Maybe a better explanation of how a semen tank works. From Virginia Tech Dairy Sciences:
Semen Tank Management
The semen storage tank is actually a large, metal, vacuum sealed bottle encased within an extremely efficient insulation system. The vacuum bottle construction allows for maintenance of -320oF (liquid nitrogen temperature) when at least 2 inches of liquid nitrogen are present. Technical advances in design and construction have produced storage tanks with liquid nitrogen holding times of six to nine months. Although semen storage tanks are well constructed, they are still susceptible to damage from mishandling. Semen tanks should be kept in clean, dry, well ventilated areas. Avoid excessive movement of the tank. The inner chamber, which contains the liquid nitrogen, is actually suspended from the outer shell by the neck tube. Any abnormal stress on the neck tube, which can be caused by sudden jarring or any excessive swinging motion, can possibly crack the tube and result in vacuum loss from the outer chamber.
To increase holding time, keep the tank in a cool location away from direct sunlight. Avoiding drafts from milk coolers, furnaces, and outside air also helps prevent excessive nitrogen evaporation. However, make sure there is sufficient ventilation to prevent possible suffocation, which can be caused by too high a proportion of nitrogen gas in the air you breathe. Protect the tank from corrosion by keeping it elevated above concrete or wet floors with boards or pallets. Locate the tank where it will be safe from children and vandals; but don't hide the tank--it must be placed where it can be seen daily and monitored for excessive nitrogen usage by detecting escaping vapor. Finally, always be watchful for a lid that is left off and for frost or sweat on the tank. Give particular attention to the neck and vacuum fitting. Frost indicates that the vacuum insulation has been lost and liquid nitrogen has been or is evaporating rapidly. If this should happen, use a wooden yardstick to measure the amount of liquid in the tank. If the tank still contains liquid nitrogen, the semen is probably still good but should be transferred to a good tank immediately. Should the tank be empty of liquid nitrogen, it is doubtful that the semen is viable.
