Test Carbon Monoxide inside Vanagon. 1990 MultiVan with camper top up for 4 hours.
April 4, 2004  Time 7:15 pm
Temp 42 degs in 40 out  
Full Moon
Driver side sliding window open to first notch. Curtain drawn shut.
Heat--Colman platinum catalytic "sport-cat" with full tank.
Temp measured from center of van at lower roof latch with digital thermometer.
CO measuring with "home model Nighthawk" 120 volt detector placed on seating area. And professional type hand held type-- always powered on, set on dash.
The Nighthawk will detect and store highest reading.  

Did not open doors.
I was not inside van.
after 2 hours 55 degs in, 35 out
CO at 10-11 ppm

after 4 hours 55 degs in, 30 out
CO at 10 ppm

The Nighthawk registered 0 during checks because it is designed to ignore low and harmless readings.

The reading of 10-11 ppm is same that I have observed while camping for short times (1-2 hours)with same professional detector and is not harmful even if over 8 hours.
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note:
Upon checking highest stored reading of home model "Nighthawk" CO detector, found highest reading was erroneous at 351ppm.  That was result of having detector plugged into extension cord outside while I pulled up in van with cold engine and I did not clear memory. At that time the hand held detector was over 900ppm.
The 900 ppm is most likely from hydrocarbons and not actual CO.

Inside temp may have also been affected by the heat sink of the roof latch the temp probe was resting in.