PUTTING SENSORS   TO WORK

Navy Pilots Catch Their Breath with a
New Oxygen Regulator

Fighter pilots routinely push both the aircraft’s performance envelope and their own physical limits. The CRU-103 oxygen regulator, with its low-spring-rate electrodeposited nickel bellows, helps give them an extra edge.

Paul Hazlitt, Servometer Corp.

Lt. Carol Watts (left) describes for Lt. Lyndsi Bates her nighttime strike against Iraq on 17 December 1998, after returning to the aircraft carrier USS Enterprise (CBN 65) during Operation Desert Fox. Watts is an F/A-18C Hornet pilot from Strike Fighter Squadron 37, Naval Air Station Cecil Field, FL. Her oxygen regulator can be seen beneath her mask. (DoD photo by Petty Officer 3rd Class Tedrick E. Fryman III, U.S. Navy.)

Maneuvering for position in air-to-air combat, fighter pilots who are able to pull more g-loads (the force of gravitational acceleration) without losing consciousness have a powerful advantage. Engineers at Carleton Technologies Inc. have designed an aircrew oxygen regulator that automatically applies positive pressure for breathing under high g forces. With fewer parts than previous devices that perform similar functions, the regulator costs about one-fourth as much as its precursors and has proven reliable in service. At the heart of the design is a low-spring-rate electrodeposited nickel bellows from Servometer.

Photo 1. Responding to aircraft maneuvers, the CRU-103 oxygen regulator sends oxygen under pressure to the wearer's mask and upper-body g-vest. At the heart of the regulator is a low-spring-rate electrodeposited nickel bellows.

The CRU-103 chest-mounted regulator (see Photo 1) is worn on the parachute harnesses of U.S. Navy pilots and Naval Flight Officers flying tactical jets. The 13 oz. device routes breathing oxygen under pressure to the wearer’s mask and to an upper-body g-vest in response to aircraft maneuvers. The more g’s the pilot pulls, the greater the oxygen pressure applied to mask and vest.

Between the g-sensing air supply valve on the aircraft and the pressure valve in the regulator are highly flexible metal bellows (see Photo 2). As the bellows fill with air and expand, they close the regulator valve and increase the pressure of oxygen going to the pilot. According to Carleton design engineer Jim Talty, the thin-walled electrodeposited nickel bellows provided exceptionally low

Photo 2. This is a representative sample of the various types of electrodeposited nickel bellows made by Servometer. Some have been gold plated, as indicated by the color.

spring rates for a quick, consistent response. This reliable mechanical solution is lighter and smaller than the alternatives.

Yank and Bank
Banking and turning an aircraft at high speed increases g-loads on pilots to several times the normal force of gravity in the vertical axis. Today’s agile fighters can very rapidly attain nine or more g’s. As blood drains from the head to the lower body, cerebral perfusion pressure in the brain falls and nerve cells, denied oxygen, shut down. The pilot’s vision turns gray and then black. With sustained or rapidly increased g-force, g-loss of consciousness (G-LOC) can totally disable the pilot for up to 40 s. Full recovery from G-LOC may take as much as 3 min. In a high-performance aircraft maneuvering near the ground or in a sky full of enemies, even momentary loss of consciousness can be fatal.

Positive-pressure breathing to compensate for g-forces delays the onset of gray-out and G-LOC, and makes the anti-g training exercises commonly performed by maneuvering pilots less tiring. Increasing the partial pressure of oxygen in the blood compensates for the reduced blood pressure in the brain. Boosting intrathoracic pressure through breathing oxygen and a counter-pressure vest can hike pilots’ g-tolerance by ~2 g’s. The positive pressure, however, must be matched to the increasing g-load by an automatic regulator with a quick response.

Until recently, Navy pilots used three different oxygen regulators with their life support equipment. The least expensive cost only $300, but it varied breathing oxygen pressure only in response to altitude, not g-force. It could not provide enough oxygen to meet peak demand, and would cause breathing discomfort in pilots when they needed oxygen most. Despite its simplicity, the aneroid (containing no liquid) regulator also proved unreliable and costly to maintain. Two more sophisticated regulators could adjust their output pressures to both altitude and g-forces, but they cost around $4000 each.

The Navy sponsored Carleton’s development of the CRU-103 regulator as part of an Advanced Tactical Life Support System. To meet Navy requirements, engineers in Carleton’s Life Support Group looked for a reliable pressure-translating device that could reduce parts count.

Responsive Regulator
After evaluating pistons and springs, Carleton partnered with Servometer to create a solution based on a precision metal bellows. Servometer engineers helped analyze the regulator requirements, including travel distance with pressure change. Most of Carleton’s products used hydroformed brass bellows. Servometer offered custom-designed electrodeposited nickel bellows with far lower spring rates to minimize resistance. The force required to extend and compress the bellows convolutions, the bellows spring rate, is a function of inside diameter, outside diameter, number of convolutions, material of construction, and wall thickness.

Electrodeposited nickel bellows are widely used as the sensing elements in pneumatic regulators, switches, gauges, actuators, and pressure compensators. Compared with brass and other bellows materials, nickel combines high yield strength (110,000 psi min.) and high tensile strength (125,000 psi min.). The electrodeposition process maintains high chemical purity, and retains the mechanical properties of the metal. The bellows in the CRU-103 oxygen regulator have a projected service life >100,000 cycles.

Electrodeposition routinely produces walls one-quarter the thickness made by mechanical hydroforming. Electrodeposited nickel bellows typically provide one-fifth to one-tenth the spring rate of hydroformed brass bellows of the same size. When the nickel bellows expand, the amount of force lost in stretching the bellows is very low. The force also stays consistent from one regulator to the next.

The bellows assembly used in Carleton’s oxygen regulators is a 0.153-in.-long cylinder with walls 0.0015 in. thick and an o.d. of 0.375 in. The convolution pitch of the bellows is 0.032 in. with a depth of 0.025 in. The assembled regulator measures 3 in. high by 4 in. wide by 2 in. deep.

In operation, the bellows are typically triggered at 3.5 g’s, releasing oxygen under pressure at 0.1 psi. Supply pressure increases gradually to 1.0 psi at 9 g’s. The regulator’s goal is to give pilots exactly the oxygen they need without waste. Pressure to the mask and the compression vest is kept the same to maintain equilibrium inside and outside the pilot’s chest. The valve-actuating end cap soldered onto the bellows makes it easy for life support technicians to adjust the regulator. A maintenance kit includes a replacement bellows, diaphragm, and O-ring.

Broader Acceptance
Initially, the Navy issued the CRU-103 regulator to the crews of only the highest performance fighters and attack aircraft. The lower cost of the regulator ($1100 vs. $4000 each) and its performance, however, have led to the CRU-103’s being used on all U.S. Navy aircraft. Fleet experience, in terms of reliability and reduced need for repairs, has been excellent.

For further reading on this and related topics, see these Sensors articles. "Enhancing Overpressure Tolerance with Bellows," February 2001 "Fundamentals of Pressure Sensor Technology," November 1998 "Choosing the Right Low-Pressure Sensor," September 1998

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