Closing the O2 Loop

A smart system that monitors how much oxygen you need then dispenses the exact amount. Clever but expensive.

When last we examined portable oxygen systems-see the August and November 2001 issues of Aviation Consumer-we concluded that what the world really wants isn’t available: a smart system that delivers the oxygen you really need, without any intervention by the user. Evidently, we werent the only ones thinking along these lines.

Controlled Oxygen Delivery Systems introduced a product called Aviation Oxygen Regulator Model SMS100 at Sun n Fun in April. Headed by Dr. Brent Blue, who first brought pulse oximetry to GA via his Web site, CODS has engineered an elegant and practical, if not inexpensive, solution to automatic oxygen delivery.

The SMS100 combines a pulse oximeter with a sophisticated regulating system to maintain your oxygen saturation (SO2) at normal levels with a real-time closed feedback loop. Via an integrated digital processor, the pulse oximeter measures your oxygenation level and then adjusts the amount of oxygen delivered, increasing or decreasing it, so as to maintain a set SO2.

The SMS100 is assembled into a somewhat bulky and not particularly lightweight plastic box (7 by 3.5 by 1.8 inches, 20 ounces with batteries) with pressure-sensitive tactile buttons for control on the face, along with a monochrome LCD display with adjustable contrast. The size is to a large degree dictated by the regulating valve, battery storage (four AAs) and other components attached to the circuit board. Still, smaller would be better, in our view.

At one end of the box are all the connections. The sensor probe for the pulse oximeter plug has a standard DB-9 connector. While our prototype test probes didnt have it, the production units will be equipped with a locking DB9 connector with integral screws that will prevent the probe from being inadvertently disconnected.

There’s a brass barbed oxygen supply connector and a pair of user ports, all designed for use with existing plastic tubing. Each person requires his own system; there are twin ports because its designed to be used with a dual lumen cannula, which has a pair of supply hoses instead of the single supply most of us in aviation are used to. CODS says AA-cell alkaline batteries will provide six hours of use and we feel thats likely a conservative number based on our testing. You can also use rechargeable batteries but we werent able to establish an operating life for them. A bayonet plug-in is provided for connection to the aircraft power supply (12 to 24 volts) via an included coiled/switched cigarette lighter adapter cord. It can also be hardwired.

Headset pass-through jacks will be added to the production version to easily allow for any function alarms to be heard in the headset. A variety of standard adapters are available to allow connection to built-in oxygen systems or portable systems with conventional regulators. Inlet pressure is limited to 20 to 50 PSI with a max flow rate of eight liters per minute (LPM).

The unit comes packed in a foam-padded abuse-resistant waterproof Pelican case.

How It Works
You initially set the SO2 target level that you wish to maintain. This value can vary from 87 percent to 100 percent and defaults to 97 percent. The instructions recommend using whatever your normal SO2 is at your home airport.

The units integral pulse oximeter can be used to determine this number. Once set, the SMS100 will default to that set target when powered up. The instructions warn that setting the level to a higher SO2 is not necessary, wastes oxygen and may be counterproductive.

The pulse oximeter is driven by the same chip that powers Nonins hospital-grade units. Both SO2 and pulse rate are provided to the user via the LCD display. There are a variety of probes available to interface between you and the SMS100, which we’ll get to shortly.

If the SO2 value drops from the set point, the internal valve is opened and oxygen is metered out every time you inhale, with the bolus supplied just at inhalation, similar to the way Mountain High Equipment & Supplys EDS unit operates.

A sensor registers the start of inhalation to time the operation of the valve. The big difference is that the EDS unit is pre-programmed to deliver a preset amount based on altitude and the average persons needs. As we saw in our review last year, this may not be adequate for everyone and the EDS must be manually adjusted to supply oxygen at a higher rate if needed.

With the SMS100, the amount of oxygen is adjusted by the chip depending only upon actual need. There are no arbitrary settings based on altitude. If the need is great, for example if you start the system when there’s already an oxygen deficit, the system supplies more oxygen, up to full flow, until the set point is reached.

It then throttles back to maintain the set SO2. As you climb in altitude, the chip automatically adjusts the amount delivered to accommodate the users personal increased need, determined by the pulse oximeter.

If you grasp the case, you can feel the valve opening and closing, metering the oxygen. Its easy to tell the difference between full flow and metered flow, just by whats being delivered to your nostrils. Upgraded software will reportedly allow the system to learn what it takes to bring a user back to the set point so there will be less tendency to oversupply when you start out low, thus further conserving oxygen.

The system is designed to be fail-safe; the valve is normally full open so a power failure results in full oxygen flow. If any errors are detected-the pulse oximeter probe ceasing to function, for instance-the unit also goes to full flow. The error messages are all accompanied by an aural warning which can be fed into your headset directly (once those plugs are in place), an error readout on the LCD display and a flashing red LED on the unit for the most serious failures.

We were unable to compare directly the duration of a system using the SMS100 and Mountain Highs EDS. In our experience, the EDS system at its standard setting did not adequately oxygenate all testers and we had to set it to higher flow rates to achieve a satisfactory SO2 value for many.

We expect that in most circumstances, the SMS100 will at least equal the duration performance of the EDS and may exceed it in some circumstances. In some instances, the SMS100 duration will be less than an unmonitored EDS system because the EDS system isn’t providing enough oxygen to achieve target oxygenation levels, according to our testing. In any case, it will be substantially better than conventional constant flow systems or those using oxygen-conserving cannulas.

For testing purposes and with a safety pilot flying, we took the system to 23,000 feet and we were able to maintain our set SO2 levels. The current system is not designed for use with an oxygen mask, so its only useable legally to 18,000 feet. CODS tells us they anticipate eventually introducing a model designed to work with a mask.

We could also foresee the possibility that theyll obtain approval of cannula use above 18,000 feet using this system because of the built-in SO2 monitoring. Of course, we wouldnt bet money on the FAA approving this but the system can clearly deliver enough oxygen with just a cannula. In the meantime, anyone flying over 18,000 feet with the SMS100 will have to switch to a continuous flow mask, fitted with an appropriate flow meter, to remain strictly legal.

Pulse oximeter probes all work in a similar fashion. An LED generates a beam of red and infrared light through the skin thats sensed by a pick-up module. By measuring the ratio of red and infrared light absorbed by the blood, the pulse oximeter calculates the percentage of hemoglobin thats carrying oxygen, arriving at the SO2 value.

The SMS100 comes with one sensor, of the users choice. Two take their readings from a fingertip. The simplest one is a plastic spring-loaded clip, lined with soft rubber, as is commonly used in hospitals for short-term readings. This is the type sensor used by the regular pulse oximeters we tested last year. Somewhat bulky, it can get a bit annoying over a period of time, but its easy to use. Just clip it on and youre in business.

The other fingertip sensor is smaller; a thin piece of rubber with a bump at each end containing the LED and pick-up. This is placed over the fingertip and held in place with a specially formed die-cut piece of medical grade foam tape.

This probe was less intrusive, by far. We could still operate the radio knobs and buttons with it in place. After a while, you almost (but not entirely) forget its there.

This smaller probe will fit under most gloves as well. It does require more effort and both hands or assistance to affix it, however. Its not the sort of thing youd want to do in the air solo without an autopilot. There’s also the ongoing cost of the disposable adhesive Flexwraps-a package of 25 is $18.95-to consider.

The throttle hand seemed the best choice for the finger sensor and is the least intrusive, in our estimation. Consideration to radio operation also needs thought with the finger sensors. Lead length could become an issue depending upon placement of the SMS100 and the oxygen supply/cylinder. The nice thing about using the DB-9 connectors is that an extension is easily found at computer stores.

With either sensor, we found that it was most comfortable to route the wire up our arm a ways. We tucked it under our wristwatch strap, but you could use other means, such as a Velcro strap or a clothing clip to secure it.

Another solution leaves you looking a bit like somebody out of a Sci-Fi thriller. The reflectance probe is attached to your forehead with adhesive. A disposable double-sided foam tape is used to secure the sensor to the forehead.

The most innocuous probe solution, dubbed EarSat by CODS, adds a reflectance sensor into the earcup pad of your headset. This rests against your cheek to obtain its readings.

This probe takes longer to stabilize and sharp head movement or even jaw movements, such as talking, can cause it to lose contact or read low momentarily. We tried this using both a Series II Bose and LightSpeed 20K headset with excellent results.

Surprisingly, even to developer Brent Blue, it even worked with our cropped beard. Unfortunately, when we tested it in conditions that had us sweating heavily, as might occur in heavy weather flying or in an emergency, it didnt work as we’ll with the beard. CODS is still working on the EarSat design at the time of this writing. The units we tested were custom made with the probe set into the foam earcup. Newer models affix the flat probe to the surface of the earcup with a die-cut adhesive, making installation easier, cleaner and portable as well.

On the down side, the EarSat adds yet another wire to the headset wire jungle. To help ameliorate this issue, CODS has secured the headset wires, the probe wire and the cannulas supply tubing inside some plastic spiral wrap. The spiral wrap does a good job of containing the mess in a reasonable fashion, although less frequent oxygen users will probably want to keep the bulky cannula supply separate and invest in a pack of cable ties.

We tried various arrangements to find the most comfortable; to the rear, side and front. There wasnt a clear winner. In all cases it seemed as if some sort of clothing clip to support the spiral wrapped wires and tubing, which are bulky, would make for greater comfort. We imagine each user will find the combination that works best.

One sensor is included. Puchased separately, the Earsat is $149, the flexwrap fingertip probe is $99. The EarSat probe is the way to go for most regular users. Its the least intrusive and that counts for a lot in our book.

The constant monitoring and maintenance of SO2 levels without need for user intervention represents a big gain in safety, in our opinion. The system seems we’ll thought out and it is easy to use. On the other hand, its bulky and if you have a portable GPS, it adds to the wiring mess inside the cockpit. Still, if you want this capability, its worth the nuisance factor, in our view. Its the ultimate in high-altitude safety.

But the question is, do you really want it? At $1195 per person for the SMS100, this isn’t for the sometime oxygen consumer who uses the occasional puff at night or over the mountains.

But for the serious flight level flyer who uses a lot of oxygen, this system is the logical next step, a true leap forward technically and, most important, in terms of safety. It closes the open loop in oxygen delivery, something that has raised safety concerns for a long time.

We expect that pilots interested in this level of safety monitoring will purchase one for their use and perhaps for a regular passenger or co-pilot, while relying upon less expensive traditional systems for occasional passengers.

Addresses: Controlled Oxygen Delivery Systems, 982 W. Broadway Jackson, WY 83001; 800-992-0017, 307-732-0040; Mountain High Equipment & Supply Co., 625 SE Salmon Ave. Redmond, OR 97756; 800-468-8185, 801-561-9970;

Also With This Article
Click here to view “CODS vs. EDS.”
Click here to view “Checklist.”

by Douglas S. Ritter

Doug Ritter is an Aviation Consumer contributing editor and editor and publisher of, a Web site devoted to survival equipment and issues.