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Calibrating without blowing a gasket: bellows finger makes adjustments through sealed avionics gauge.

While designing their 9A1990 liquid oxygen level gauges for high-altitude military transports, the design engineers at Transicoil Inc, Norristown, PA, were challenged to find a method for mechanically adjusting a hermetically sealed instrument from the outside--without breaching the seal. Specs for the oxygen gauges required two field-adjustment capabilities for potentiometers inside the seal: one to vary the measurement range and another for periodic calibration.

Mounted on the instrument panels of C130s and C141s, the gauges tell the pilot how much oxygen is left for their critical breathing apparatus. Each plane model has different sized liquid oxygen storage tanks, necessitating the gauges' range adjustment. Each aircraft also has its own power supply requirements, thus demanding the field calibration spec.

The hermetic seal prevents fogging of the display glass by condensation. To this end, the gauges are purged to [10.sup.-8] atm, then partially refilled with dry nitrogen. This standard practice in avionics instrumentation counters the atmospheric air "pumping" that drives water vapor into the internals of "open" instruments. Water vapor is the leading cause of damage to the instruments' viewing glass, besides making readings difficult at potentially critical moments.

To provide the field adjustment capability, Transicoil engineers designed rotary adjustable potentiometers into the instrument internals within the hermetic barrier. While the easy solution would have been to extend the adjustment knobs through the instrument shell (with O-rings providing the seal), the engineers decided that the seal integrity was too important to risk failure.

The team started looking for different methods of transmitting a mechanical movement from one side of the seal to the other. They had been using electrodeposited nickel bellows as pressure compensation and pressure actuation devices in other instruments, and were therefore familiar with the bellows' absolute impermeability, ductility and suppleness, reliability and extremely low weight. The question was how to put these properties to use in the new function.

Their solution was to create a mechanical "finger" based on a custom bellows, much like gloves in gloveboxes. Servometer, Cedar Grove, NJ, customized their bellows with a stiff conical point at one end. Thus, the bellows' main body is as supple as a Slinky, yet the conical end is stiff enough to turn a knob or linkage inside a gauge. A nib on the outside of the bellows, in turn, engages the potentiometer to make the adjustment. "You might think of the nib as a 'fingernail,'" says Chet Wasniewski of Transicoil. "Who among us hasn't turned a loose screw with a fingernail?"

To give motion to that fingernail, Transicoil used a slender 0.187-in. dia. shaft with a 45-degree bend near its end, running down the center of the bellows. The top end is exposed at the rear of the gauge and has a slotted head for easy turning. As the installer or maintenance technician turns the head, its opposite end traverses a 0.3750-in. dia. orbit. "It's like 'runout' in a bent drillbit, but very slo-mo and controlled," explains Wasnieski. "Here, runout is a good thing!" In fact, it is all that is needed to articulate the end of the bellows and adjust the potentiometers on the other side of the seal.

The finger assembly is soldered to the rear of the instrument to keep the hermetic seal intact. The conical tip is EB welded to one end of a conventional bellows, and an annular flange EB welded to the other end. The 0.125-in. hole in the center of the flange accommodates the offset shaft. The bellows trait measures 0.75-in. long by 0.25-in. dia., with 0.0007-in. walls. To ensure an airtight assembly, Servometer checks each one on a Helium Mass Spectrometer leak tester, rejecting any leakage beyond [10.sup.-9] std cc/sec.

Various materials, including copper, were considered for the bellows, but both companies returned to nickel. "Electrodeposited nickel is ideal here for its ductility, high strength-to-weight, corrosion resistance and fabricability," says Paul Hazlitt, chief engineer at Servometer. Nickel's higher elastic limit meant that the device would retain its original shape better than other materials, despite repeated deformations. The nearest competing material weighed 20% more than the nickel, which is significant in aerospace applications.

The finger solution also reduced part count. "We basically have a one-piece assembly--bellows and built-in offset screw--that engages the potentiometers directly, with no other linkages necessary," maintains Wasniewski. Hazlitt adds that this application is rather unique for bellows, "We've seen bellows used for aneroid functions, pressure compensation, seals, flexible couplings and self-locating electronic connectors, but this is one of the few as a mechanical manipulator."


Circle 261--Transicoil Inc, or connect directly to their website via the Online Reader Service Program at

Circle 262--Servometer, NDES Booth 3330, or connect directly to their website at
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Title Annotation:Revision X
Geographic Code:1USA
Date:Feb 1, 2004
Previous Article:Sealing & trimming materials.
Next Article:Precision machining.

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