Cryogenic tank level control

[porky]

Hi all

What system (best) would you guys out there recommend to measure the level of liquid nitrogen at -182ºC.... in a storage tank...

Besides Differential pressure transmitters

Thanks

 

[Tom Jenkins]

A pressure transmitter with an isolation diaphragm.

 

[Mtn_Bkng_Dave]

Hmmmmmmm

A long stick?

No seriously what is the objection to differential pressure?

Dave

 

[shooter]

I would weigh the tank.
or if any room in it put a floater inside a tube with a stick on it trhu a small hole not in the lid. On top you can mount anything you like.
maybe old fashioned but working fine.
danfoss has them for pressurized vessels.

 

[JesperMP]

Dave,
I think the problem could be that a pressure transmitter would require a very good thermal isolation to the diaphragm. -182ºC is very cold, a lot of electronics is only guaranteed to work at substantially higher temperatures.

I second the idea of weighing the entire vessel. Simple, robust.

 

[nif]

Put a scale under the tank...

 

[Steve Kemp]

Dave,

I designed a series of liquid nitrogen accessories many years ago. For level I used a 10mm dia stainless steel tube with another smaller one inside it; insulating the inner one with short lengths of nylon tube at intervals in order to prevent the two metal parts shorting together. These two parts form the timing capacitor of a monostable. As I recall, a metre of tube assembly was about 100pf. The monostable was triggerred by a fixed, free-running oscillator (about 40kHz, I think). The output from the monostable is a fixed frequency, but with a pulse width dependant on the capacitance of the tube assembly, which is immersed in the liquid nitrogen. As the dielectric constant of the gap between the tubes changes due to the variable level of the nitrogen, so the pulse width changes. Filter this signal and you have a very linear liquid level to voltage output. Of course, as it stands, this can only be used with a cylindrical vessel. So long as the coaxial lead from the top of the vessel to the oscillator stage is less than a metre or so, the probe-to-cable capacitance ratio is not a problem. I may still have a schematic somewhere if you want to roll your own monitor.

 

[porky]

Heres My Problem

We have a Nitrogen storage tank 1000m3 capacity,
It has a hight of +- 12mtrs, but the tapping for the diff. transmiter (H) is 800mm from the base of the tank...

My problem is that if I use 0% as 12m - 0.8m I will not know the actual level below 800mm

What do you recommend ... beside using a stick

 

[JesperMP]

I see now, a 1000 cubicmeter tank is relatively large. Do you really have that much liquid nitrogen !? Wow.

It can be costly and timeconsuming to put the whole tank on weigh cells.
I can also imagine that a tap in the bottom or sides is not the most comfortable solution.
I would think that an ultrasonic or radar based level sensor could be the solution. Both types can be mounted on the top, and can therefore be accessed with ease. Be sure that the beam from both types of sensor doesnt hit anything inside the tank than the liquid or the bottom. Also, dont mount them flush with the top of the tank. They need a certain distance to safely detect the liquid surface when 100% full.

 

[JohnW]

level sensing

Try a Nucleonic sensor :

http://www.vegacontrols.co.uk

I have not used one but it looks good for your application. It is non-invasive and sideways looking

 

[porky]

Thanks to all

Thanks for the input...

Will have do some research ..
thanks