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Mr. Duality
03-22-2009, 11:15 AM
I've had a carbon monoxide detector for eight years. It's the Nighthawk brand, which plugs into a wall outlet. It has never registered any number other than zero even though we have a woodstove (used on cold nights) and the wife is fond of scented candles- she sometimes burns several simultaneously.

We live in a town which has exceptionally clean air but I'd think the CO detector should read something other than zero sometimes.

Could my CO detector be defective?

Telperion
03-22-2009, 11:35 AM
Candles burn fairly cleanly, but engine exhaust should be a very good way to test it. But be sure you don't run it for too long inside the garage, just in case the detector is defective.

Q.E.D.
03-22-2009, 11:48 AM
Candles burn fairly cleanly, but engine exhaust should be a very good way to test it.

Not really, no. Modern cars emit very little CO. A better source would be burning charcoal.

zwede
03-22-2009, 12:07 PM
A car with modern emissions controls (catalytic converter) will read zero on CO. But you could stick the detector right at the tailpipe and do a cold engine start. There will be CO until the catalytic converter reaches light-off temp which will be a window of a few minutes.

Rick
03-22-2009, 12:37 PM
Make that less than a minute for a modern car. Cats light off real quick now a days.
Use a lawnmower, or a gas powered weed whacker, or light some charcoal. No cat, and lots of CO.

Una Persson
03-22-2009, 12:41 PM
Huh. Our Nighthawk detector recommended just lighting a match, blowing it out quickly, and blowing the smoke on the detector. This always generates a low to mid-level reading.

brazil84
03-22-2009, 01:25 PM
I've had a carbon monoxide detector for eight years.

As a side note, it's my understanding that Nighthawks are rated for 7 years of use. So you might want to buy a new one anyway, or at least check the manual.

Also, I would consider buying 2: One for near your sleeping area, and another for near the boiler or hot water heater.

HongKongFooey
03-22-2009, 01:27 PM
Huh. Our Nighthawk detector recommended just lighting a match, blowing it out quickly, and blowing the smoke on the detector. This always generates a low to mid-level reading.Bah. What fun is that? :D

Q.E.D.
03-22-2009, 01:57 PM
No cat, and lots of CO.
Oh, I dunno. I should think burning a cat would make lots of CO. Shave it first, though; burning fur stinks.

Mr. Duality
03-22-2009, 05:12 PM
Huh. Our Nighthawk detector recommended just lighting a match, blowing it out quickly, and blowing the smoke on the detector. This always generates a low to mid-level reading.

Tried that with the old detector which has a digital readout and peak level memory, using several matches one at a time. It never read anything but zero.

Went and got two new detectors, one with digital readout in addition to audible alarm and one with audible alarm only. Once again the match test resulted in zero and silence.

The literature accompanying the digital CO detector lists various CO levels and time at each level required for alarm response. For instance at 70 ppm the unit must alarm within 60-240 minutes, and at 400 ppm the unit must alarm within 4-15 minutes. I could find no time requirements for the audible-only alarm but that doesn't prove time is irrelevant, especially when the info was authored by someone in China.

I suppose I'll wait until we use the BBQ grill and bring some half-consumed charcoal indoors.

Shawn1767
03-22-2009, 05:23 PM
Speaking of CO detectors, where is the optimum place to have one? Low to the ground or high near the ceiling? Does CO rise or fall? Is it heavier than air or lighter? I mean, say you have some deadly gas that is heavier than air and your detector is higher than your bed. You'll be dead before the gas reaches the detector. So, which is it for CO detectors?

arseNal
03-22-2009, 06:04 PM
Well, this may or may not apply to yours, since you appear to be reading the manual. But I know the ones we use show 0 for any reading under 30 ppm.

Vlad/Igor
03-22-2009, 06:36 PM
Well, this may or may not apply to yours, since you appear to be reading the manual. But I know the ones we use show 0 for any reading under 30 ppm.Ooooh, good observation, and I should know this, since I deal with measurement systems. Any given measuring system will have a lower limit of detection and of quantitation, which will usually not be the same figure. Unless you get above the limit of quantitation, the device will not be able to separate the signal from random noise. IOW, you need a largish source of CO that will set off the device and not land you in the hospital. You also need to know the LOD and LOQ for that device.

Vlad/Igor

Una Persson
03-22-2009, 06:41 PM
Speaking of CO detectors, where is the optimum place to have one? Low to the ground or high near the ceiling? Does CO rise or fall? Is it heavier than air or lighter? I mean, say you have some deadly gas that is heavier than air and your detector is higher than your bed. You'll be dead before the gas reaches the detector. So, which is it for CO detectors?
CO has a molecular weight of 28, which is slightly higher than the mean molecular weight of air in general. So one would think that it should be slightly less dense, and therefore tend to drift for the roof. However, I've seen people get into knock-down drag-out fights over the "kinematic mixture viscosity"or something of CO, claiming it should either be slightly heavier, the same, or slightly less density in effect than air. Really, IMO I would say it's pretty much close to air and that as long as the meter is in the same room as you are, with standard mixing from HVAC and the motion of humans in it it shouldn't matter too much where you place it.

And then there's the idea of "should it be near your bed, or near the furnace/water heater"...

Chronos
03-22-2009, 07:17 PM
CO has a molecular weight of 28, which is slightly higher than the mean molecular weight of air in general.Erratum: Slightly lower than the mean molecular weight of air. Air has an average molecular weight of around 29 (28 for N2, 32 for O2). And of course, most things that produce CO are hot, so that would also lead to it being less dense.

Gases tend to mix pretty well, given time. But to the (very small) extent that CO would be separated out at all, it should be at the top of a room, since it's lighter.

Also note that all gases at a given pressure and density have the same number of particles per volume, so comparing density of gases really is as simple as comparing their molecular weights. This is not true for solids and liquids, where you have to worry about the spacing between the atoms, too.

Shawn1767
03-22-2009, 07:33 PM
Thanks for the answers to my question. However, I used to have a CO detector that plugged in to an outlet. Because most outlets are near the floor, then my detector would most likely be near the floor, right? That's lower than the surface of my bed where I'd be sleeping if there were a CO leak somewhere. But I guess what you're saying is that it'll mix with the air enough anyway to be detected right?

Una Persson
03-22-2009, 07:55 PM
Erratum: Slightly lower than the mean molecular weight of air.
You're exactly right, and not only do I know that, but I've answered this question (correctly) before. I really messed up in that post - I was thinking "higher in the room" and typed "higher." :smack:

Shawn1767, despite my mis-type in the post prior which Chronos corrected me on, I really do think that plugged into your outlet will probably be good enough, provided your room has adequate airflow.

Quartz
03-23-2009, 09:31 AM
With regards to placement, when I spoke to the gas man, he recommended putting the CO meter in the same room as my boiler but on the other side of the room, above the door.

Una Persson
03-23-2009, 10:22 AM
With regards to placement, when I spoke to the gas man, he recommended putting the CO meter in the same room as my boiler but on the other side of the room, above the door.
That's not a bad place to put a detector (there's some argument over whether the CO would be more likely to enter the flue or else leak through a leaky heat exchanger), but if your bedroom is too far away to hear it, then you should consider getting a couple of them.

beowulff
03-23-2009, 10:33 AM
Ooooh, good observation, and I should know this, since I deal with measurement systems. Any given measuring system will have a lower limit of detection and of quantitation, which will usually not be the same figure. Unless you get above the limit of quantitation, the device will not be able to separate the signal from random noise. IOW, you need a largish source of CO that will set off the device and not land you in the hospital. You also need to know the LOD and LOQ for that device.

Vlad/Igor

I was at a Rocky Horror party a few years ago. The homeowner fired up his motorcycle in the house (in tribute to Eddie), and it set the CO detector off in seconds...

Mr. Duality
03-23-2009, 12:06 PM
I took all three CO detectors outdoors and exposed them to exhaust from my '87 Dodge pickup. (The new ones have battery backup and I used an extension cord for the older one.) All three alarmed after a minute or two in the smoke. I should mention the ol' Dodge burns some oil until the engine warms up. I find it curious that the two with digital display would not display any numbers while alarming but I didn't want to play with them for a long time because it's cold & snowy outside today.

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