Friday, May 14, 2010

Zero and Beyond!
Thermistor Vacuum Gauge Construction.

I had been thinking for a long time about getting a gauge for very low pressures. I came across this little project a year or two ago but it was beyond my understanding and abilities at the time.

It came from

Even with this nice circuit to work from I still had the problem of electrical feed-throughs. Then one day not too long ago the solution pop into my head.

Strip fluorescent lights!

I had a T4 size fluorescent tube which is just under 16mm dia. Now that's just larger the the external measurement of a 1/2” copper pipe. So the brass fitting could be adapted to fit. Using an O–ring to keep it air tight.
Strip lights have two electrodes nicely sealed in to each end! Perfect! So I cut the tube on the lath and with a hot wire. I then washed out the fluorescent powder. I believe these lamps have may have mercury vapor in them so I didn't breath while I was doing it...

Then I covered the electrodes with a bit of Al. foil to protect them as I rounded the cut end with a flame to de-stress it a bit.

I popped it all in some safety pickle and got the Dumet wires back to nice clean copper. I could then , with a bit of flux, solder the thermistor to the inside.

Once I fitted this to my home-made ‘QF16 from plumbing pipe fitting’ my feed through was complete!

This is turned from a 1/2" tank fitting. I'll go into this in another post.

I'm just waiting for my Viton o-rings to arrive and I'll hook it up.

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