Charging water droplets and detecting the charge cm away

[TimB]

Hi all

Sorry obscure subject line but you will understand what I mean when I explain

I have a orifice ejecting water droplets, some very small (10 nm) what I want to do is gauge on how many are being ejected making sort of a real time meter.

I was thinking if you could charge the droplets as they left the orifice then detect the charge a few 10's of mm away as they slam into a plate. The more droplets the higher the electrostatic charge transfer.

One issue I just thought of is that a larger number of small drops would = a larger charge than one big drop.

Any ideas on where to look for info on how to do it?

Cheers
Tim

[shantanu@india]

Is this an academic exercise or a real demand by your customer? From an academic angle you need to measure the rate of flow, the pressure and the nozzle dia to evaluate the degree of atomization.
From the perspective of a real customer demand maybe you can place a jewellery load cell behind the plate and measure the force that is acting on the plate. Force being rate of change of momentum, maybe you can calculate the impact velocity using hydraulic equations. This velocity would be the vectorial addition of all the elemental velocity of the water particles. If you measure the velocity of the fluid at the exit of the nozzle using a standard flowmeter, then from the principle of conservation of energy, the kinetic energy of the total water exiting the nozzle should be more or less equal to the kinetic energy of the atomized spray hitting the plate. This won't give you directly the number of droplets but might put you on the right track.

[TimB]

Thanks

Its for me. It would be impossible to gauge the atomisation as the flow is already atomised and may just pass right through the orifice without further atomisation.

I have heard of force detection using a detector but then you need to know the velocity of the flow and that is not knowable. I was more thinking about getting some idea of the amount of droplets and hence charge.

After talking it through with a friend I came to the conclusion that the best system (not the static charge) is the one my patent application covers. I was just looking to fill in the gaps between the periodic measurements my system uses. Hence the rough gauge.

Thanks
Tim

[chris_cb_uk]

is it possible to direct the droplets through a dark chamber? I'm thinking along the lines of optical smoke detectors which rely on reflecting the infrared from a hidden chamber and being picked up by a detector.  The logic being the more droplets the greater reflection? You could modulate the IR source to reduce ambient light issues.

[TimB]

Hi Chris

Yes I have seriously looked at this. Using say reflectivity or light transmission. With this I'm not looking for high accuracy just repeatable signal.
One issue I'm looking at is the contamination of the detectors, but yes it's a possibility.

If I charged the droplets then detected the charge I may not have the contamination issue.

[chris_cb_uk]

I think if you were looking at passing a laser or light through the path of the  atomisation or relying on reflection behind the outlet you wouldn't get too many issues with contamination.

I wonder if you could use a thermopile sensor to detect the infrared heat difference from the ambient surround compared to the atomised outlet.
One thermopile as a reference and one pointing at the atomised outlet, just trying to think outside the box!

Charging is a way but my concern would be buildup on the detector plate.

I know accuracy isn't of greatest concern but it's useful to be able to have a reference and I'm not sure how easy it would be to measure a changing reference for a charge on a contaminated plate

[top204]

How about using a tiny piezo on the plate that the droplet falls upon? Each drop will cause a very small voltage to be generated, which can then be amplified, or matched. Temperature differences that will cause the piezo to give different voltages can be compensated for and no matter how misty it gets, the piezo will always create something when hit.

[TimB]

HI Les

The issue is that the vibration is also velocity dependent and I will not be able to know the velocity.

Cheers

Tim