On the other hand, given a a good RMS meter operated in a frequency range where it's spec'ed to work and a known circuit impedance will give you a very good measurement of the power: P=Vrms²/R. And analog meters make for rather pretty demos, to be honest.
In this case being able to supply 8 volts into a meter with a low internal resistance is actually a plus ;)
That said, the current is still tiny, even a crappy volt meter will pull no more than a few hundred micro-amps from the circuit that it is measuring. So let's be generous and say that it pulls a whopping 10 mA (very likely on the high side (edit: it turned out to be 37 uA)), in that case the power generated would be 0.08 W. Not enough to charge your phone by a long shot, but enough for a pretty demo with a moving needle.
Finally, these cheap meters tend to output complete nonsense values when presented with high frequency AC, one would expect that they would at a minimum measure power transfered using a dummy load and some kind of dedicated power meter.
All in all this does not make things look any better for uBeam. I'd really like to know who did their technical due diligence.
What's really bad to me - but I'm a nitpicker - is that in the video the lady refers to the volt meter as a power meter, which it definitely is not. It measures potential, not power, any power that it takes from the circuit can be chalked up to imperfect design and the laws of physics. Maybe the person doing the demo was not knowledgeable about the physics behind the equipment but that's betraying a complete mis-understanding of the basic concepts involved.
The transducers used appear to be of the piezo-electric type, those have a fairly high internal resistance and the voltages produced by those devices would be relatively high, the currents correspondingly tiny.
What gets me about this whole saga is that uBeam makes it seem as if they invented wireless power transfer using ultra-sonics, but any sound is wireless power transfer and that includes ultra-sound. The whole crux is how much power can be generated and whether or not there is a practical way to overcome all the obstacles.
To transfer some power doesn't prove a thing, when I whisper in your ear I'm transferring power using soundwaves...
Also: For low-level AC measurements you invariably need active rectification. So your meter will never be directly connected (or through a series resistor) to the source.
The guts of that meter register 37uA for full scale (I found some specs), at 10V that's an internal resistance of 270K so probably it really is driving the meter directly.
For 8 V that translates into 0.25 mW of power produced, which I don't think will be charging any cell-phones.
Exactly. These transducers look just like low-cost piezos used for ultrasonic ranging in air (e.g. car parking sensors, alarms, etc.). I have not seen any in that package that would be made for any significant amount of power. High power transducers look quite different.
The one the parent mentioned has 10kOhm/Volt sensitivity, so for 10V range it gives 100kOhms
http://support.radioshack.com/support_meters/doc73/73794.pdf
Correct, but it is going to measure something (it most likely rectifies and sends it to the meter, should work fine in the 100khz range)
> a volt meter is simply the wrong tool for the job
I don't disagree with this
The biggest factor are the ways in which the sample is taken and what kind of algorithm (if any...) is used to determine RMS. Simple meters simply rectify the AC current, add a small capacitor and a resistor to drain it and measure the DC voltage across the resistor. More advanced meters will use an isolation stage with a pre-amp driven from the auto ranger that will present the wave-form in one piece and suitably scaled to a DA stage where the sampling will take place. This allows for pretty good determination of the actual voltage (which in the case of AC has been agreed upon as root-mean-square, whereas, if the circuitry is bunk could easily become 'peak' or even 'nonsense').
Anyway, we're all in violent agreement that this measurement is bogus and since that is all the quantitative info there is I'll let it go here.
http://www.analog.com/en/products/linear-products/rms-to-dc-...
You could even attach a ultra lower power microcontroller dev board to it[1] and show the less technical audience "look! computer powered by spooky magic!"
1: like this: https://www.silabs.com/products/mcu/lowpower/Pages/efm32gg-s... super low power microcontroller demo board that runs off of all sorts of energy harvesting rigs like tiny solar cells, thermoelectric piles, and so on.