Cockpit CO Detectors

SafeTest 90 is the one to beat but AIMs 935 does nearly as much for a fraction of the cost.

Remember the great radon gas scare of the 1980s? No one had ever heard of the stuff and seemingly, overnight, you could buy a radon gas testing kit at the corner 7-11, along with a six-pack and a gallon of milk. Just as quickly, the panic over radon faded.

Worries about carbon monoxide (CO) have followed a similar path. Not to suggest that the household CO threat should be understated. Carbon monoxide poisoning is said to be the leading cause of accidental poisoning in the U.S., with some 1500 deaths annually. Its deadly serious.

The inevitable aviation connection is that aircraft heaters rely on scavenged exhaust heat and thus CO can get into the cockpit. As noted in the sidebar on page 14, CO-related accidents and incidents are neither common nor rare. The hazard is real if not great and therein lies a market.

There are at least a half dozen CO detectors suitable if not designed for aviation use, with more on the way. Ranging in price from under $5 to nearly $500, theres protection at every price level. But if you invest in a CO detector of any kind, youre signing up for the long haul: The inexpensive spot detectors require periodic replacement and the pricier electronic models need to be calibrated, tested and/or replaced periodically and kept in fresh batteries.

What Standards?
The FAA has no useful standards for aircraft carbon monoxide detection systems, whether portable or installed. But thats not to say the agency has no standards at all. Theres actually an ancient (1957) TSO C48 describing minimum operational standards for aircraft carbon monoxide detection systems, when theyre required. Obviously, CO detection is not required in small aircraft operated under Part 91. That means youre on your own in selecting and installing a detector. (Only one of the models currently on the market requires installation more involved than merely placing the detector in the cabin or sticking a spot to the panel.) As for allowable levels of carbon monoxide, for airworthiness requirements under FAR Part 23, aircraft cabins must be suitably ventilated and carbon monoxide concentrations cant exceed one part per 20,000 parts of air or 50 parts per million. However, these are certification requirements only. Even though airframes are supposed to remain airworthy to the original standards, we doubt if any subsequent CO intrusion tests are done in the field.

The Underwriters Lab standard, which was recently revised upward, requires CO alarms to signal well ahead of the onset of symptoms for the average person. (That in itself may be a moving target, since response to CO varies by individual.)

Specifically, UL requires an 85-decibel alarm signal to sound within 189 minutes if 70 parts per million CO is detected. If the level is 150 PPM, the alarm limit is 50 minutes and its 15 minutes at 400 PPM. The old requirement was 90 minutes for 100 PPM and 35 minutes for 200 PPM. Exposures higher than the new standards are considered cause to seek immediate fresh air.

With these values in mind and with no other relevant aviation standards in place, buyers are left to set their own criteria. In our view, a passive detector ought to at least noticeably discolor at the lower level exposure levels and active designs-which require periodic calibration or replacement-ought to alarm at the lower levels, self-test and annunciate any failures.

Last, form factor: A detector ought to be unobtrusive, easy to install and use and require next to no maintenance, except for periodic replacement.

Two Types
CO detectors sort into two broad types, so-called biomimetric and electrochemical detectors. Biomimetric devices-also called gel cell or litmus-employ a gel that changes color when exposed to CO. The visible spot contains silica and a small amount of palladium chloride, which turns dark in the presence of carbon monoxide, although other chemicals may also be used.

The gel is designed to absorb CO at about the same rate as human hemoglobin, hence the term biomimetric. The more CO it absorbs, the darker it gets. It also recovers and returns to normal at about the same rate that the bloodstream returns to normal oxygen levels.

Some electronic detectors use gel technology by simply shining a light beam through the gel tablet toward a sensor, which measures the color change. Biomimetric designs have to be replaced periodically because the gel can become contaminated. (Its unclear how real this concern is, however.)

An electrochemical detector uses a logic chip to annunciate when it sees a resistance value or voltage across a CO-sensitive semiconductor. Some of the more sophisticated detectors even give the concentration in parts per million on a digital display.

In general, biomimetric sensors are less expensive and essentially make the $100 or cheaper-sometimes, much cheaper-detector practical. Electrochemical designs are more sensitive but cost three to 10 times as much.After obtaining a sample of most of the popular CO detectors marketed for aviation, we fabricated a plastic test chamber and exposed each detector to carbon monoxide from automotive exhaust and from a cylinder of pure CO. Our intent was to confirm that these devices detect CO, not compare them at confirmed calibrated levels.

Nonetheless, we felt that two of the detectors-the SafeTest 90 and AIM 935-were accurate enough to serve as benchmarks on actual CO concentrations, since both generally agreed on gas levels on all our tests.

Dead Spot
This passive detector has been on the market for at least a decade and is distributed by JL Sims Co. and sold by Sportys Pilot Shop. It consists of a biomimetric spot affixed to a 2-inch square plastic badge with an adhesive on the back. The fresh spot is about the color of an emery board but darkens to near black with CO exposure.

Having seen these in dozens of aircraft, nary a one darkened, we assumed they were nothing more than a feel-good gimmick, albeit a cheap one. Thats hardly the case, however. When exposed to CO in our test device, the Dead Stop turned noticeably darker after about five minutes of exposure at around 125 PPM, a low to mid-level of CO. At 500 and above, the patch darkens fast enough to watch, turning pitch black.

Thats not bad performance for a $4.95 product, albeit slow compared the electrochemical types. However, if youre worried about low-level CO-say less than 50 PPM-the Dead Stop wont be your pick. Its too slow to pick up low levels and the discoloration is too slight to notice. (One solution is to buy two detectors at a time and keep one in its clear wrapper as a color reference.) In short, you might notice symptoms before the Dead Spot blows the whistle. But at 100 PPM and above, it does the job.

Sportys catalog says the Dead Stop lasts for 30 days while the instructions on the back advise to replace it after 90 days. In our estimation, the longer replacement cycle is reasonable but well be conducting some long-term tests to see how the spot wears. The Dead Stop is shipped in a sealed plastic envelope and has a shelf life of three years. (Bought in lots of five or more, the price is $3.95.) Once overexposed, it wont recover its sensitivity.

Quantum Eye
Next up the price chain, at $9.95, is the Quantum Eye from the Quantum Group, another passive patch. The Quantum Eye is billed as a multi-level dectector in that it uses three colors to signal CO level: Yellow for normal, dark green for caution and dark blue/black for danger.

According to the product data sheet, it will respond to 100 PPM of CO in 45 minutes, 200 PPM in 19 minutes and 400 PPM in 9 minutes. (See the sidebar on page 11 for what those levels mean.)

Exposing the Quantum eye to our test CO showed that it seems to measure up to the claims but the response is slower than the Dead Stop. We gave it a good blast-measured 600 PPM on the SafeTest 90-and watched the Dead Stop blacken in seconds. Over the course of 10 minutes, the Quantum Eye slowly turned greenish, or at least darker. At levels below 100 PPM, we couldnt see much color change in an hour or so and even at that, the color change is hard to interpret.

Conclusion: For an extra five bucks, we didnt think it outperformed the Dead Stop and, in fact, was slower to respond to low levels of CO. The Quantum Eyes usable life is claimed to be 18 months.

Also from Quantum is the COStar, a product recently introduced for aviation and general vehicle use. Its a biomimetric design powered by an internal 9-volt battery. The COStar is about the size of a pack of cigarettes and retails for $60 from Sportys. (Buy direct from Quantum for $39.95) According to the product data sheet, the COStars lowest threshold is 70 PPM, which it will sense for up to 240 minutes before alarming. At 150 PPM, the alarm limit is 50 minutes and at 400 PPM, its 15 minutes. These are maximum times; it actually seems to perform better than that.

But like the patch-type sensors, the COStars shortcoming is response time. When we dosed it with 1500 PPM plus, the SafeTest 90 squalled in seconds while the COStar took nearly two minutes to respond. The alarm is an urgent beeping about as loud as a pager. When taken to fresh air, the SafeTest stopped alarming immediately, while the COStar kept at it for 10 minutes. Holding the test button silences it for four minutes. The test button flashes every 30 seconds to verify power-up and to self-test the sensor, which it does at 10-minute intervals. (In our unit, it was closer to a minute. Its easier to hold the test button to confirm operation.)

On the one hand, if you were pumping enough exhaust into the cabin to reach 1500 PPM, the COStar would alarm soon enough to save the day. At lower levels, however, you could be exposed for quite some time with no alarming. With the additive effects of altitude, it seems possible that you might notice symptoms before that. The Dead Stop and Quantum Eye will begin to darken at these levels. Conclusion: We view this device as little more than an automated patch-type design that will get your attention with an alarm. We like its small size and the price is right. However, if you want rapid low-level detection, this unit comes up short, in our view.

AIM 696/935
AIM Safe-Air, a Canadian company, makes two residential-type detectors, the 696 and 935, both of which are electrochemical sensor designs. At $44.95 retail, the 696 is intended strictly for home use but can be pressed into cockpit service.

The $74.95 935 is a low-level detector suitable for residential use but also intended for vehicle and aircraft applications. The 696 meets UL requirements but also the more stringent American Gas Associations 6-96 standard. The higher-priced 935 is not presently certified under UL or the American Gas standard but this appears to be a technicality, thus its called a monitor not a detector.

Both are sealed units with non-replaceable batteries; once you activate the unit with a twist seal, its on duty. AIM recommends replacing the entire detector every three to four years. (A built-in timer flashes an error message to remind you.)

Both AIM units are smallish triangular plastic devices, 5 inches on a side and an 1 1/4 inch thick, making them small enough to sit on the cockpit floor unobtrusively. They have a piercing pager-type signal readily audible, even with headsets, plus a flashing red warning light. Both also have self-diagnostic and test features that flag a hard failed unit, a unique feature.

The lower-priced 696 senses CO levels as low as 1 PPM but only displays values of 30 PPM and higher. One of two alarms is activated if the calculated CO saturation exceeds 6.5 percent or if the instantaneous CO reading exceeds 550 PPM.

The 935 uses the same hardware but has more sophisticated software. It annunciates LO for CO concentrations below 5 PPM and HI for CO concentrations above 150 PPM. It also reads the direct value between 6 and 150 PPM. Unlike the 696, the 935 shows discrete values between 5 and 30 PPM. In our trials, we were impressed with how the AIM detectors performed. With the slightest whiff of CO, both detected and displayed the concentration. We did note, however, that compared to the SafeTest 90, the AIM units require longer to display the max CO concentration. When they do, however, they agree closely with each other and with the SafeTest.

In flying with the 935 in a Cardinal RG, we smelled exhaust at gear extension and immediately noted a 23 PPM reading on the display. Quite impressive. On the other hand, weve heard several user reports of these units failing and, indeed, our 696 bit the dust during testing.

SafeTest 90
In the gas and mining industries, CO detectors are nothing new, although theyre more sophisticated and expensive than the household variety. The SafeTest 90 from Quest Technologies springs from this market.

At $295 retail from Sportys, it occupies the upper price strata but is the most sophisticated detector of this group, with one exception: Its self-diagnostics arent as good as the AIM detectors. Using electrochemical sensing, it samples once per second, self-tests every 30 seconds with a flash and reads values from 1 to 1500 PPM. The SafeTest 90 is housed in a nicely made silver plastic case measuring 1.3 X 3.0 X 3.9 inches. (The silver coating is to provide protection against extraneous radiation.)

With a 9-volt alkaline battery, 5000 hours of continuous use is claimed. Using a lithium battery doubles that. Quest says the sensor has to be replaced every two years at a cost of $185, a task the user can perform.

This device has the capability to measure time weighted averages (TWA) and short-term exposure limits (STEL) and digitally displays CO concentration in its LCD window. It emits a medium-loud chirping signal and flashes warning lights when levels are exceeded. Specifically, its high alarm sounds at 200 PPM, the time weighted average alarms at 35 PPM after an hour and the STEL limit is 100 PPM. In short, this is a serious, industrial-class detector youd expect to see clipped to the belt of a mine safety inspector.

Operation is simple; merely switch it on and it performs a full self-diagnostic test, complete with chirps. Alarm levels can be customized and/or suppressed by entering a set-up mode. As noted, the SafeTest senses much more rapidly and its scale extends from 0 to 1500 PPM, so it has greater resolution and range than the other products. Its alarm-a piercing warble-would wake the dead. Because it lacks the self-diagnostics of the AIM units, the company recommends bump testing by occasional exposure to CO. (Car exhaust will do as will taxiing with the door open. Its that sensitive.)

New to the market is the KOMeter from Simulation Systems and Applications, the only one of these devices intended strictly for aircraft use and the only one that can be hardwired for operation between 9 and 28 volts. At $497, its also the most expensive. The KOMeter is housed in a rugged octagonal plastic case 2 1/2 inches across by 1 inch deep, with a pair of wires for the power source. It has a three-year warranty and a five-year design life, according to the company. Its design was guided by TSO C48 but its neither FAA certified nor approved and in fact exceeds the CO sensitivity requirements of the TSO.

The KOMeter is supposed to be installed by an A&P and should be in sight of the pilot, since it has a digital display with clock and an aural warning. The company recommends using Velcro to attach the device and wiring is simple-a power and ground wire. But it does require a Form 337 and log entry. A sticky point: The instrument should be removed or covered in plastic if aerosol sprays are used near the cockpit.

According to the specifications in the installation manual, the KOMeter alarms at 75 PPM in 10 minutes, in five minutes at 150 PPM and instantly above 400 PPM of CO.

When it alarms, the KOMeter flashes VENT, displays the CO level and emits an aural alarm, whose volume we found anemic at best. We doubt if youd hear it through a headset but you probably would see the VENT warning.

Ideally, the alarm should wire through the audio panel but theres no provision for that. (A follow-on panel-mount KOMeter will have this feature but no production schedule was available at press time.)

The KOMeters stated specs are somewhat misleading, in that it seems to detect and display lower concentrations without flashing the vent warning or the alarm. If you were scanning it, youd see the lower CO value, but sans alarm. The KOMeter doesnt react quite as fast as either the AIM units or SafeTest, but eventually catches up with a comparable CO reading. In its favor, the KOMeter serves as both digital clock and CO monitor, displaying UTC alternating with CO to indicate on-duty status.

Its possible to go a little nuts with this CO detector business. If you decide you need one-not necessarily a given-you have to determine what CO concentration represents a hazard. Aeromedix.coms Brent Blue, an AME, believes the threshold should be 10 PPM, at which judgment impairment may arise, complicated by effects of altitude. Only two of the detectors tested will rapidly measure concentrations that low, the SafeTest 90 and the AIM 935. The KOMeter senses that low, but not as quickly.

At $295 plus $185 every two years for a new sensor, the SafeTest 90 is a first-rate instrument but pricey, in our view. It also must be tested occasionally and the self-diagnostics arent as good as the AIM units. It does appear to be more durable.

Closer to the surface of planet earth, the AIM 935, at $74.95, is a better value, since it does almost as much for a quarter the price. If you dont buy the argument that low-level CO concentrations need to be monitored, the AIM 696, at $44.95, strikes us a resonable compromise. It detects above 35 PPM and would thus confirm CO as the problem if symptoms arise in the cockpit at concentrations above that. On the downside, we have concerns about durability of both AIM detectors. We know of users who have had one or more failures and our 696 failed in just a few days. Aeromedix and AIM have stood by these under warranty.

Valuewise, we see the other active detectors as also rans. The passive patch detectors worked better than we expected, even if they dont provide significant low-level protection but youll get a lung full of CO before they show signs of trouble. Okay if you think you can hack CO poisoning symptoms without losing awareness. But some pilots, rightfully, wont be comfortable with that.

The COStar and KOMeter were less impressive. Although we like its small size, the COStar costs more than the AIM 696 but doesnt perform as well. At $497, the KOMeters performance is fine but not quite the equal of the SafeTest 90, which costs less. We see fussing with a Form 337 as perhaps more hassle than its worth. But if you want a CO detector powered by ships power, this is the only choice for now.

Also With This Article
Click here to view the Checklist.
Click here to view “Exposure Standards? Take Your Pick.”
Click here to view “CO Accidents and Incidents: Neither Rare Nor Common.”
Click here to view the Addresses.