Carbon Monoxide Around the House: Comparing Ambient CO Monitors

July 01, 2011
July/August 2011
A version of this article appears in the July/August 2011 issue of Home Energy Magazine.
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Carbon monoxide (CO) is really nasty stuff. It’s odorless, colorless, and neutrally buoyant, and it can make you stupid before it kills you. It’s not the kind of stuff you want floating around in your home. That’s why BPI, RESNET, and other organizations require thorough testing and monitoring of CO.

Paul Raymer is chief investigator of Heyoka Solutions, a company he cofounded in 2006. He has been wandering through the mysteries of building science since 1977. He has multiple BPI certifications and is a HERS Rater. (April Wise Photos)

In this article, I am going to tackle personal CO detectors and save the larger units with probes that are used for measuring the undiluted CO in combustion appliances for a future review. These personal CO detectors come in a wide variety of styles—from units that clip on your belt or hard hat to watch-like units that wrap around your wrist. You should carry one of these dedicated units with you to continuously monitor ambient CO throughout a house test. Adding this to a combustion analyzer for undiluted CO testing means that you will constantly be monitoring the air around you, protecting yourself for the price of a couple of good dinners out, about $200.


CO detectors use an electrochemical sensor that has a standard life of three to five years. (Household CO detectors should be replaced every three to five years, because their sensors fade as well.) These sensors are like batteries and gradually become less sensitive. The sensors on some of these devices can be replaced, but a new sensor typically costs around $175, which is almost the cost of a new unit. Therefore, I suggest that you consider these as personal safety devices that need to be replaced every two to three years. Personal safety is not a bad investment.

Manufacturers recommend regular calibration of these devices; some recommend calibration as frequently as once a month. That is a bit extreme for the less hostile environments that building analysts find themselves in, but it shows how serious it is to get the readings right. Many of these devices have been designed for people who come in contact with elevated levels of nasty gases regularly—such as firefighters and people working in steel mills. We shouldn’t be finding a lot of CO in homes, but the fact is that we do. Some of the manufacturers recommend some sort of “bump” test on a daily basis, a simple indication of whether or not the detector is functioning. Some of them have kits to attach to the detector with a bottle of CO. Some of them suggest performing the daily test with smoke from a blown-out match.

To test the alarms on these devices, I took a page from John Tooley’s book of wisdom.(John discovered this while grinding coffee in his kitchen.) I ground up some coffee beans and put them in a little cloth bag. I put the bag in a larger zippered plastic bag, along with the detectors that I was testing. I was surprised when the readings leapt up to levels ranging from 7 to 53 ppm. After letting the bag sit for about four hours, I put the detectors back in, and the readings soared to levels ranging from 113 to 336 ppm! It is possible that the gas in the bag was not actually carbon monoxide. Although the sensors in these detectors are tuned to CO, they respond to other gases as well. Perhaps it was just an unusual bump test, but no matter what it was, if you find those levels in a house, get out of that house as quickly as possible.

Table 1. Side-By-Side Comparison of CO Detectors

For the remainder of the article I am going to describe six CO detectors that are readily available on the market. See Table 1 for a side-by-side comparison of these six detectors.

Sensit P100. The Sensit P100 is a two-year (or so) product that you throw away at the end of that time. Since the sensors in these devices have about a three-year life (on average), replacing the detector after two years is not necessarily a bad thing. It does have an on-off switch, which maintains its battery life.

The Sensit P100 CO has been fabricated for industrial use, with a bright-yellow, rubberized case and a no-nonsense clip that attaches to whatever part of your clothing you want to attach it to. When you turn the unit on, the screen goes through a series of displays, including the temperature in °C, the months of operating life remaining, and the low alarm (preset at 9 ppm) and the high alarm (preset at 35 ppm). And the alarms are serious—the light blinks, the unit chirps, and the housing vibrates—making it difficult to ignore. Since CO is a cumulative poison that builds up in the body over time, the Sensit P100 also tracks a time-weighted average (TWA) over eight hours and will alarm when the TWA is greater than 50 ppm. It has a 100-event memory that can be downloaded to a computer with an optional adaptor download kit.

Manual fresh-air zeroing can be done at any time by pressing the zero button for two to three seconds. This resets the zero level to its present reading, so if the reading outside the building is 10 ppm, pressing the zero button will reset so that you can enter the house at zero level. If you hold the zero button down too long, the P100 goes into its calibration mode. Holding the on-off switch for six seconds turns the detector off. It will not shut off until you tell it to. Since an energy survey of a home requires continuous monitoring of the level of ambient CO, detectors that switch off automatically can be frustrating and potentially dangerous.

Testo 317-3. The Testo 317-3 (known as the CO Stick) is 6 inches long and about 1½ inches wide. The kit comes with a case that includes a belt clip, batteries, a wrist lanyard, an earphone, and instructions.

Operation is simple. There are three buttons on the face of the unit: on-off, max/hold, and the alarm on-off. Pushing the on-off button turns the unit on and starts the display segment test. Within five seconds, the 317-3 is ready to display the CO level. To turn the unit off, you simply push the on-off button a second time. There is no automatic off, so it will continue to operate until you tell it not to, or until the batteries die, which is about 150 hours (or six days) with the alarm switched off.

One of the interesting things about the Testo 317-3 is that it doesn’t require zeroing, and a self-test mode “verifies CO sensor function anytime, anywhere without the need for test gas,” according to the manufacturer. It is good practice with all of these detectors to zero the unit in ambient, or clean, air. For example, if the outside air is at 10 ppm due to street pollution, and you go into a home and start measuring, you won’t know if something inside the home is producing the CO at 10 ppm or less. The 317-3 doesn’t require a purge or countdown procedure. The sensor is temperature compensated as well, which keeps it from being affected by changes in temperature that could cause it to pick up on false CO values as it heats or cools.

Of the units tested, the 317-3 was the most sensitive and had the fastest response time. It doesn’t include a backlit display, which may make it difficult to read in a dark crawl space or basement, but the alarm is audible and the unit comes with an earphone, which makes it possible to hear it in noisy places. The alarm can be switched off, which might be helpful if the homeowner is close by and you don’t want to scare him or her.

UEI CO71A. The UEI CO71A has three buttons and a display on its face. Pressing the on-off button or bar activates the detector and starts a countdown to an initial zero level. A second quick press of the on-off button activates the backlight. Pressing the max button activates the maximum reading that the unit sensed the last time it was activated. Pressing the hold button freezes the display. The sensor grille is at the bottom of the unit. The CO71A has an audible alarm, along with a blinking LED on the top of the unit that changes color as different levels of CO are reached—green for 2–9 ppm, amber for 10–34 ppm, and red for 35 ppm or higher.

To turn the unit off, you simply push the on-off button. It does not shut itself off, according to the instructions, to “avoid interruption during a test.” The 9V battery is easily replaced, and the manufacturer states that sensor life is five years, although the warranty is for three years. UEI suggests recalibrating the CO71A every 12 months at a certified service location. The unit is sold with a case that has a belt clip on the side. This prevents your belt or clothing from covering the sensor when the CO71A is clipped on a belt. The display screen, controls, and indicator LED are easily seen when the unit is in its case.

Extech CO10. The Extech CO10 comes in a rugged, molded orange housing accompanied by a black fabric case with a belt loop. This case has no openings for the display, or the control buttons, or the sensor. There are three buttons on the face of the unit: on-off, mode, and sel. Pressing the on-off button starts the self-test mode, and the detector counts down for the ten-second start-up time. The mode button cycles through a maximum CO level display, a data recall display for ten logged data points, the data store mode, the alarm set point mode, a display of the time the unit has been operating, and the audible alarm on-off selector. Pressing the sel button in various modes recalls data, stores data, or turns the alarm sound on or off. A four-second press of the sel button in the normal reading mode will turn on the display backlight. A quick press of the on-off button will turn the unit off; or it will automatically power off in 15 minutes.

Although the maximum reading is lost when the CO10 is shut off, the levels recorded in the ten log locations remain. This makes possible a survey of key points in a house with an electronic means of recording the readings (as long as you remember where they were recorded).

The alarm is fixed at 35 ppm. Any level above that will gradually increase the speed of the beeper. If the reading reaches 200 ppm, the alarm will sound continuously.

Amprobe CM100A. The Amprobe CM100A is slightly bigger than the Testo 317-3 but is approximately the same shape. It has two control buttons: on-off and backlight. The display has two lines of information that generally display the current CO level along with the maximum reading. You can adjust the alarm to 25, 30, 35, 45, 50, 70, 100, or 200 ppm (although it is not clear in the manual exactly which button you need to depress to accomplish this). The manual says that the unit can self-calibrate if the detector is located in an “area free of CO” and placed in the calibration mode. It doesn’t say anything about sending it back to the manufacturer for calibration or doing a bump test regularly. The unit does shut off automatically after 15 minutes.

The CM100A comes with a black fabric case on a belt loop. There are no openings in this case to provide a means of reading the display, or to allow the sensor to be in contact with the ambient air.

Bacharach Monoxor III. The Monoxor III is really in a different class, and I have included it here only because it is often used both for ambient and for undiluted CO testing. One drawback to that approach is that the unit cannot be used for both purposes at once. If the probe is in the throat of the water heater measuring the undiluted CO, it can’t be measuring the ambient CO in the room. Nor is the Monoxor III well suited to walking around the house taking ambient CO readings. The pump is running all the time, which is a bit noisy; it is somewhat bulky; and it is particularly awkward if the hose and probe are attached to the body of the unit. The unit has no alarm function, so you have to be looking at it to monitor CO levels in the house.

However, in order to conduct a complete building analysis, you need a means of measuring the undiluted CO and the ambient CO. Although it is a compromise, the Monoxor III is one of the least expensive ways of measuring both. Since this review is about dedicated ambient CO detectors, I’ll save my more-detailed review of the Monoxor III for my next column, which will be dedicated to combustion analyzers.

Conclusion. At first blush, $200 or so seems like a bunch of money to spend on something that’s only going to last a couple of years. Carbon monoxide isn’t something to take lightly, however. And how long does a cell phone or a computer last these days? At least with one of these detectors you are protecting your health, and your clients’ health too, and you’re accepting the fact up front that the device will need to be replaced. The investment is worth it.

Paul Raymer is chief investigator of Heyoka Solutions (, a company he cofounded in 2006. He has been wandering through the mysteries of building science since 1977. He has multiple BPI certifications and is a HERS Rater.

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