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MG MGB Technical - Exhaust Gas Analyzer Question

This is a nitpicky question but one that has been bothering me about my gunson Gas Tester.

I have ben using it as per the instructions and so adjusting it or "zeroing it" to 2% CO before inserting it into the tailpipe.

But what has been buggin me is this:

Does my final reading equal the total on the display or the total - the initial 2%?

Say for instance I set it at 2%, then take my readings and the settle in at 5%.

Is my CO level 5% or 3%?

Thanks,
Erik.
erik

In your example, your reading is what is indicated ie 5%. I don't fully understand why the gadget has to be "zeroed" at 2%, but the reading displayed is correct. I have actually proved this by using my Gunsons immediately after having the car tested at an official testing station. It was surprisingly close to the pukka instrument. If you try the unit on a car with a catalyst, you can get readings well below 2%, so it obviously isn't a true zero.

Mike
Mike Howlett

I thought 2% was the control measurement found in regular air...my Gunsons works pretty well also....5% is kinda high BTW
Pete
P J KELLY

I remember the instructions explaining something about 2% being the ambient measurement for air. PJ Kelly is correct.
From that I would assume your final measurement is exactly as it reads, 5%, as you don't subtract the ambient 2% from regular air.
D O'Brien

Are we talking about CO or CO2? 2% CO is a high number for ambient, even for Australia! RIC
R E L Lloyd

Yes 2% for Carbon Dioxide would be about right, but would way over the top for Carbon monoxide. Typical permissible exposure limits (PEL) for carbon monoxide are around 50 parts per million (ppm) parts of air. 2% would be 2 parts per hundred and probably lethal for any length of time.


Matthew Crabb

Calibration at 2% must be for some other reason rather than "zeroing" at atmospheric pressure:

Components of atmospheric Air
(excluding water vapor)
Content percent by volume

N2 78.084
O2 20.946
CO2 0.033
Ar 0.934

Handbook of Chemistry and Physics
68th Edition

Edd Weninger

It does register CO, the Gunsons manual says that calibration to 2% in air is because it is coincidental that air measures the same as exhaust gas with 2% CO, which isn't to say that air contains 2% CO, just that the instrument measures it that way. 5% isn't that high, it is only 0.5% above the maximum allowed for 1980-ish UK cars. My Gunsons is about 2.5% pessimistic (i.e. it reads higher than the exhaust actually is) as confirmed by using it immediately before and after a testing station visit. I also find that the instrument gives wildly varying readings with just small changes in orientation, and high readings if placed on warm tarmac rather than being raised up in the air. All pretty crappy, but knowing the rules at least it allows me to ensure the V8 will pass emissions and not have the testing station fiddle with the carbs, which then require setting-up from scratch again afterwards.
Paul Hunt

This thread was discussed between 24/11/2003 and 25/11/2003

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