# Theory/construction of conductance measuring device (films)?

1. Dec 5, 2016

### Milchy

Hi,

So I have this situation, I'm totally inexperienced with anything electricity. But as faith has it sometimes, suddenly I need a device that would be able to measure the electrical conductivity of a thin film.

Is there anyone who could guide me what would be the essential parts of a system to do this kind of thing? Mainly the "box" that would be sending and receiving the signals - what is it and what are the main characteristics it would need inputing? If Im buying it - how fancy and expensive can it get? And if I decide to wire one - is it possible to a total newbie without getting myself killed?

Also depending on the above, how do i choose the wiring and material to make contact with the films, etc? I imagine the plates/electrodes would be like a sorta sandwich maker: where I put the film in between them. The films would be of thickness around 1 mm and less, the materials are household polymers: PE, PP, PP, PVC etc..

So far what I have understood (actually i dont understand, but it seems this is what i should do):

Apply a single frequency voltage and then measure the amplitude and phase shift of the resulting current. (But is it AC or DC? How high do I need the voltage to be - or is it something I'll have to gradually increase till I get some readings?)

Also, the films arent really conductors, depending on what I decide to do to them, whatever I have to measure will probably be low in amplitude/size of units I'll probably also need to amplify the signal somehow?

Sorry in advance if I said lots of stupid things.

EDIT:

On a side note, whatever answers I get, would the same technique be applicable for conductance measuring of liquid solutions of the mentioned polymers? It should be the same? I probably just need the main device (brains) of the operation and the rest is just the right electrodes/plates

Last edited: Dec 5, 2016
2. Dec 5, 2016

### Nidum

The conductivity of most common materials can be found from reference sources - no need to measure anything .

If you want to try some experiments then put a small metal plate on top and bottom of a same size sample and measure resistance using a test meter . Simple sums then give the actual conductivity .

I have assumed that you only want to find the through thickness conductivity ?

3. Dec 5, 2016

### Milchy

Yes, I need it for some experiments as the films will be embedded with different substances. Will a standard test meter even work? I mean, aren`t those generally made for more conductive materials?

4. Dec 5, 2016

### Nidum

An off the shelf test meter will measure down to about 50MΩ . Special insulation meters can measure down into the GΩ range .

Once the through thickness resistance of a small sample of sheet material drops much below about 1 MΩ the actual value doesn't usually matter very much anyway in practical applications .

You now seem to be asking a different question . What is it that you actually want to do ?

5. Dec 5, 2016

### Milchy

Different question? The question is the same, how do I measure the electrical conductance of a film. But if its as easy as connecting a few metal plates to a test meter and slipping the film between them, then I might have been overthinking this. I've never held a test meter in my hands, but can it vary the amplitude and frequency of the current that you pass through the film?

6. Dec 5, 2016

7. Dec 5, 2016

### Nidum

At frequencies significantly above DC and where the sheet material is practically an insulator then you are really into the area of measuring capacitance rather than simple resistance . If this is relevant to what you are doing then let me know .

8. Dec 5, 2016

### Milchy

Im not sure about capacitance, probably not something I need. But a question, supposedly the film is uniform in its electrical properties and thickness across the whole specimen. Would I observe any different reading from measuring it the displayed ways?

9. Dec 5, 2016

### f95toli

You have to more specific about what you want.

*Do you need the surface conductivity of the actual film (i,e, conductivity per square)?
*Are there (good) contacts on the film?
*Will you be using the van der Pauw method?
*What is the expected conductance range?
*Which frequency range?

This is very much a standard measurement and there are lots of commercial solutions. A "normal" measurement of e.g. a metal thin film would use a four-point I-V measurement using van der Pauw method. The instrumentation should ideally be something like a Sourcemeter; or at the very least a benchtop multimeter which will allow you to do four-point measurements

Also, what you've drawing looks more like a foil. "Thin film" usually implies something deposited on some type of substrate.

10. Dec 5, 2016

### Nidum

The through thickness resistance value depends on the conduction area . So if you have large area contact electrodes top and bottom the resistance will be lower than if you had small area contact electrodes .

If you are working on some idea or actual device where this problem matters then tell us how it works and we'll probably be able to give you more focussed answers .

Last edited: Dec 5, 2016
11. Dec 8, 2016

### Milchy

It is a film - its a transparent polymer slide with a thin film of particle coating on top

1) Yes I would want the surface conductivity
2) No specific contacts, plain surface area is all I got
3) Yes
4) I expect 0.01 μS - 1 S/m
5) ~ 0.1 - 10kHz
6) Uncertain at the moment, I would like to know my options and get away with the least amount of spending.

Last edited: Dec 8, 2016
12. Dec 8, 2016

### Baluncore

How did you get the idea that you needed to measure the electrical conductivity of a thin film ?
Do you have any references to where such a technique is being used ?

Are you studying the polymer, what soaks into or is forced into the polymer, or what is coated or deposited onto the surface of the polymer ?

13. Dec 8, 2016

I thought I had responded to this - but do not see my post...It would be best ( in addition to very helpful) to identify a test specification for what you are wanting to do, for example : ASTM D257, if you are in an industry these can often be found by looking at competitive, or similar products datasheets.

With the appropriate standard in hand you can then find an instrument manufacturer : TREK

14. Dec 9, 2016

### Milchy

I need to measure the conductivity in order to evaluate the suitable application of the films, also to tailor the deposition process for better results.

I´m studying the deposited coat on the polymer surface.

15. Dec 9, 2016

### Baluncore

From that I expect you should be measuring conduction along the film on one polymer surface.

The type of measurement, DC/AC, the frequency and what voltage and current to use will depend on the film material. If it is a carbon or inert metal film then DC measurements will probably be OK. If the film is an ionic gel then you will need to use an AC measurement with carefully regulated voltages to prevent electro-chemistry or polarisation while making the measurement.

For inert films you could start experimenting by placing two parallel electrodes on the film surface. The electrodes would be about the same length as their separation. The electrode separation should be maybe 100 to 1000 times the thickness of the film. Doubling the electrode separation doubles the resistance, doubling the electrode length halves the resistance, so for a thin film, the scale would not be important so long as the electrode separation to length ratio remains constant. In effect you will be measuring the relative thickness of the film.
You will need some way to maintain contact along the full length of the electrodes. That will require a flexible pressure pad or flexible electrodes. You should make some form of simple electrodes and start measuring resistance with a cheap digital multimeter. Let us know what range of resistance measurements you get.

The geometry of the fringing fields at the ends of the electrodes, their proximity to the edge of the film and contact resistance will cause some variation in readings. If higher accuracy is required there are ways to eliminate those variations.

What type of film materials might be involved? How thick are the films?