Some of our customers have expressed concern over experiencing sudden vacuum loss to a few of their dewars. These were returned to the factory and immediately leak checked. They were then revac’d and LN2 tested. The results of this investigation were; the vast majority of these vacuum losses were due mostly to an incorrect filling procedure (overfilling). This article will explain why it is imperative that careful filling protocol be followed. I am addressing the subject in this months Tech Tip to help explain why overfilling dewars during the charging process can be detrimental to the vacuum integrity of the aluminum dewar.

The purpose of the pumpout plug is two-fold. The main purpose (and most obvious) is to maintain the vacuum in the annular space of the dewar. As long as there is a vacuum, the pumpout plug will remain inserted into the pumpout body. The second purpose is to act as a pressure relief plug in the event of a catastrophic inner leak. If a leak develops to the inner section of the dewar, allowing liquid nitrogen to enter the annular space, this plug will fall out relieving any pressure build up. This pressure increase in the annular space is created when the liquid nitrogen evaporates into a gas and begins to expand.

There are two o-rings that seal the plug to the pumpout body. These are coated with a vacuum grease film to ensure longevity. The pumpouts are then covered with a flexible composite cap to keep dirt and contaminates from affecting the o-rings.

The problem from overfilling occurs when LN2 comes in contact with the pumpout plug or body for an extended period of time. The extreme cold temperatures cause the o-rings to contract and harden. This can cause the o-rings to temporarily lose their sealing properties. This problematic scenario can occur on dewars from all manufacturers whereas all utilize the same o-ring design. The plastic cap will not prevent this from happening but will extend the time required for failure to begin. It is imperative that the pumpout remains covered. Although they may crack and fall off if nitrogen has been spilled on it, (a sign that improper filling has occurred), a new replacement cap should be applied to the pumpout immediately. They are inexpensive and MVE will supply replacement pumpout caps when so desired.

Years ago, MVE utilized a metal cap that adhered to the pumpout body. Although this kept dirt particles from affecting the o-rings, it did not relieve pressure adequately and became a liability issue. We had installed for a short while, a hard plastic cover that would relieve pressure but did not protect the plug from dirt. The design we currently use appears to be the safest and easiest method of protecting the orings without adding to the cost of the dewar. This appears to be the general design practiced throughout the industry although there are different styles depending on the manufacturer. This filling precaution is particularly important on vapor shippers where the window for failure recovery is shorter.

Vapor shippers are usually warm when LN2 charging is required. Because of the smaller inner volume and the additional liner to be cooled down, the initial flash off is far greater then if the dewar was cold. It is due to this fact that the shippers usually require additional filling during the charging cycle.