Jim wrote:
I need some help in trying to calculate the percentage of oxygen existing in a closed room. This is partly just for my interest, but there is a
practical side to my question also.
I live in a recently built house that is one of these "super-insulated" structures with little outside air exchange except what is forced via fans. During the winter months I live primarily in a couple of rooms closed off from the main house to save on heating bills.
I have equipment that very accurately measures the carbon dioxide level in real time, that is located in my main living area. I live at 3000 feet elevation, and the house is all electric; no oxygen consuming heating, not even a wood buring stove. I don't have any way of directly measuring o2 in the house.
The measured co2 in my living area ranges from approximately 500 ppm to over 2000 ppm. The higher concentrations generally occur during the winter
months when I am spending more of my time indoors, and am preserving the
heat by running the forced air intake fans less. (This will be changing in the future, as I will be forcing air from a solar heated porch into the
house during some winter daylight hours)
My question is, from knowing the co2 concentration (and humidity percent also) in a closed space, how can I calculate the oxygen concentration? I know that the o2 conentration of normal dry air is around 20.95 percent, but suspect that the relationship is not perfectly direct between the increase
in co2 and the decrease of 02.
Any help on this math problem would be appreciated.
Thanks
Jim
Hi Jim,
The relationship between CO2 buildup and oxygen depletion isn't one-to-one because human respiration affects these gases differently. When we exhale, we convert oxygen to carbon dioxide at roughly a 1:1 ratio in terms of molecular exchange, but the
absolute concentration changes differ significantly due to their different starting concentrations in air.
Here's how you can estimate the oxygen concentration:
Basic Calculation:
Start with normal outdoor air composition:
O2: 20.95%
CO2: ~400 ppm (0.04%)
N2 and others: ~79%
For every 100 ppm increase in CO2 above outdoor levels, oxygen decreases by about 21 ppm (0.0021%). This ratio comes from respiratory exchange ratios.
So if your CO2 reaches 2000 ppm (1600 ppm above outdoor levels), the oxygen would be roughly:
20.95% - (1600 0.0021%) = 20.95% - 3.36% = ~17.6% O2
Important Caveats:
This assumes no other oxygen-consuming processes (like combustion) and no plants actively photosynthesizing
At 3000 feet elevation, the starting oxygen is about 19.5% instead of 20.95% (due to lower atmospheric pressure)
Humidity affects gas concentrations slightly, but not enough to significantly change the O2 calculation
These are estimates - actual oxygen sensors would be more precise
Health Considerations:
While 17.6% O2 is still breathable (normal range is 19.5-23.5%), your high CO2 levels (2000 ppm) are more concerning from an air quality perspective. Current building science recommends keeping CO2 below 1000 ppm for optimal cognitive function and
comfort. Your plan to increase ventilation via solar-heated air sounds excellent.
For more accurate monitoring, you might consider:
A simple oxygen sensor (available for under $100 now)
Tracking the "air change rate" in your home
Monitoring how quickly CO2 builds up when the house is closed
The math gets more complex if you want to account for:
Room volume
Number of occupants
Exact ventilation rates
But your CO2 measurements already capture the net effect of all these factors.
This is a response to the post seen at:
http://www.jlaforums.com/viewtopic.php?p=9014517#9014517
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