Simple capacitance meter, preferably analog?

Leader make some pretty neat bridges:

This one's obviously kaput but you get the idea....

You need to get that draft fixed.

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My Ph.D. thesis describes an LVDT-based pressure sensor, but goes on to poi nt out that a stretched diaphragm pressure sensor with capacitance position sensing would be a lot nicer. If you throw in electrostatic force feedback - with a very small gap - it gets even nicer.

MKS Baratron got into selling capacitance manometers at much the same time, so my first insight wasn't exactly unique - the Philips central laboratory in the Netherlands published a description of such a capacitance-based pre ssure sensor in 1954 (IIRR) so it certainly wasn't original. The electrosta tic force balance approach doesn't seem to have been commercialised yet.

It was just an open slider door for ventilation.

Ceramic caps can be excellent temperature sensors.

I have one of those around here somewhere. Pretty nice thing, in its day.

I think he's saying it's being a microphone.

If it's a one-of. You (Joerg) could use the AD630 and a sig. gen... my ~$350 rigol does two channels with a phase sync button... You'd still need a LP.

George H.

It reacted to pressure. I could make a loud noise and you'd see that.

It's simpler than that, I am using an LC circuit here which is followed by a commercial low-cost USB sound digitizer for rock bands, a PC or laptop, and on that I run the pro version of Daqarta. The software is rigged into displaying phase in a graph with frequency and all that selectable. If you keep the frequency locked to the 48kHz sample rate of the digitizer the sensitivity is excellent.

However, the semiconductor folks want something more simple. Buy, plug in, connect a single cable, see results.

Nice... I always feel silly suggesting things to you, since I figure you'll have ten ideas to my one. But hey, even the blind pig can find an acorn... And I also figure that others reading the thread might find it useful.

George H.

It's possible there is an old one out there that does meet your spec. They produces tons of these machines over decades. I've not seen a new one in person- the old ones never seem to die.

Analog capacitance meter would be a difficult design. The basic design principle is to exploit the time it takes to charge or discharge the capacitor -- how would one achieve this task without counters, registers and a lo power microcontroller.

That's one design principle. There are others. Transformer-based impedance bridges are another way of measuring an unknown capacitance, and they have been around since long before microcontrollers were invented.

How, is straightforward; one measures phase and amplitude of the response (current) with respect to the stimulus (applied voltage). Phase can be as simple as an XOR and a lowpass filter, and amplitude can be handled with AGC amplification (the amplitude being represented by the gain-control signal). Massaging a phase-sensing meter reading is just a matter of inscribing the appropriate scale on the meter face; analog is easier than digital, some times..

Sine wave, tuned amp, phase-sensitive detector.

that's one way to do it.

The other is to feed it a fixed voltage and frequency and measure the current it draws.

Capacitance meters didn't exist before microprocessors?

There are many ways to measure capacitance but even if restricted to using your method - charge a capacitor and stop charging (or zero) when the voltage reaches a threshold (do this with a fixed period); measure average voltage.

The guys bought a Boonton 72C off of Ebay, should be there next week. I'll post here once we know how it fares. Nice thing is, aside from the analog meter it also has an analog output so one could log capacitance change using a simple digitizer such as a Labjack.

It should literally be plug-and-play.