The daily maintenance of liquid nitrogen tanks should revolve around cleaning, liquid level monitoring, sealing inspection, and environmental control, while vacuum recovery needs to be achieved through professional testing and targeted repair. The specific techniques are as follows:
Key points of daily maintenance
Cleaning and drying
Regularly clean the outer surface of the tank to prevent dust accumulation from affecting heat dissipation. Internal cleaning requires emptying liquid nitrogen and allowing it to stand at room temperature. Use a neutral cleaning agent (pH=7-8) and a soft cloth to wipe the inner wall. Do not use steel wire balls or strong acid or alkali solutions to prevent damage to the inner container. After cleaning, blow the inner container with dry nitrogen to ensure there is no residual moisture and prevent freezing and blockage of the pipeline.
Liquid level monitoring and supplementation
Regularly check the liquid nitrogen level using a liquid level gauge or liquid level monitoring system, and replenish it promptly when the remaining liquid nitrogen is one-third of the total capacity to avoid sample damage due to liquid nitrogen depletion. For a 10 liter tank, if the distance between the liquid level and the upper opening is greater than 11 centimeters or the depth from the liquid level to the bottom of the tank is less than 18 centimeters, it needs to be replenished immediately.
leak test
Check the sealing of the neck plug daily, and feel slight resistance when rotating the neck plug. If the neck plug is loose or leaking, it needs to be replaced with a new one. It is strictly prohibited to use other plugs instead of specialized tank caps, and it is even more forbidden to use sealed plugs, in order to prevent the continuous evaporation of liquid nitrogen from causing an increase in nitrogen pressure and damaging the container.
environmental control
Place the liquid nitrogen tank in a well ventilated, cool, and dry place, avoiding direct sunlight and high temperature environments (the ambient temperature should not exceed 50 ℃). For every 10 ℃ increase, the evaporation rate of liquid nitrogen increases by about 30%. Therefore, it is necessary to maintain air circulation around the tank (ventilation ≥ 8 times/hour) to reduce the generation of condensation on the surface of the tank.
Vacuum recovery techniques
Quickly determine the decrease in vacuum degree
Mark the position on the outside of the tank corresponding to the liquid nitrogen level, and observe the height of the liquid level drop at the same time every day. If the liquid level drops significantly above the normal average range on a certain day (such as dropping more than 1 centimeter more than usual), it may be due to a decrease in vacuum degree.
Professional testing and repair
Helium mass spectrometer leak detector detection: If there is suspicion of vacuum layer failure, use a helium mass spectrometer leak detector to detect the interlayer vacuum degree (normal ≤ 5 × 10 ⁻ ³ Pa). If the vacuum degree is greater than 2 × 10 ⁻ ² Pa, it needs to be returned to the factory for repair.
Re evacuate: For situations where the vacuum degree does not decrease significantly and there is no obvious damage to the tank body, send the liquid nitrogen tank to a professional maintenance organization to re evacuate and restore its vacuum degree. After returning to the factory, the performance of the tank can be restored to over 90% by re vacuuming.
Replace sealing components: If the decrease in vacuum degree is caused by aging of the sealing gasket or valve failure, it is necessary to replace the sealing gasket or valve with a new one in a timely manner to ensure the sealing of the tank.
Temporary remedial measures
If immediate repair is not possible, the replenishment cycle can be shortened (such as changing from once a month to once every two weeks), and the tank should be avoided from being placed in a high-temperature environment to slow down the evaporation rate of liquid nitrogen.