40 yr old thought experiment

Where does the charge "go" when you disassemble a Leyden jar?

Mark L. Fergerson

It went into my finger. Yeouch!

Cheers

Phil Hobbs

MHD? ...Jim Thompson

So if I have this right, randomly polarised molecules flow between the plates and are polarised by the field - that takes energy. The faster the flow, the more molecules get polarised, the more energy.

On the face of it, that should work. Maybe the effect is small and dominated by impurities, but aren't we all?

Cheers

For conductive liquids, there are MHD flow meters.

There are also vortex shedding meters, doppler meters (need particles in the fluid) and a weird inertia-based thing, a vibrating U-shaped tube somehow. And orifice plate/restriction delta-P of course. And turbines.

And more!

There might be a way to charge an insulating fluid and measure the charge downstream, and use the transport delay to compute velocity. Sort of like tossing rose petals into a stream.

You could squirt temperature signals into a fluid.

Imagine the fluid is a chain of capacitors, in line waiting to be charged.

If the cap was 1 cm long in the flow direction and 100 pF, then at 1 m/sec and 100 volts bias, the current would be 1 uA. That's a good signal. One could also have another cap (or maybe the same one) to measure the fluid dielectric constant and correct for that.

Nice idea.

That's a density measurement, IIRC. You shake a known volume to measure its (inertial) mass.

I've seen a prototype for that for downhole use. It was miserably inaccurate, but might work in the right circumstances.

Cheers

What, you don't pump the thing down first? (With a charge pump, of course).

a Coriolis flow meter measures mass flow, if you want volume flow you need to measure density too

pretty much every car a has a variation of that to measure air mass flow into the engine, basically measuring how much power it takes to keep a resistor in the air stream at a certain temperature

-Lasse

Another way would be to look at the force on the U; downward-flowing liquid goes in, upward-flowing liquid comes out, so there's a net force (to cause the momentum change). You also have to measure static pressure (head) to complete the calculation.

OK, I'm thinking now that there will be no net charges induced into the fluid downstream of the capacitor plates, so no net current into the plates, so it won't work. But I'm not sure.

Mark L. Fergerson "

Ask that over in sci.electronics.basics and you will get an answer.

This was different - you apply a pulse of heat to the fluid, then measure the time it takes for the 'hot part' you've just made to flow past one or more temperature sensors some small distance away using a correlation algorithm. It can be more complicated, with the heating being a continuous oscillation, but that's the principle.

The advantage, if it works, is that it copes with a wide range of fluids. As you can imagine, the heat spreads and dissipates by conduction and convection which is just one problem.

Cheers

What if the liquid you actually want to measure isn't comprised of polarized molecules? It seems to me the type of liquid is already determined before you decide to measure the flow.

Water is comprised of polarized molecules, but don't they tend to align to each other rather strongly?

Everything but vacuuum has a dielectric constant > 1.

I don't think you need particles in the fluid to measure the fluid speed. That was what the Michelson Morley experiment did to detect movement in/of the Aether. Any fluid flow would be the same.

yes, it would be like the ultra sonic wind speed sensors recently discussed

-Lasse

Time-of-flight doesn't need particles. Doppler does.

Uhhh... sure about Doppler? Or do you consider "molecules" to be "particles". I'm thinking of the classic sound doppler of approaching/receding sound source. In which case the OP's fluid certainly has "particles".