re: LN Loss from Germanium Detectors

[JDT4%RP%DCPP@bangate.pge.com]

Gary Kramer asks:

>I want to measure the rate of loss of LN from the Dewars attached to our 
>Germanium detectors by looking at the gas flow from the vent tube of the 
>dewar.
>
>Does anyone know of a source for sensitive gas flow meters? 

We use a gas flow device for measuring long term environmental air sampling 
processes in the field (I mean REALLY in the fields - cow pastures, corn 
fields, beaches, parking lots, etc.)  We are using flow meters just like what 
your local gas company uses to meter the natural gas to your house.  They are 
accurate to within five percent, generally about two percent, and reliable as 
hell (two prime considerations if you happen to be in the business of selling 
natural gas for profit).  They operate at differential pressures on the 
order of 1 to 2 psi.  The devices are manufactured by Rockwell.

Just as a curiosity, though:  Assuming you're trying to determine the 
optimum fill frequency, most dewar systems use a weight sensitive high voltage 
cutoff switch to protect the detector in the event of loss of coolant.  Why 
not use a "weight change per unit time" calculation to determine the volume of 
LN2?  If you're trying to determine something like breathing air quality in a 
confined space, a similar approach could be used taking into account the 
density of LN2 vs. N2 gas and come up with something that way.  Of course, if 
you're operating in a confined space, you probably aren't all that keen on  
introducing a large supply of inert gas...

Jack Topper
jdt4@pge.com
My employer completely disavows any and all knowledge of everything I do...