Zero Maintenance Pressure Instrumentation
Authors: Garnett Lawrence T. 
Published: October 2008

Reliable electronic packages are achieved with careful sealing of components against corrosive agents and by protection of parts against mechanical damage. An electrical pressure transmitter with welded enclosure and magnet-drive of zero and span adjustments is described.

ENVIRONMENTAL FACTORS IN PACKAGING

A difficult, environmental condition under which pressure instruments for aircraft and missiles must operate is high vibration. A difficult environment for electronic circuits is high humidity and condensing conditions prevalent along the Gulf Coast. The construction of pressure sensors for aerospace service comprises solidly encapsulate electronic circuits in a metal can, with welded housing. This is appropriate for both conditions of high vibration and high humidity. An aerospace pressure sensor is shown in cutaway in Figure 1.

For resistance to vibration, the laws of scale dictate in favor of smallness, and transducer design has trended in that direction as vibration levels move higher. Thin film technology has made it possible to produce not only very small and integrated electrical circuits, but also has provided strain gage sensors without significant mass. This low mass, coupled with intimate bonding to the substrate, eliminates parts which can move under shock and vibration. The small size of thin film strain sensors arid integrated circuits permit packaging in small welded enclosures with glass feedthroughs. Interconnections between I. C. s now become the major sources of failure due to mechanical fatigue and/or corrosion.

A similar vibration and humidity-resistant design for industrial use is demonstrated in the pressure sensor shown in Figure 2. Motion of the sensing diaphragms is limited to a few mils of displacement so that mechanical natural frequencies are kept high (the lowest natural frequency is 3000 Hz for the lowest range of 15 psi). Long-term stability is achieved by intimate molecular bonding of insulating film and resistance film to the metal deflection number. An alternative construction is to deposit the resistance film directly on the diaphragm, but economics of manufacture are achieved by use of a common deflection sensor. For resistance to moisture and corrosive vapors and gases which might be present in the environment, the strain gage sensor is housed in a welded enclosure with glass-to-metal seals for electrical leads. The enclosure also serves as a redundant pressure containment if the sensing diaphragm fails due to overpressure or corrosion. A second welded enclosure with glass-to-metal feedthroughs is used to contain the electronic amplifier needed for high level signal transmission.

To provide comparative values in packaging, a load cell is made by Gould for laboratory weighing applications. This is a very price-conscious market, where the environment is also much less severe than on offshore drilling platforms. A generally open construction has been used, and sealing of electrical parts of the strain gage against moisture intrusion is by a thin conformal insulating coating (Parylene). A thin coating is needed to avoid strains in the electrical film which can cause an unwanted output signal.