# Capacitance of Transducers with a water-filled beaker

1. Jun 22, 2015

### nst.john

I am making a sonoluminescence experiment and part of it is two transducers touching a glass beaker with water. However, I don't know exactly how to measure the capacitance. With the picture attached, the places marked in green with "C?" written on them are where I was told there would be capacitance in the overall circuit to measure. Can you tell me if these places are right, and if there are more places where capacitance will be held, and how to measure the capacitance of these places accurately. I need this so I can make an inductor coil based on the capacitave reactance of the transducers so I can achieve electrical resonance in the circuit. Thank you!

2. Jun 23, 2015

### Jeff Rosenbury

What you have should work. The capacitance will be very small. There are capacitance meters to make the measurement.

Is the capacitor supposed to be the transponder?

Also you should be aware that water has a highly variable dielectric constant. Due to hydrogen bonding with surfaces (or pretty much anything else) it can change due to geometry/composition/ temperature/whatever.

Also if the transducers are electric, they might mess with your measurements. You might consider isolating them while measuring the capacitance.

3. Jun 23, 2015

### nst.john

How should I isolate them. And is there a way to figure out the dieletric constant of the water with the glass then? Using an equation of something. Lastly, is there a simple way to measure capacitance without an LCR meter? Because I don't want to spend too much money on an LCR meter and I know there are cheap ones but I don't know which cheap ones are of good quality. Could you recommend an LCR meter as well?

4. Jun 23, 2015

### nst.john

The capacitor is supposed to be the transducers

5. Jun 23, 2015

### tech99

Some digital multimeters have a capacitance function, which I have found to work quite well. I am sure you will need to have a means of adjusting the inductance a little in order to hit the resonance. You need to adjust the circuit so the electrical resonance coincides with the mechanical resonance. Maybe use a CRO with high impedance probe and a variable signal generator?

6. Jun 23, 2015

### nst.john

7. Jun 23, 2015

### nst.john

And how can I tell if the electrical resonance coincides with the mechanical resonance? You mean making sure the frequency I'm using is used?

8. Jun 24, 2015

### tech99

The LCR meter you show looks very useful for your task. It can read down to 1pF and 1uH. Remember these are the values measured at some arbitrary low frequency, so will change a little at higher frequencies.
The system you are using has a mechanical resonance in the transducer and an electrical resonance caused by the inductor and stray capacitance. I think you could locate these frequencies using a signal generator with a suitable RF voltmeter. The objective is that the two resonances coincide, so that is where you need to adjust L slightly.

9. Jun 24, 2015

### nst.john

Sorry just to clarify. I'm using it at a frequency of 40KHz would it still be accurate at that high a frequency? And that's true I'll measure for that

10. Jun 24, 2015

### nst.john

So how would I exactly measure these frequencies?

11. Jun 24, 2015

### nst.john

Also, is there a way to calculate the slight change of inductance and capacitance at higher frequencies. The LCR meter does it at 1KHz, so how would I check the values at 40KHz?

12. Jun 24, 2015

### tech99

The problem arises because across the inductor itself there is a small stray capacitance. This has the effect of inflating the inductive reactance as the frequency is raised. I think at 1 kHz the values will be pretty close, but as I have mentioned, I feel that slight adjustment, for instance by moving the core slightly, would be a good idea.

13. Jun 24, 2015

### nst.john

Ok that sounds good I will tinker around with it. Thank you. After I get the LCR meter it should make it easier

14. Jun 24, 2015

### Baluncore

Since the transducer will be resonant at some frequency, transducer reactance will be a tuned function that is dependent on frequency. It will not be possible to accurately measure the transducer capacitive reactance at a frequency other than that you intend to use.

I would expect your transducers will need to operate at a frequency that generates standing pressure waves in the water. Once you have found a resonant frequency of the sphere of water, you should then adjust the inductive matching reactance to minimise the drive current to the transducer. That minimum will occur when the drive voltage and current are in phase.

15. Jun 24, 2015

### nst.john

Sounds like a plan. Thanks a lot for the help!