The Fifteen-Year Nitrogen Number: What a Storage Tank Really Costs to Own

Every cryogenic vessel carries two price tags. The one on the quotation is paid once. The other, the static evaporation rate on the data sheet, is paid in liquid nitrogen every week for the fifteen to twenty years the vessel stays in service. Laboratories negotiate the first number hard and rarely look at the second, yet over the vessel's life the second is usually bigger.

What the data sheets say

Taken from published product data sheets: a 30 litre storage dewar at 0.21 litres per day evaporates about 77 litres a year standing still. A 47 litre dewar at 0.36 litres per day loses about 131 litres. A 95 litre racked tank at 0.81 litres per day loses about 296, and the 175 litre version at 0.87 litres per day loses about 318. A dispensing reservoir at 0.23 litres per day loses about 84. An open benchtop dewar left filled would lose more than 1,000 litres a year, which is why open dewars are only ever filled for the task in hand.

Those are static figures, measured closed and undisturbed at roughly 20 degrees Celsius. Add lid openings and retrievals and a working laboratory consumes 1.5 to 2 times the static rate.

Litres per year is the wrong unit

Ranking vessels by litres lost per year makes the biggest tank look the most expensive. Ranking them by nitrogen per stored sample reverses the table. The 47 litre dewar holds roughly 7,000 quarter-cc straws, so it uses about 18 millilitres of nitrogen per straw per year. The 175 litre racked tank holds up to 72,000 straws and uses about 4 millilitres per straw per year. Consolidating three or four small tanks into one large one also cuts lid openings, which widens the gap further in real use.

Run your own number

Static rate times 365, times a workload multiplier of 1.5 to 2, times your contract price per litre, times fifteen years, divided by the samples the vessel will actually hold. Run it for every vessel on the shortlist before the purchase order goes in. The full table and a worked example for a 12,000 straw laboratory are in the source article: The Real Annual Cost of Liquid Nitrogen.