Vacuum Gauge Data

July 19, 2017 at 1:55 pm

One of the main features of the Fusor is remote operation, or as close to that as is possible without a huge amount of extra work.  I should be able to operate the Fusor without having to reach in and physically adjust or observe anything directly i.e. operate it while sitting with a laptop a couple of meters away.  This is necessary due to the Faraday cage, but is a great benefit in that it makes data collection and analysis much easier.  Instead of observing say, pressure values, from gauge readouts, all data will be sent to and logged in a computer, where it will be available for future examination.


Pirani gauge Calibration, fusor


This requires adapting all instrumentation to send its data to the computer.  While there is a plethora of different units used in measurement instruments – force, pressure, voltage, luminosity, amount of radiation – a lot of them work by measuring a voltage/current/resistance at some point and then converting this to an appropriate value of, we’ll say pressure, that the user wants to know.  If you can get a look at this voltage and figure out the formula that converts voltage to pressure, then recording the voltage is equivalent to recording the pressure.  Getting a computer to record voltage and convert it to pressure with a formula to give a second by second readout of pressure is far easier than doing it manually.  All that’s left is to find a convenient spot to read the voltage to the computer from.


At the moment I’m doing the above for one of the vacuum gauges, the Pirani gauge.  This measures low vacuum, from atmospheric pressure to about 1 millionth of an atmosphere (1000 mbar to 0.001 mbar).  (Here’s a good rundown on high vacuum gauges by CERN)  Normally, one would read the pressure indicated on the controller’s needle gauge, but that won’t cut it here.  Making things much easier for me, there are two screws on the back that a voltage corresponding to pressure can be read from.  With a multimeter I was able to record various values of voltage corresponding to pressure and feed these into a Numbers chart to get an exponential line of best fit.  The values weren’t completely accurate across the whole range (I had to read the pressure myself by judging when the needle was in the best position) but they were pretty good in the 0.001 mbar – 0.1 mbar, which is all I really care about for fusor purposes.


Pirani gauge Calibration, fusor Pirani gauge Calibration, fusor


Now, this isn’t complete, but the guts of the work is done.  When I have the myRIO (provided by National Instruments) up and running, the myRIO will constantly read the voltage from the Pirani gauge, and use the formula to convert it to a real-time readout of chamber pressure.  Cool beans.