Sampling in the Face of Variability

May 7, 2011

Technology, and more importantly, budget constraints may prevent us from logging data over time.  That means we must show up on a given day and measure that point in time.  This method gives us a random reading from a normal day.

If we only have one shot at taking a measurement, some advocate aggressive sampling to simulate the absolute worst case maximum scenario.  Let me give you some examples of trying to simulate the worst case maximum scenario:

  • Measuring carbon monoxide: Turn every single combustion appliance on at once.  Everything that creates a negative pressure in the home is turned on including kitchen and bathroom exhaust fans the clothes drier and the centralized vacuuming system.
  • Measuring airborne mold spores: A leaf blower blows air over carpeting, upholstered furniture, and in the ductwork.
  • Measuring VOCs: Although the cleaning crew normally cleans after hours, you have the cleaning crew clean an office while it is occupied.  You have the occupant wear the highest amount of perfume possible and wear dry cleaned clothes.  Oh, and don’t forget adding in an air freshener and shutting down the HVAC system.

Most of these extreme situations create a worst case maximum scenario that is unlikely to ever happen.  If it did happen, it would likely be for a very short period of time.

Is there some in-between ground?  Yes. We can adjust some of the variables, within reason, to simulate what I call a “worst case normal” scenario.  A worst case normal scenario is different from a worst case maximum scenario in that you are only adjusting variables within the parameters of normal building operation.

For example, this might mean having the commercial building be at minimum ventilation before measuring VOCs.  This would be accomplished by having the VAV system at minimum supply and ensure the outdoor intake dampers are at the minimum opening.  Or it might mean having the home owner close the windows and turn on the HVAC system before taking air samples for mold.