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Re: small tube filling pressures

Jeff,  you'll have to excuse my less than complete mastery of Paschen's Law;
 I think that it's showing that there is a curve which has a point of max
light output, and falls down (at different rates) when the pressure is above
or below this ideal pressure.
Regardless, we have run several tests in the our shop having to do with
filling pressures:
in the firet set of tests we were using identical 10 inch OPEN signs (11 mm).
 Filling them in pairs we filled the first set to12mm pressure, the second to
6 and the last pair were filled to 24 mm of neon.  They all looked the same
(none brighter than others).  When we found a trans former that gave us as
close to 80% load as possible with our "normal fill pressure - 12 mm" we put
the other two pair on to see how it effected the mA meter.  There was
something like 1 mA total swing from such great filling pressure differences.
 
In another series of tests we were doing for some company selling to the auto
industry,  we were pumping and filling some 6mm tubes (red) and were filling
between 20 and 200 mm.  There was VERY little difference in operating temp or
light output.
In Neon Techniques they talk about underfilling and cleanup (pgs.15-16; 183)
and how quickly it can shorten the life of the tube.  There are some
references to overfilling and resultant hotter running tubes and
"compression" of the arc stream.  Yes it happens: in fact I have a 20 mm 4
foot tube that was fill by opening the new red gas bottle and letting as much
in as will go (guessing around 3/4 atm); it runs hot and never stops snaking.
 It is little different in light output.
The most enlightening information I  think comes from a study published by
the British Sign Assn from another study on tube life where they come up with
Life hours for both neon and argon. (And yes I did do an article for Sign
Business - July 94)  
 

Life (hours)= (Axp^4xV)/ (kxi^2)
where A= inside surface area of each electrode
         p=filling pressure of neon
         V= tube volume
         i= tube current
         k= empeical factor based on the coating material

Note the real killer:  pressure is to the fourth power
                          
I'm sorry that Wayne is not on line now, but maybe has co-worker would help
out in that Wayne has also been involved with the tail light industry and can
shed some more (red)light here.

Morgan