Engineers' Resources - NER Calculator

THE NER TEST

Normal evaporation rate (NER) is a measure of the amount of liquid product that boils off to vapor in a given period of time.

The NER rate is normally expressed in terms of a percent of the total volume of liquid turned into vapor within a 24-hour period (at 70°F ambient temperature at 9 psig contents pressure) relative to the manufacturer's rated full vessel capacity.

Within a closed vessel, some of the liquid that boils off to vapor occupies the vapor space of the vessel.  The remaining vapor vents out of the vessel to the atmosphere.

The change in weight of the closed vessel is not necessarily an accurate measure of the NER.  The NER is actually the change in liquid volume inside the vessel.

How to execute an NER test using change in weight

Place the vessel filled to the manufacturers rated capacity on a scale.  The vessel contents are vented to atmosphere at 0 psig.  Record the change in weight over time until a stabilized weight change rate is established.  This typically requires several days to make sure the vessel is completely cooled to the contents temperature.