pressures but also with vibration. Size of the tubular system was another factor in the introduction of new resilient based pressure gauges.
In today’s world, the mechanical gauges offer the simplest and least expensive way of supervising the flow of pressure.
In 1848, the Bourdon tube gauge was introduced to the world and has proven itself to be a reliable, inexpensive basis for monitoring pressure.
Bourdon gauges do not require an external source of power or at best a small internal battery. Such systems are often prone to failure with the malfunction of the electrical energy source,
Mechanical gauges use the energy they are measuring to provide an indication of the power they are using.Quite often the pressure is converted to motion through a series of gears which in turn drives the pointer.
The Bourdon system employs a sealed flexible tube that expands & contracts changing its shape slightly in reaction to pressure. With one end of the tube fastened to the source of energy to be monitored, the free or opposite end of the tube which is sealed moves a only a fraction of inch. The tube itself is usually formed in a “C” shape although small diameter spirals and other shapes can be constructed to provide sufficient movement to be transferred through a series of gears to rotate an indicating pointer. Accuracy of the Bourdon system can be as high as 0.1% over a pressure range of -30″ HG Mercury to as much as 15,000 or 20,000 psi. Gauge dial diameters vary from as little as 1 1/2″ to a maximum of 16 “. The Bourdon system does not operate well at pressures below 12 PSI as there is not enough force to cause the tube to flex sufficiently.
If excessive pressure is applied the tube will become permanently distorted. Once permanently distorted the tube will be unable to return to zero rendering the gauge useless. The tube must be constructed of a material that will withstand a minimum of 200,000 cycles.
For low pressure applications below 12 to 15 psi a bellows or diaphragm is often used. This system was first patented in 1850, just two years after the introduction of the Bourdon system.
The basic diaphragm system was modified later to today’s capsule system. In theses applications, the elastic chamber is usually fashioned from very light and/or thin sheet metal formed into a capsule. Operation reliability & increased longevity of the diaphragm capsule is often enhanced by use of a coil spring.