On Thu, 30 Oct 2008 23:43:59 -0500, Don Foreman
wrote:

On Thu, 30 Oct 2008 17:33:17 -0800, bart wrote:


All occupied spaces must be ventilated to some extent or the occupants
would eventually suffocate. HVAC systems handle this for normal
occupancy.

Fresh air has a natural CO2 content of about 385 ppm.
Due to the health risks associated with carbon dioxide exposure, the
U.S. Occupational Safety and Health Administration says that average
exposure for healthy adults during an eight-hour work day should not
exceed 5,000 ppm (0.5%).

0.5% - 385ppm = 0.462%. An amount of CO2 necessary to approach this
concentration could be released in a space once per air change. I
think OSHA regs call for a minimum of 4 air changes per hour but there
are various conditions that require more. Ask an HVAC expert about
that or do a bit of web research on the subject.

If you release 125 cubic feet of CO2 in 15 minutes (about one 5' tall
cylinder, 4 air changes per hour), the room/space would need to
contain about 27,000 cu ft of air to achieve the necessary dilution.
With 10 ft ceilings that would be an area of 2700 sq ft.

Scale accordingly for use of less CO2. I strongly doubt that your box
will use CO2 at anywhere near this rate of 500 cu ft/hr. That'd be
about 61 lb of liquid or dry ice.

I recall using boxes like this 40 years ago. Some used liquid CO2,
others dry ice. They weren't very big; a few cubic feet of capacity
inside. They worked surprisingly well, holding set temp to within a
couple of degrees. No special ventilation measures were used. A 10
lb block of dry ice lasted most of an 8 hour shift.


Further to your comments, 40 years ago the rooms may have been far more
"leaky" than they tend to be nowadays.

5% CO2 will cause unconsciousness in some individuals and 10% will eventually
cause unconsciousness and death in all individuals.

I speak as one who lost consciousness due to an environment that was later
measured with a Dreager tester to have 5% CO2. Since that environment happened
to be a very well sealed full-face crash helmet in a rain storm, I was
extremely lucky to get away with only a broken leg and a written off
motorcycle...


Mark Rand
RTFM