Fusion Message Board

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Subject: Re: Idea - Cheap Pressure Gauge
Date: Aug 05, 8:50 pm
Poster: Dave Cooper

On Aug 05, 8:50 pm, Dave Cooper wrote:

>Idea - what if some highly skilled amateur device maker (a.k.a. not me) were to create a feedthrough with a small piezoelectric device (damn it Jim, I'm an engineer, not an English major)that formed a bridge between the fusor chamber and the room. I envision a pair of free moving cylindrical plates with the piezo in between them. The pressure difference would pull against the piezo rather than pushing, but would still cause a current. Even at very low pressures, I would think it would be sensitive. Piezos are cheap these days too. Comments?
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Here's a little bit more to think about on this idea. The piezo device as noted by the other posts produces a potential when stressed mechanically. More correctly, it produces a displacement current as charges move within the structure. When the mechanical stress is removed, the charges redistribute and the potential goes away.

Now suppose the gate of a MOSFET device is coupled to the hypothesized piezo pressure sensor at atmospheric pressure. Then, evacuation of the chamber proceeds, resulting in stresses on the sensor element, and a resulting displacement current. Other than the miniscule leakage currents of the MOSFET, the potential on the sensor and the MOSFET gate, should remain constant as long as the chamber is under vacuum.

The circuit would be extremely simple, a battery to power the indicator (UAmmeter or DPM), the MOSFET itself, a capacitor and the sensor. A few more parts to make it user friendly and you might have an instrument. The big issue will be how low the MOSFET leakage actually is. For junction capacitances of a few pF, an integrating capacitor of a few hundred pF would probably be needed to handle all the sensor displacment charge. With typical leakage currents of 10^-16 A, the circuit time constant appears adequately long to give useful working times. It does not look too good for extended operations, however.

You might also think about a simple tungsten filament (such as from a flashlight bulb). A small constant current slightly heats the filament. The readout circuit is a simple voltmeter reading across the filament. As the pressure decreases, the filament gets hotter, its resistance rises, and the voltage across it increases. I built one that works down below 10 microns. The original used a multimeter on the resistance scale to read pressure!

Dave C.