Kelton Joyner wrote in message ...
Parallax,
You seem to like figuring out solutions. How about this one.
I have a stainless cylinder 8 inches in diameter, 3 feet high, closed at
the bottom and open on the top.
I want to keep the cylinder filled with liquid nitrogen to within 6
inches of the top and never less than 6 inches from the bottom. I also
want to know when it is half full.
I have a computer with two parallel ports, a solenoid operated cryogenic
valve with an interface to operate from a parallel port, necessary
plumbing to connect to a 1000 gal dewer of liquid nitrogen.
What other parts to I need to fabricate something to measure the liquid
nitrogen in the cylinder.
krj

Parallax wrote:

When i bought my boat, it was 10 yrs old and even then the fuel
sending unit didn't work. Over the years, I have replaced it 4 times
and it has NEVER worked right. Those damned sending units are such
crappy technology...electrical/resistive/ mechanical/floating going to
a meter, and they really expect this to work for any length of time on
a boat? So....
Useless idea #3741 (I am really going to try this one)

Take two pieces of flexible tubing maybe 1/2" diameter filled with
hydraulic fluid connedcted to a T. The base of the T has a 1/4" clear
hose going up to where it is visible in the cockpit. The two pieces
of 1/2" hose support the fuel tank whose weight compresses the fluid
causing it to rise. As the tank empties, the level in the clear
tubing falls. So, it is really measuring weight. But, no
electricity, no moving parts (except the fluid) and absolute
simplicity. Now, the problem, most fuel tanks are fastened down
firmly so when this thing is installed, it must be able to move a tiny
amount up and down (maybe should use 1/8" clear tubing).

Kelton:

I have to keep x-ray detectors cooled with LN2 and I check the level
with a dipstick. basically, I took a piece of plastic and painted it
black and I dip it into the LN2 Dewar and pull it out almost
immediately. A very thin and whitish layer of frost forms up to the
top level of the LN2 because the plastic is a poor thermal conductor.

However, I assume you want an automated method. So: I use an
ultrasonic level guage on a plating tank that simply attaches to the
side of the tank, it closes a relay when the level falls below the
detector. You would use one on the top to tell when it is overfilled
and one on the bottom to show if it falls below the minimum 6" and one
in the middle. They werent tooooooooo expensive but I cannot remember
how much. They do require that you have access to the tank wall
immediately adjacent to the fluid and if this is a dewar, it could be
a problem.

So...You might try a thermocouple probe just above the max level
(Dunno if they will go down to LN2 temps). My x-ray detector dewars
have a low LN2 indicator that could be modified for max level too. I
dont know how it works because I simply use my dipstik method.

If you really want to get complicated, you might try ultrasonic
methods (time delay reflectometry) to detect both the top of the fluid
level. I have seen such devices and they werent too expensive.