Dewar Issues

[Chuchnit]

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]

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]

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]

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]

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]

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]

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]

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]

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? :laughing::thumbsup:

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]

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.

[Chuchnit]

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.

That's pretty cool man. Yah, I knew you lived close. Hmm, this makes me want to risk it and just fill the sucker without a lid. Seriously I just don't understand how no top will evap less than with something covering the neck. Buckeye...... got any info on this? You are the resident dewar, SSD, and who the hell knows what else genius around here.

[Gutterz]

can u remove the spout an put a cork on it? Some of the ones Ive seen on Ebay just have a cork with threaded rod to hold it secure from the sides.

[ocgmj]

My dewar didn't come with a plug so I use something like this: Link (http://www.newmantools.com/cob/nylon.htm)

[Gutterz]

Thats pretty sweet.

also chuch if u need somthing now, you can check the auto parts they have replacement rubber freeze plugs for motors. Their similar to what Ocgmj posted but they tighten down with a nut. They worked well when I had to leak test banshee motors after rebuild.

http://images.outdoorpros.com/images/prod/5/Dorman-02603-rw-49849-61834.jpg

[Neuromancer]

You can pick them up at pool supply ships too, although they will probably charge more for them.

Glad you guys were mentioning this though as I was wondering myself what to get to top a dewar :)

[HITandRUN]

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.

Yeah I had one of those as well! It was a 20L dewar. You could fill it up to the top and leave it there for a week without barley loosing 1/4 of it. I dont see how the relief valve would do it. I think its more bad vacuum in this case.

Seriously I just don't understand how no top will evap less than with something covering the neck.

Pressurizing the dewar with a bad vacuum will make it leak out faster so leaving it not pressurized will last longer.

[Chuchnit]

Yeah I had one of those as well! It was a 20L dewar. You could fill it up to the top and leave it there for a week without barley loosing 1/4 of it. I dont see how the relief valve would do it. I think its more bad vacuum in this case.



Pressurizing the dewar with a bad vacuum will make it leak out faster so leaving it not pressurized will less longer.

Interesting info there HitandRun. So if it is a bad vacuum, how fast would you guess my evap rate is going to be on the dewar? I literally live right by the gas company so I could start benching pretty fast.

[2chesapeakes]

What the guys at Air gas told me was to fill the dewar all the way. If the outside of the dewar frosts up or condensation forms on the outside of the deward chances are the vacum is no good. If no rost or condensation is present. When I was researching my dewar the manufacturer had listed a normal evaporation rate. If i remember correctly the way to test mine is to fill it weigh it leave the vent open and weigh it 24hrs later. The has listed the normal evaporation rate in the manual. My dewar is what is consider a low pressure dewar. It is designed to maintain 22psi so once filled it only takes a few minutes to build enough pressure to pour. The one I had borrowed was a storage dewar it is not designed to build pressure. It took 1.5 days for enough pressure to build to be able to usethe withdrawal device on the dewar. If I wanted to use it right away I would have to pour from the dewar to the thermos.

[kikicoco1334]

i have the same dewar i think it's just where you keep it it may be different cuz i got my lab 30 a about 3~4 month ago maybe longer and i only fuled it once and weather is very cold in tennessee back then for me at least and there was ln2 left over till like 2 weeks ago i used most of it when i got it and had about 5L left in it....
make sure the cap on the dewar is good and it's in a place that's cold
and my mother in her work they had to fule it every week may be that's because the amount of usage idk i am baned @ work so... yeah,,, GL BRO

[Buckeye]

The one I had borrowed was a storage dewar it is not designed to build pressure. It took 1.5 days for enough pressure to build to be able to usethe withdrawal device on the dewar.


That sounds like how mine is, it never vents either but it only takes over night to build up pressure.

I had a 160L rental that was that way, took for ever to let pressure rebuild. But I was filling my 50L from it so I was really dumping pressure.

I am not sure why LN2 doesnt evap right away with open Dewars. I think it because its inside the Dewar which has been chilled down so it keeps the mass of LN2 cold for long periods of time.

I know most don't worry about it as most likely they get one filled and then use it all up over a weekend. I don't think it would last a long time tho., what bencher in there right mind would let a Dewar with LN2 sit and not use it for something, like Ice Drinks, Cooking With LN2, freezing bugs, cats tails etc :)

[Neuromancer]

Umm geez guys google LN2 and letthe teachers show you (I watched a bunch of video from the science teachers) and the thing is (and why you can reach your hand into a tube with LN2)

The cold is so cold it actually stays at teh bottom.

Coreolis effect anyone? (however you spell it) simple physics.

Temp at hte top of a half empty dewar is hugely different than the bootom... and air is so nirogren rich I guess... no thats not right...

geez dunno why LN2 does not boil away in a semi enclosed container.

[lesstutrey]

My guess as to why it doesn't, would be a very low level of entropy. Heat rises, cold sinks, so the gas that comes off the top is still colder than surrounding air, and is thus sinking below the air, and sort of creating a vacuum seal of it's own (like forcing an upside down cup into a aquarium).

My dewar operates at 12psi (the heater on it auto-shuts off once 12 psi is reached, not that i've gotten to use the dang thing) but that's just mine.. they make the same model with a slightly different manifold and it goes to whatever psi you dial in. More manifolds need a feature i didn't even realize i was taking for granted, a fill level indicator. Even after months (possibly years) of storage my dewar still has "something" making it read not quite empty and shoots out gas if i open the vent.

[HITandRUN]

Interesting info there HitandRun. So if it is a bad vacuum, how fast would you guess my evap rate is going to be on the dewar? I literally live right by the gas company so I could start benching pretty fast.


No clue mate since I never had a leaking one. I mean you could test it yourself by purging it with pressured air. Make up rig that fits the top with a gauge and a ball valve and than pressurize this thing.