Accuracy is what the ...
Published by Adrian Ovezea, Executive Director of Sales and Marketing - Europe at Environmental Dynamics International (EDI)
Accuracy is what the manufacturer has to demonstrate before getting its method and instrument approved for a requested class of precision (accuracy). Accuracy classes are defined and used in IEC and ANSI standards.
The engineer opts for a class, and afterwards for the instrumentation capable inside the class.
The accuracy of the measurement on site will be subject of respecting basic rules of engineering after having a method implemented. Errors might come from external factors like random variation of light, humidity, pressure, just name it.
Accepted tolerance has to stay inside the accuracy class ( if we wanted to measure 1000m³/h within a tolerance of +- 0.1% then we need to be able to have the accuracy of class 0.1 by using an instrument which was certified (and calibrated accordingly), where 1m³/h will be the margin of error (plus or minus), resulting in a range of uncertainty of 2m³/h...
Usually a measuring instrument has to be 3-10 times more accurate then the lowest tolerance margin (10 times more accurate for metrology calibrations, 3 times at least in op field). Many times these rules are skipped for the sake of price savings.
Coming now to the two methods... volumetric and inferential.
See this link and its subclasses. The complexity of the PD makes them expensive, accurate and kind of reliable. The others may be price convenient, but if they must fit the same accuracy class, they will get complex and expensive, too.
The trend is to minimize the gap in price using latest achievements. Unfortunately, many public companies using public funds cannot afford the risk of experimenting, and they are conservative. (not all of them :) )