R sqare

Resistance to what conduction, friction ....

I am looking for the resistance in Ohms Ω per square it is a conductive cloth used for creating heating elements.

To do this test on the table with the cut out swatches. How do you preform this test?

Our current testing jig is a frame with spring loaded clamps to hold the sample it is cut in a rectangle of 220MM x 100MM the test leads are mounted to the metallic base plates so it take a reading across the sample. We then divide the reading by two to get the R-Square of the material This seems to give us accurate results but it is a destructive test that we would like to work around by testing on the web. Rough sketch below

Some where on you equipment the sheet runs over rollers. Could two spring loaded arms hold contact against the sheet at the crown of one of these roller.

Or install two isolated rollers hook test equipment to them. Taking the resistance between them as the sheet is run thru.

I like the rollers idea. Wouldn't need to be too sophisticated either.

Is there a "non-contact" way to do this?

Seen this at my old job on semiconductive web ....They used a hand-held volt mater, but, I'm not sure about the sensors. I would try leaf or brush and see if the results correlate.

The terminology and units being used don't seem to make sense.

Hi Tornado,

We used to do the same kind of test where I used to work. I was asking "ohms per square WHAT?"

This article explains it:

http://en.wikipedia.org/wiki/Sheet_resistance

But R square ≠ R per square....

Actually, you even spelled "sqare" correctly for him; but how many OPs do you know that get the thread title right?

You could try contacting Strandberg Engineering, they have a moisture monitor that uses resistance to measure moisture content in moving fabric.It would be simple to convert units to ohms instead.

You will need to provide them with the normal resistance of the material.

http://www.strandberg.com/

Good Luck!

There is a paper for measuring resistivity and I don't have a copy of it anymore, but it basically involves putting down 4 probes. On diagonals, you put a current source and a voltage source. You have a way to rotate the probes 90 deg electrically.

We actually took two I-V curves in the 0 and 90 positions and then processed it through a formula. It is independent of geometry of the 4 probes.

There is another technique which is easier. It basically requires 4 in-line probes with the same spacing. The outer two are fed with a current source. A voltage is measured on the middle two. These probes were extremely close together in our application. It was just a contact block with spring loaded probes in it.

You may be able to use PCB probing probes like those from pylon. Google "pogo probes"

Again, I forget the formulas involved, but the meter read directly in ohms/sq because of the current chosen. We always checked +- currents.

So you would just have to stop the line. Place the contact block anywhere. Mak the measurement and go.

If you had an infinite sheet, the resistance from any point to any other point would be the same (and you can regard the two points as opposite corners of a square). That's why this measure is a convenient one. The way you are doing the test at the moment conveniently "removes" the infinite nature of the sheet in a way which happens to give you double the resistance.

Picking any two convenient points as the cloth crosses a roller and working out the multiplying factor will be just as valid (as the way you're doing it now).

Choosing two points which are as close as practically possible to each other will give a result which is closest to the real value.

Thank you all for the help with the problem and the spelling. My wife tells me I do no proof read enough, "her proof is here" Anyway we are going to start some prototype work with the roller setups you have suggested and see what I can come up with.

Best regards

R W

RW, I don't know if it will help at all, I haven't checked it out, but I just got an email from Keithley Instruments (I get way too much junk mail!) that is promoting a new webinar they have available, titled:

"Making Resistivity measurements in bulk materials"

If you go to their site, and don't find the info needed to get to that webinar, send me a PM and I'll forward the email to you, that should get you there.

Tom D.

I'd also suggest the roller system. We set one up to measure inductance of rolling grain to monitor the moisture. I'd also add that whilst prototyping, that you conjure up a device to "tag" the spot when it gets into area of the run that doesn't meet spec. S simple dye shooter which would trip when the alarm trips would do. It won't do much good to know the R failure without knowing where.

We measure area resistance with a 4 point probe similar to that described by K.I.S.S. The reading is direct, no calculation involved. Your roll speed is not that fast, could you find a point where the probe could contact the material & travel with it for a few seconds to take a reading. On a flat surface this could even be done by hand.