Semen tank question

My cousin called me this morning and told me he left the lid off of his semen tank for about three days.  It was night when he bred a cow and he forgot to put the stopper back in.  He asked me if his semen was still good. He also told me that he checked and there was plenty of liquid nitrogen left in the tank. 

I told him that I thought he was fine as long as his tank didn't go dry.  Is this correct?  He's got a group of cows that he's synchronized and I would hate for him to shoot dead semen in them and waste more money.

My dad and the ai tech just did this yesterday.  Plug was out for right at 24 hours....tank went from 19 cm on Monday down to 13 yesterday...if was all frosted at the top.  Talked to two different semen reps and both told me that we should be fine (stated as long as nothing was within 3 inches of the top of the tank....)

We did take to units to a microscope as a precaution and both were fine....should be good as long as you didn't get well below 10 cm's....

He told me he had frost on the top too.  He found the tank open when he went to breed a cow this morning.  He still bred the cow and used a cane that had only 1 straw left in it and said there was nitrogen still in the plastic holders(don't know the name) in the cane.  I figured that he ok as long as the semen was still emersed in nitrogen but just didn't know for sure.

Good thing his tank was filled last week, huh?  (lol)

If it would have been me it would have been dry and I would have lost all of my embryos and semen.

As evidenced, he will have a farely rapid Ln2 use compared to normal, but as long as there was liquid in the tank, and was not extremely low, he will be just fine. Keep in mind however, from the top to the bottom with a very low liquid level, there would be as much as a 100 degree increase in temp.

IE --  -296 could be as high as -200 in this case. Still extremely cold, but if any of you remember Brad Strouds EFD show, a 100 degree increase can injure the semen, even if it was solid frozen, it will start to crystalize the remaining water in the cells -- and could slighty damage the moltility due to cut tails. It would have to get pretty warm (Like -100 to do that)

If this happens to any one elses tank, just get it refilled ASAP and you'll never know it happened. The frost on top was due to rapid evaporation of the Ln2 and will do no damage to the tank!

Terry