Uncertainty has to be considered for each individual test done by the customer themselves. There are a number of variables in the uncertainty calculation that Crystal does not control such as the physical setup, operational procedures, and ambient environment. Therefore, the customer must come up with their own calculations that they must justify. However, here is what we looked at.
Our products have an influence on the expanded uncertainty as does the DUT (device under test). For instance, each DUT has a different resolution of readability that directly affects the operator's ability to set the pressure at the correct point in the pressure profile. This will be unique for every series of pressure gauges from every different manufacturer so a user would need to take this information under consideration. It must be noted that we reported an expanded uncertainty with a k=2 for a 95% confidence level. As a result we can suggest the different areas that should be considered for each component and system. They are:
XP2i, M1, 30 Series: For these products we found that the main drivers of the expanded uncertainty to be: Reference pressure uncertainty (we use various pressure controllers and deadweight testers with different expanded uncertainties); resolution of digital indication uncertainty (depending on the range of the gauge, the resolution of readability changes); repeatability uncertainty (this is the ability of the gauge to repeat measurements such as trying to avoid hysteresis and non linearities). Additionally we considered the following but found their contributions to be minimal at our site due to the construction of our new facility: laboratory ambient temperature, laboratory relative humidity, laboratory ambient pressure. Our expanded uncertainty numbers reflect all of these factors combined together.
GaugeCalXP: the following areas should be considered as to whether they are significant or not: head correction (difference between sensing elements of XP2I and DUT, although considered a bias it is an uncertainty); fluid media (the compressibility of the fluid media is based on the bulk modulus so water is better than alcohol, for instance); ability to purge the comparator of air (at high pressures the gases will dissolve into the liquid media, a poorly purged system will have greater adiabatic behavior); repeatability of the operator to apply pressure using the GaugeCalXP (the ability to actually apply pressure to the desired set point in repeatable manner (this can be evaluated by doing a gauge repeatability and reproducibility test)).
DUT: The DUT has different graduations that should be notated. An operator must be able to determine the lowest readability of the gauge such as 1/5 of a graduation.
System level: Would be affected by: lab temperature, relative humidity and pressure conditions.