q = [n([DELTA] h)(85-t1 )] / [ts - tf ]
Where:
q = calculated heat transfer rate to cargo tank with lading, Btu/hr.
n = normal evaporation rate (NER), which is the rate of evaporation, determined by the test of a test cryogenic liquid in a cargo tank maintained at a pressure of approximately one atmosphere, absolute, lb/hr.
[DELTA] h = latent heat of vaporization of test fluid at test pressure, Btu/lb.
ts = average temperature of outer shell during test, °F.
t1 = equilibrium temperature of lading at maximum loading pressure, °F.
tf = equilibrium temperature of test fluid at one atmosphere, °F.
RHT = [(U2 - U1 ) W] / q
Where:
RHT = rated holding time, in hours
U1 and U2 = internal energy for the combined liquid and vapor lading at the pressure offered for transportation, and the set pressure of the applicable pressure control valve or pressure relief valve, respectively, Btu/lb.
W = total weight of the combined liquid and vapor lading in the cargo tank, pounds.
q = calculated heat transfer rate to cargo tank with lading, Btu/hr.
49 C.F.R. §178.338-9