Uncertainties in air exchange using continuous-injection, long-term sampling tracer gas methods

The PerFluorocarbon Tracer (PFT) method is a low-cost approach commonly used for measuring air exchange in buildings using tracer gases.  It is a specific application of the more general  Continuous-Injection, Long-Term Sampling (CILTS)  method. The technique  is widely used but there has been little work on understanding the uncertainties (both precision and bias) associated with its use, particularly given that it is typically deployed by untrained or lightly trained people to minimize experimental costs.  In this article we will conduct a first-principles error analysis to estimate the uncertainties and then compare that analysis to CILTS measurements that were over-sampled, through the use of multiple tracers and emitter and sampler distribution patterns, in three houses.  We find that the CILTS method can have an overall uncertainty of 10-15% in ideal circumstances, but that even in highly controlled field experiments done by trained experimenters expected uncertainties are about 20%.  In addition, there are many field conditions (such as open windows) where CILTS is not likely to provide any quantitative data.  Even avoiding the worst situations of assumption violations CILTS should be considered as having a something like a “factor of two” uncertainty for the broad field trials that it is typically used in.  We provide guidance on how to deploy CILTS and design the experiment to minimize uncertainties.