The cryogenic industry term Normal Evaporation Rate (or NER) describes the amount of cryogenic liquid that boils off to vapor during a given period of time.
This boil off rate can become a method to compare the heat transfer rate between different containers or vessels (assuming the same conditions). Normal Evaporation Rate is expressed in volume/unit of time, for example dewars are often in liters/day. NER is often calculated by weight however since that is easier to measure. To calculate the NER you can measure the weight change over a 24 hour period and divide it by the liquid latent heat of vaporization. Normal Evaporation Rate can be an easy method of checking the heat leak since NER is often published. You can also determine a dewar’s holding time at the same time.
An example may help:
You’ve acquired a 10 liter dewar and want to determine the Normal Evaporation Rate and subsquently the hold time for the dewar if it is filled with liquid nitrogen. You have an electronic scale that will weigh a full dewar (which would weigh just under 14 kg or about 30 lbs) and a source of liquid nitrogen.
After adding liquid nitrogen to the dewar and letting the dewar stablize at least an hour, you start recording the weight of the dewar in 1 hour increments. You find that the weight of the dewar has changed 3.8 kg over a 12 hour period. Using the National Institute of Standards and Technology fluids database, you see that liquid nitrogen at ambient (1 atm, 20° C) has mass of .86 kg/liter. Thus the NER would be:
3.8 kg change / 1/2 day = 7.6 kg/day
7.6 kg/day / 0.86 kg/liter = 8.8 liter/day Normal Evaporation Rate
Calculating the actual hold time for the dewar is now also easy. For the vessel you simply divide the vessel size by the calculated Normal Evaporation Rate (assuming 10L usable space):
10 liters / 8.8 liters/day = 1.1 days holding time.
Again your value may vary slightly from published data since you may not be at the same conditions and if you fill an unchilled vessel the intial evaporation rate will be greater.
Finally to calculate the heat leak for the dewar you need to know the latent heat of vaporization. For our example if you use the NIST database again, liquid nitrogen is 11.4 watts/kg. Thus the heat leak for this device would be:
3.8 kg/12 hours x 11.4 watts/kg = 63.3 watts/Hr