Good morning, DavidR:

During our indoor air quality investigations, we are frequently asked to calculate the mixing factor or decay rate for a given study area, (this are normal standard studies while investigating IAQ issues). To do this, we release a quantity of a known material and measure the concentration over time.

Often, we will use CO2; dropping a couple of pounds of dry ice into a five gallon bucket (very scientific), can quickly raise the CO2 concentration in an office setting to 5,000 ppm.

As we drop the blocks of dry ice into the bucket, one can literally see the CO2 spilling down the outside of the bucket, and spilling out along the floor (for a short distance). It does this primarily because it is very cold, and therefore dense, and therefore it “sinks.” While this is occurring, the highest concentrations of CO2 are found along the ground.

Air currents, ventilation system, normal convection cells and the movement of occupants quickly increase the mixing and disrupt the temporary layer of CO2. However, in practical terms, the CO2 NEVER reaches equilibrium in all parts of the room – why not?

A standard indoor air quality algorithm used to model concentrations of airborne materials is given as:



Wherein:
Ct is the dynamic concentration (C) at any time (t);
Co is the original (starting) concentration;
Q is the amount of outside air being brought into the room via ventilation;
V is the volume of the study area-
k is the mixing factor.

The last term, k, is the reason that we never see equal concentrations in any study area, Pascal’s Law notwithstanding. The air in a room is always in motion, and there are two competing parameters: Sources and Sinks. Sources are supplying contaminant into the system, and Sinks are removing the contaminant.

The term k can range from 0.1 for a normal living room to 0.999 for a large empty airplane hanger with massive fans stirring the air in all directions. So, whenever one measures the concentration of something in the air, there are two huge variables: spatial variations and temporal variations.

There reason your measurements vary is because you are using a single measuring device, in a single location at a specific moment in time. By moving the device around, you will get differing readings (spatial variations); by keeping the device in one place, you will get differing readings (temporal variation).

One of my normal activities (whether I am measuring the airborne concentrations of mould spores, radon, or even determining the methamphetamine concentration in an house), is to be able to walk into a structure and be able to at least qualitatively determine the k, and determine where airflow is going, and why.

Having said all that, by the way, it is also important to know that most home CO monitors on the market (such as the ones originally under discussion in this thread), don’t actually measure CO!

Cheers!
Caoimhín P. Connell
Forensic Industrial Hygienist
Forensic Industrial Hygiene

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG