addressing uncertainty in oil and natural gas industry greenhouse

4.4 Quantifying Measurement Uncertainty
Ideally, one would develop emission estimates and associated uncertainty from measured facility data.
This data could be obtained either from periodic sampling or by continuous monitoring. The decision tree
provided in Figure 4-4 directs the user to the appropriate equations for quantifying measurement
uncertainty. Following the decision tree provided, we will first examine the equations for aggregating
uncertainty for a single measurement point.
Are the data based on a single
point measurement or multiple
measurements?
Single point
Determine what parameters
contribute to uncertainty. Note that
bias may be one of the parameters.
Are the uncertainties in the
measurements independent? (See text
for guidance)
Yes
Apply “Square root of the sum of the
squares” to aggregate uncertainty
parameters that are independent
(Equation 4-4)
No
Apply Equation 4-5 to aggregate the
uncertainty for parameters that are
correlated.
Although there is only a single measurement value in this application, such as the cumulative flow rate
through a totalizer meter, there may be more than one parameter that contributes to the uncertainty of the
measurement. For example, Section 3.2 indicates that the following items need to be considered when
estimating measurement uncertainty:
• Uncertainty of the measurement instrument;
• Additional uncertainty of “context specific” factors (discussed previously in Section 3.2); and
• Uncertainty of measurement corrections, e.g., pressure and temperature corrections for gas meters
measurements.
The aggregated uncertainty from these parameters is calculated by either applying Equation 4-4 for
independent parameters or Equation 4-5 for correlated parameters.
Example –Single Flow Measurement
The following example demonstrates the uncertainty calculation for estimating CO 2 emissions resulting
from the combustion of produced natural gas. The scenario presented is based on a single measurement of
the flow.
Pilot Version, September 2009 4-16