Liquid Nitrogen Boil-Off Calculator
Estimation Results
Understanding Liquid Nitrogen Boil-Off Rates
Liquid Nitrogen (LN2) is a cryogenic fluid with a boiling point of -195.8°C (77 K). Because its temperature is significantly lower than ambient room temperature, it is constantly absorbing heat from its surroundings. This heat transfer causes the liquid to transition into gas—a process commonly referred to as boil-off.
The Static Evaporation Rate (SER)
Every cryogenic Dewar or storage tank has a Static Evaporation Rate (SER). This is the amount of liquid nitrogen that evaporates in a 24-hour period while the tank is stationary and unopened. The SER is determined by the quality of the tank's vacuum insulation and the materials used in the neck tube construction.
Factors Influencing Boil-Off
- Vacuum Integrity: A degraded vacuum significantly increases heat leak and boil-off.
- Ambient Temperature: Higher external temperatures increase the thermal gradient, speeding up evaporation.
- Working Rate: Opening the tank to retrieve samples introduces warm air and moisture, often doubling or tripling the "static" rate.
- Liquid Level: As the liquid level drops, the "wetted" surface area changes, though the SER usually remains relatively constant until the tank is nearly empty.
Formula Used in this Calculator
The calculation is based on the following physical relationship:
Total Loss (L) = Static Evaporation Rate (L/day) × Time (days)
Remaining Volume (L) = Current Volume (L) - Total Loss (L)
Static Holding Time (days) = Total Capacity (L) / Static Evaporation Rate (L/day)
Practical Example
If you have a 35-liter Dewar with a manufacturer-rated static evaporation rate of 0.22 liters per day, and it is currently half-full (17.5 liters):
| Parameter | Value |
|---|---|
| Current Volume | 17.5 Liters |
| Evaporation Rate | 0.22 L/day |
| Duration | 10 Days |
| Total Loss | 2.2 Liters |
| Remaining after 10 days | 15.3 Liters |
Safety and Monitoring
Never allow a liquid nitrogen tank to go completely dry if it is storing biological samples. Monitoring the boil-off rate helps you establish a refill schedule. If you notice a sudden increase in the boil-off rate (indicated by frost on the outer shell of the Dewar), it likely indicates a vacuum failure, and samples should be transferred immediately.