Safety Tidbit 4.29 - Calculating Outside Air

Safety Tidbit 4.29 – Calculating Outside Air

Reference: TSI Application Note: TSI-138

As I introduced in the previous Safety Tidbit, carbon dioxide concentration is common indicator of indoor ventilation.  We can measure the carbon dioxide easily enough (I like the TSI Model 7575). Since each of us is a CO₂ generator, how do we determine if there is enough outside air brought in to dilute the build-up of CO₂?

Percent outdoor air (%OA) is the percent of the total volume of delivered air that is outdoor air. There are a few ways to determine %OA such as using sulfur hexafluoride tracer gas or, similarly, measuring the concentration of CO₂. However, I think taking three measurements of CO₂ and calculating the %OA is the easiest (temperature can be use in place of CO₂). The three locations to take the measurements include outdoor air, supply air, and return air.

To calculate the %OA follow:

Where:

Xr = CO₂ concentration in the return air

            Xs = CO₂ concentration in the supply air

            Xo = CO₂ concentration in the outside air

Now all you need to do is take the three measurements. A few common pitfalls to taking readings include:

·      Supply air is mixing with room air

·      Return air is not mixed well

·      Outdoor air measurement is tainted

·      Not enough occupants to generate high enough CO₂ levels

Remember if you have a room where several people will be working for extended periods of time think of their health and measure the carbon dioxide levels during the height of the workday to ensure the HVAC system is performing adequately. Otherwise, your folks could be nodding off on the job.

Hope this was helpful and thank you for reading my Safety Tidbits! Comments and questions are always welcome. ~ Bryan

P.S. If you have a new safety or health question, please let me know.