I have the "Model 300 Pocket CO" detector sold through Aircraft Spruce in my 170B. So I thought I would give you a product review. Here's a link to it:
http://www.aircraftspruce.com/catalog/p ... ckkey=6595 (Links to pdf copies of the manual and spec sheet for the unit are under the tabs on the Spruce page.) Here's a link to the manufacturer of the unit, who is located in California:
http://www.kwjengineering.com/
This is a long post, so I apologize in advance. But I thought others may be interested in the information.
When I bought my plane a little over a year ago, it had a card/spot CO detector that was so old and faded as to be useless. So I decided to replace it with something more substantial. After researching the options, I settled on the Model 300 for a few reasons. It was an actual detector rather than a passive card. It was portable and relatively inexpensive (compared to other industry portable detectors, and especially compared to panel-mounted aircraft units). And it had an extended useful life with a user-replaceable battery. (Many of the other and more expensive portable CO detectors are "one and done" units. The battery cannot be replaced and the entire detector must be discarded/replaced in 2 years.)
As seen in the picture on the Spruce web page, the Model 300 is small (OK tiny) and rather cheesy in appearance. It's 2 3/4"H x 1 1/2"W x 5/8"T, the size of a key fob, and is designed as a "personal" detector " to be "carried on key ring, clipped to a worker's shirt pocket, or attached to the dashboard of a vehicle or piece of equipment." It has a single oval button in the middle, a small LCD display (with backlight), a buzzer and a red LED alarm light. It's simple to operate -- just press the button and it will perform a brief self test (including battery check), show the current CO reading and go into "12 hour mode." I like the 12 hour mode, because the unit automatically shuts off after 12 hours if you forget to manually turn it off. (If you push the button twice, it goes into "continuous" mode and does not automatically shut off. Also, it does not record or report dosimetry information in continuous mode.)
In addition to giving a low battery and other warnings, the display shows any current CO exposure in PPM (range: 5 to 500 PPM in increments of 1 PPM). The display is updated every 5 seconds. The unit instantaneously alarms (82 dB buzzer at 24", red LED light, backlight and vibrator) at or above 50 PPM, 125 PPM and 400 PPM. In other words, if it detects exposure at those levels, even for an instant, the alarm goes off. The "pulses" of the alarm are different at each level. (The 50 PPM alarm is optional and may be shut off). Additionally, and I think just as important, the unit alarms if it detects the equivalent of 25 PPM exposure over an 8 hour period using a time-weighted average. In other words, if the detector calculates that your total level of exposure so far meets or exceeds the equivalent of an exposure of 25 PPM over 8 hrs. (even if your present exposure has been less than 8 hours), it sets off the alarm. (I think I explained that right). How it calculates this is explained in the manual. So according to the manual if the detector "has been on for less than 8 hours, it will also issue an hourly warning if your current TWA {time weighted average] puts you on track for exceeding this limit in 8 hours." (This is a pretty low threshold in my plane given that I cannot fly it anywhere near 8 hours. Also, since the standards for CO exposure are usually expressed in exposure levels over time (e.g., X PPM over X hours), I understand that this additional threshold alarm more realistically tracks exposure risk. But believe me, I'm no expert.)
As alluded to above, in the 12 hour mode, the unit keeps track of dosimetry information, which includes your total-exposure and time-weighted average exposure.
The unit uses a standard CR2450 coin battery, a feature I really like. It can easily be replaced by the user. (According to the Q&A tab on the Spruce web page, the manufacturer offers a $79 service which includes replacing the sensor if needed, calibration, one year additional warranty, and battery replacement.)
One downside, however, is the temperature range of the unit. It cannot be used below 32 deg. F, a potential problem for those of you in Alaska and other really cold areas unless you can keep the cabin above freezing. Another downside is that when you hang it from the keychain loop on the unit, it hangs upside down. Not a big problem for me, but just so you know.
This is just a rough summary of some of the features. Other features and details are described in the manual and tech sheet identified above.
All in all, even though the unit doesn't seem like much at first glance, I'm satisfied with it. It has a lot of features for a tiny unit. And its far better than a spot card. While it's not exactly cheap, its not nearly as expensive as other detectors with similar functionality. I have it hanging off my pilot-side door entry hand strap in my peripheral vision so I can see the red LED if it alarms. (An advantage of its size is that you can hang it pretty much anywhere.) It seems to work well, though I must admit that most times when I'm flying I have the ventilator at least partly open. I'm not necessarily endorsing it --just trying to accurately highlight its features.
I hope this helps.