Gases & Instrumentation - March/April 2008

range, the amount of thermal expansion can be significant, affecting a larger percentage of the actual operating stroke of the poppet.

For example, the full stroke of a proportional valve with an effective orifice of 0.003 inches can be 75 ten thousandths of an inch (0.000075 inches) while the effective stroke of a valve with an 0.065 inch orifice may be 0.0162 inches. A thermal expansion totaling only half a thousandths of an inch (.0005) would reduce the stroke of the larger valve by only 3 percent, but it would completely close the orifice of the smaller valve. A closed loop feedback circuit would attempt to increase the amount of voltage or current to the valve to correct this problem, but merely add more heat to the equation.

A properly designed thermally compensated valve uses materials that reduce the effect of elastomer swell due to permeation by gases such as helium and hydrogen at high pressure. Trace gases such as chlorine that cause fluoroelastomers to swell are also adjusted for within certain parameters.

Richard Parker and Otis

Conclusion

Regardless of ambient operating conditions, as soon as voltage is applied to a valve, heat becomes a factor for any critical application. By selecting a valve with well-designed thermal compensation features as well as a vendor with many years of experience in applying proportional valves in critical fluidic applications, engineers can get the highest performance and reliable results for their projects. G&I

RICHARD PARKER IS FLUIDIC CONTROLS PRODUCT MANAGER WITH PARKER HANNIFIN, PNEUTRONICS DIVISION, 26 CLINTON DRIVE, UNIT 103, HOLLIS, NH 03049. RICHARD PARKER HAS BEEN INVOLVED IN FLUIDIC CONTROL APPLICATIONS PRIMARILY RELATED TO THE MEDICAL AND ANALYTICAL INSTRUMENTATION MARKETS FOR ALMOST 20 YEARS. HE HAS HELD THE POSITION AS PRODUCT MANAGER FOR THE PAST 11 YEARS WITH PARKER HANNIFIN, PNEUTRONICS DIVISION. PRIOR TO PNEUTRONICS, HE WAS RESPONSIBLE FOR DEVELOPING PA TIENT-RELA TED SURGICAL AND MEDICAL DEVICES FOR A HARVARD UNIVERSIT Y TEACHING HOSPITAL. HE CAN BE REACHED AT 603.578.1562 OR RPARKER@PARKER.COM.

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March/April 2008

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