addressing uncertainty in oil and natural gas industry greenhouse

3.3.2 Quantifying Sampling Precision
Quantifying sampling precision requires that primary samples be collected according to a defined
protocol, but randomized in some way for each sample (in either space or time). For example, a preselected
percentage of the total number of samples can be collected in duplicate and the repeatability of
the measurement determined from these duplicate samples. Additionally, duplicate gaseous samples can
also be analyzed in duplicate and thus a full record of both sampling and analysis system variations can be
obtained. In this case, the sampling component of the variance represents the natural variability of the
sampling target as well as any errors in the sample collection and preparation.
When in situ measurement techniques are used (e.g., infrared gas analyzer), both the sampling and
analysis are addressed at the same time. The determination of sampling bias, or the difference between
the mean of several measurements and the true value, is more difficult. Biases could arise from several
causes such as sample fouling, inappropriate handling, or unrepresentative sampling. The “true” value for
the concentration of unknown species in a sample is never known since it is impossible to construct a true
reference laboratory standard for an unknown mixture of gases.
The importance of sampling uncertainty is a relatively new concept whose importance is slowly
beginning to be recognized. Recent research has shown that sampling uncertainty is often far greater than
analytical uncertainty. Therefore, combining sampling and analytical uncertainties to provide an estimate
of measurement uncertainty is an important component of quantifying the overall uncertainty of GHG
estimations that rely on sampling gaseous fuels of varying composition.
3.4 Carbon Content Measurement Practices
Different types of gas chromatography (GC) systems are normally used to analyze the carbon content of
gaseous streams. The GC systems might be laboratory based or set up as an online device for automated
collection of samples and their analysis. The systems are typically set up to analyze the individual
components in the sampled gas and provide detailed reports of properties including composition, calorific
value, and density.
The results of the determination of individual – or groups – of carbon-containing species are then used to
assess the total emissions of CO 2 upon combustion of such a fuel. Several key considerations include:
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If the method is capable of determining CO 2 content with the rest of the carbon containing
species, no further correction of the carbon content data is required in order to properly account
for all CO 2 emissions;
Pilot Version, September 2009 3-15