Dewar Issues

Chuchnit · #1 02-02-2010

So at the office we have a MVE Lab 30 dewar with a withdrawl device. I was given the big OK to get it tested for functionality and rights to use it when it's not needed for the medical practice.

Two weeks ago we put a couple liters in and found that the rubber seal at the neck was dry rotted and quickly replaced. I finally found time today to get it filled a little today to see if the seal was our only problem. We weighed the dewar to measure the loss rate, but ended up with bigger problems.

@ 9am empty dewar - 30.4lb
@ ~9:10am dewar + ln2 - 53lb
@ ~3pm dewar + ln2 - 41lb

So considering ln2 weighs ~1.78lbs per liter, that makes 12.7 liters at ~9:10am. This fill was just partial to test the loss rate and withdrawl device. After some issues we got the head to seal and start building pressure. Yet the thing built pressure stupid fast. Nothing like Buckeye explained about his setup. To make a long story short, I show back up at ~3pm to weigh the dewar. It only weighed 41lbs. Total loss of 12lbs or 6.74 liters in a few short hours. When I got there the whole darn head was frozen and even the gauge had a quarter inch of frost on it. The stupid hand valve seems to be frozen, and the pressure release valve is constantly staying open. One of the guys there said it was doing that all day. Now when we filled it outside it was raining so water did get on the head unit, but I can't see that causing it to stay at 6psi and constantly bleeding pressure. Anybody have any clue as to what is f$@ked up? I would love for it to be something easy so I can bench with it this weekend.

Gunslinger · #2 02-02-2010

The loss rate becomes greater as the level drops in the dewar. But it does sound like something is wrong. Have you had the vacuum checked on it?

Chuchnit · #3 02-02-2010

Nope haven't had the vacuum checked. The guy there says it has a good vacuum. Honestly I'm clueless to these things though. All I care about is being able to bench with it. I thought they operated at a higher rate than 6psi anyways. I thought it was more along the lines of 14psi. So you are saying that its losing tons because the release valve is popping off at 6psi? The other problem is the damn head freezing solid to where you can turn the hand valve to withdrawl the ln2 :(

Gunslinger · #4 02-02-2010

6 psi seems pretty low, I think most are set around 22psi, but not positive. If work is paying for repairs, I'd have them check the relief valve first, then verify the vacuum is good. Then fill it up full and check your loss rate.

Chuchnit · #5 02-02-2010

Well I just called MVE/Chart Biomed. They seem to think that it has a bad vacuum :( I doubt they will want to pay for repairs since they start at $225 + $50 inspection fee. I just wonder how darn fast it will lose the ln2 with a bad vacuum and a cork stopper? If its not too bad I could just fill it up and use it right away.

Buckeye · #6 02-02-2010

Take some vasoline and spread it all over the o-ring, it helps a great deal.
I put a rather large amount all around the seals on mine.

My Dewar is a low pressure type, yours might be a high pressure type. High pressure types build up pressure very fast.

Some pictures would be great.

Is it like this one ?

http://www.bid-on-equipment.com/detail~id~51551.htm

Chuchnit · #7 02-02-2010

Quote:

Originally Posted by Buckeye

Take some vasoline and spread it all over the o-ring, it helps a great deal.
I put a rather large amount all around the seals on mine.

My Dewar is a low pressure type, yours might be a high pressure type. High pressure types build up pressure very fast.

Some pictures would be great.

Is it like this one ?

http://www.bid-on-equipment.com/detail~id~51551.htm

It looks identical to that minus the extension on the withdrawl hose. Plus mine is green. It is rather old. I was thinking about taking apart the valve and inspecting the orings tomorrow. Man I really want to use this thing this weekend at the latest.

Gutterz · #8 02-02-2010

I wonder if the pressure is making it leak faster? When I was down at Kingpins the other day he doesnt put tops on his their just open. I asked him about evap an he said it dont evap that much an told me to look in the one that it should still be full from the day before an it was. So im realy not sure whats up.

Chuchnit · #9 02-02-2010

Quote:

Originally Posted by Gutterz

I wonder if the pressure is making it leak faster? When I was down at Kingpins the other day he doesnt put tops on his their just open. I asked him about evap an he said it dont evap that much an told me to look in the one that it should still be full from the day before an it was. So im realy not sure whats up.

So we just go and hang out with Vince now huh?

Anyways I was wondering about that because I mean I can just fill the sucker and bench it the same day. I just don't wanna lose half my ln2 in no time flat.

Gutterz · #10 02-02-2010

Im only 20 minutes away I just been down their once to pick up an F1 hung out for a few hours. Im suppose to go back when shamino is in town. We shall see he is a busy man.
But I hear what your saying no sense in having money evaporate away. Im waiting on a temp reader
an then Im ready to get some LN2.