Surgical tubing

How can I sense/measure the force applied, or the length, of thick rubber surgical tubing as it is pulled/stretched? It will be consistent everywhere along the length as the thickness is the same. Let's assume we only have 1ft to work with before it's pulled to 4ft max. Durable, Cheap...

If you put a stripe mark on it, the mark will move and get wider when the tube is stretched.
You can measure the tube diameter at a fixed distance from one end, it should get smaller with pull or at least my old wrist rocket tubes did.
If you attach one end to a spring and can find how much that stretches, you can know.
You can put a magnet in the tube and measure the field change at a stationary Hall sensor.
You can see if the air pressure inside the tube changes relative to outside.
You can have the pull compress a piezo and check the charge difference across the leads but that’s far trickier than all the above.

Every conductor and semi-conductor I know of changes resistance when under stress (one of the many ways resistance can be changed), that’s how at least some solid state scales work.

Some great ideas thanks! How can I measure the magic marker stripe getting wider cheaply? How can I measure the tube diameter? It's the same nearly everywhere. I don't need a spring, the tube is similar enough. I can just shove a magnet and hall sensor in the tube. They don't need to be stationary with respect to the outside world. How far apart would they work? If only 5mm then they could start at only 1mm apart. How many different values would I get when stretching 4x the length? In other words what's the resolution of this method? Air pressure would be cool, but more expensive? How can a pull, compress a piezo? Mechanically. Do you think rubber would change it's resistance when stretched? It is getting thinner.

Great ideas for me to start thinking about. More details please? Cheap and durable.

How about alternating black/clear stripes. You could count how many go past a sensor. They would get wider as the tube was stretched, but could still give a useful indication.

Great idea! But I'd like the sensor to be inside the tube. There is no stationary point with a different frame of reference, outside of the tube. The sensor could be outside, but there's nothing to attach it to, besides the tube itself. There is no other object besides the tube. Understand? You could put your idea inside it, but what if the marks passed very slowly? The light would change slowly. Maybe not as much as the external lighting.

Ok. I didn't get the lack of a support structure. What is the internal diameter of the tubing in the unstretched state? Is it (or could it be made) wide enough to fit an ultrasonic sensor in it? Some of them are reasonably small.

I could buy thicker tubing, there is not reasonable limit there. Then what?

I wondered if you could use the ultrasonic sensor inside the tubing. The far end would need to be closed, but you might get an echo back. I haven't put a lot of thought into it. Just putting ideas into the hat.

That's a great idea! Simple. It wouldn't have to go 4 ft. Just start it 2" away. When it stretches it'll be only 8". It may have to be in a straight line, but that's not a problem, it will be. I wonder what the precision would be at this distance? Will try it! What is the smallest sensor you can find? I can use the cheap circuit board, with a different transducer. I have some small ones. Wow perfect!

This has only one sender/receiver: http://adafruit.com/products/172?gclid=CL3U--7Ft68CFbMEQAod_EePgg

The cheap ones have 2. Can I modify those to work with 1?

This seems quite reasonable.

http://www.maplin.co.uk/ultrasonic-transceiver-40khz-10mm-open-aluminium-591660

I would think you could get something similar in your part of the world.

I have plenty of those, but which circuit? The ones from Ebay, I have, require 2.

The spec says it can be used as a transmitter and receiver. You'll need one of the electronics experts for any more info. I'm certainly not of of those.

Could you not use something like this:

http://adafruit.com/products/519?gclid=CMjGo5zNt68CFWcHRQoduxrShw

?

Just as a matter of interest, what is pulling on the tubing?

I hadn't thought about putting the Hall in the tube. That's good too but I thought rather on the post you slip the end of the tube over if it was a wrist rocket () or made -as- the end of the post. That kind of fitting doesn't pinch or otherwise deform the tube. You'd be measuring stretch at the end.

Oh, hang on. Wrist rockets aren't like they were in the 60's. These are: http://www.amazon.com/Barnett-17018-Black-Widow-Slingshot/dp/B000PWBIKY/ref=pd_sxp_grid_pt_1_1

But if you were just going to pull the tube with your fingers then yeah that's good, put it inside! When the tube stretches the distance between the Hall and magnet widens, viola! But make sure there's enough extra untangled, sure to be free-moving wire going to the sensor so power and signal wires don't get 'stretched'. Maybe something with graphite powder?

A linear Hall sensor can measure very small fields and a small neodymium magnet should be good to inches at least and resolution 0-1023 (field drop off is inverse cube though, maybe you want mms at start).

You could watch a mark move and widen with a 1k linear photo-array.

@crosh- Would have been the best solution, but it takes a minute to shrink back.

I wanted to use it as an input device like a mouse, but now I'm having other ideas too! R/C glider launch tension. Paraglider control lines. I like the hall effect sensor best so far. Any more creative ideas before I start hooking it up? Will try the ultrasonic sensor too.

You might make a strain gage for one end of the tube using a force sensing resistor like below.

http://www.google.com/search?num=100&hl=en&lr=&newwindow=1&safe=off&tbs=p_ord%3Ar&tbm=shop&q=force+sensing+resistor&oq=force+sencing+resistor&aq=0l&aqi=g-l1&aql=&gs_l=serp.1.0.0i13.75313l90250l0l93594l13l13l0l0l0l0l234l1595l6j6j1l13l0.frgbld.

sbright33: @crosh- Would have been the best solution, but it takes a minute to shrink back.

Hmm - didn't see that, but I could see it would be a problem; I'm not sure if this is the same stuff or not, but I didn't read of the same problem (though it is mentioned that return of the readings after release to the original unstretched value takes time, but not how much time):

http://www.imagesco.com/sensors/stretch-sensor.html

Have you attempted to measure the resistance of the tubing itself (likely infinite, but you could try)? Or it's capacitance (if any)? Or maybe you could make a form of capacitor by adding conductors to the ends (and/or adding some kind of conductive gel to the inside of the tubing)? Not sure if the change (if any) would be linear or something else, but it might be worth looking into...

sbright33: How can I sense/measure the force applied, or the length, of thick rubber surgical tubing as it is pulled/stretched?

You could measure the force using a load cell (force transducer).

You could measure the length using a string pot to detect relative movement between two points (or use any other distance sensor you prefer).