A common measurement model for a gas operated piston-cylinder based pressure standard
effective area is the well known integral equation formulation originally developed by
Dadson of the NPL. However a problem with directly applying this exact mathematical model
is that it cannot be easily cast into a functional form suitable for application of the
Guide to the expression of Uncertainty in Measurement (GUM) which is reliant on the
concept of sensitivity coefficients without various simplifications. In this paper, we
examine the standard approximations that are currently necessary in order to directly
apply the GUM for a pressure standard effective area uncertainty determination. We also
compare and contrast this to the exact effective area uncertainty results obtained through
the direct application of the Monte Carlo Method (MCM) which has recently been published
as Supplement 1 to the GUM. Based on these investigations we also draw some preliminary
conclusions on the relative merits on the extent to which the shape of the piston and
cylinder radii and whose uncertainties may vary along the engagement length of the
piston-cylinder may be modeled and incorporated into a piston-cylinder’s effective area
uncertainty calculation.