Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Idea - Cheap Pressure Gauge
Date: Aug 09, 2:02 am
Poster: Dave Cooper

On Aug 09, 2:02 am, Dave Cooper wrote:

>Also known as a Pirani gage... A ThermoCouple gage (or TC gage) is similar in function.. The Amateur Scientist had a column about building such a thing last year or the year before.

(I read the article after I posted,unfortunately.)

The Sci. Amer. article used a mode airplane glow plug as the sense element. It is very robust, threaded and as such, is a nice shape for a vacuum element. But it does take a bit more current. The flshlight bulb sensor uses about 50 mA, the glow plug sensor between 1.0 and 1.5 amps. No big thing for a DC supply, however.
>And, for that matter, the next step is an ion gage, ....

> You could build the sensor yourself, but then you'd have to calibrate it, which is no easy feat. Spend the few tens of bucks (or a hundred) to get a real ion gage tube. ($60-100 from Leske, Duniway is probably cheaper, but I can't find my catalog here)

I can't put my hands on it at the moment, but there is a simple calibration equation for the standard Bayard Alpert type triode Ionization gage. Will try to find it..

The ion gage tube needs a steady filament drive, and a couple hundred volt potential to collect the ions, at about 8 - 10 mA maximum.

The ion gage readout is just an electrometer. You would not need to go much below nanoamps. A simple Digital Volt Meter ... (we see the Cen Tech brand offered as low as $10 at times) reading accross a 1 MEG ohm resistor will get you to sub-nanoamp levels.

A standard 3 -1/2 digit 200 mV digital panel meter costs about $10-$15 . Typically they operate on 5V, 9V or 12V supplies. (9 Volt battery is great.) The various pressure ranges would require only some simply resistors and a range switch.

However, for these systems, a filament protection circuit is a must. Otherwise, a leak or a sudden pump shutdown can kill your gage's filament in an eye blink.

The gages I remember, used a triac in the filament circuit to give a simple AC filament interrupter. The triac was turned off, if the ion current exceeded set points. (about 0.1 to 1.0 micron)

This part might be subtle enough to stall some folks.. so perhaps looking for a surplus gage system is the best approach. They will run a couple hundred dollars, unless you get really lucky,like Richard.

You might be also able to find old tube type electronic instruments, like the Veeco, etc. You could either fix and use or gut the interior and rebuild it with a simple solid state chip amplifier, The original meters (analog) with their nice scales give you a reasonably accurate gage.
Dave Cooper