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How might I go about measuring the tension in a line of fishing wire? I'm mostly interested in determining when the line is in tension vs when it's slack.

The application is a boat-mounted depth-finder. A weighted line could be lowered into the water via a stepper motor. When the weight hits the bottom, the line goes slack and the depth is determined by the stepper motor's rotations.

I'm considering the feasibility of this approach vs a sonar/fishfinder-type setup. A mechanical measurement would be kind of fun, in a way, despite its many flaws (accuracy, waterproofing..). Thanks!

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    \$\begingroup\$ Considering you are trying to do something that has been largely superceded by "contactless" methods for what I can only see as obvious reasons, you are obviously into the fun side of things... Tried and tested method is to count the knots as it descends under gravity with a little friction in the operatives hand holding it back; as soon as it hits the bottom the hand-friction prevents the rope descending further. Depth = number of knots. I hope there are better answers!! \$\endgroup\$ – Andy aka Mar 28 '13 at 22:39
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    \$\begingroup\$ No stepper motors! It would take years to reach the bottom. Use a encoder. \$\endgroup\$ – drzymala Mar 28 '13 at 22:45
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    \$\begingroup\$ @Andyaka, love it! And the mechanism rings a bell at each knot, and an Aurduino (sp?) sound recognition routine counts the clangs.. \$\endgroup\$ – Bobbi Bennett Mar 28 '13 at 22:52
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    \$\begingroup\$ Assuming that you're going to be lowering your mass off the gunwale (side of the boat), a moderate swell will rock the boat enough such that the line will go from periods of high tension (accelerating the mass upward as the boat tips away from the line) to slack or near-slack as the boat tips toward the line. You may need a low pass filter to eliminate swell-induced transients (even if you're just using a switch). \$\endgroup\$ – HikeOnPast Mar 29 '13 at 2:20
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    \$\begingroup\$ Paint the rope different colours every 10cm. Use RGB LED and photodiode to measure colour - changing colour forward sequence: depth += 10cm, colour static: hit bottom. :P \$\endgroup\$ – geometrikal Mar 29 '13 at 3:09
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This really needs a drawing, but alas, I don't have time to draw one up. Here's a super simple method of detecting a slack line:

Start with two plates/pulleys/rods/whatever. Let's assume they are plates. There is a hole in each plate. Run the line through the hole in each plate. When the line is tight, the line forms a nice straight line between the two holes. When the line is slack, it is not a straight line.

Halfway between the plates, put a microswitch. When the line is tight, the switch is pressed. When the line is not tight, the switch is not pressed. You might need a lever or something for the line to push against-- which in turn pushes the switch. I would probably make this lever have a small hole in it that the line goes through. The length of the lever, and the overall mechanics of things, will determine how much force is required to press the switch (and thus how much weight you need on the line).

This would be super simple to 3-D print. Just saying.

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  • \$\begingroup\$ Yes! Use a microswitch. Keep it simple. \$\endgroup\$ – Rocketmagnet Mar 28 '13 at 23:45
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If I were brainstorming this, a buckle transducer would certainly come up. This is sometimes use to measure tension in muscles. A muscle would be pulled through some sort of frame, and a pin inserted in such a way as to act like a fulcrum. Strain gages can be placed just about anywhere. Not necessarily an entirely thought out solution. For example, you'd need to rig this up so the line can go through freely. Maybe you could introduce rollers someplace. This might limit sites for strain gages. Probably plenty of other problems, but there may be enough here to merit some thought.

Dennerlein, Journal of Hand Therapy.  18(2005) 120-127
Dennerlein, Journal of Hand Therapy. 18(2005) 120-127

Science Direct Link

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  • \$\begingroup\$ I personally would not want my muscles pulled through a frame! :) Otherwise, if you replace the strain gauge with a microswitch then this is identical to my solution. \$\endgroup\$ – user3624 Mar 29 '13 at 0:40
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    \$\begingroup\$ Looking back now, I get it. Plusses and minuses to both methods, but I've seen so much gear fail on fishing boats, I'll stick to having less moving parts. \$\endgroup\$ – Scott Seidman Mar 29 '13 at 1:24
  • \$\begingroup\$ Cool! Another 3d-printable idea..thanks! \$\endgroup\$ – Matt Mar 29 '13 at 19:51
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For measuring the depth use a encoder. And for sensing the bottom you can take two weights separated by a very short line. And on this line there you can place a "pull the wire" switch. So when the weights are floating the switch is on. When the first weight touches the bottom, the wire between those weights is loose so it turns the switch off. Or use a "normally closed switch".

But don't take stepping mottors into account, because they are too slow.


Edit:

Fiber Optics!

A better approach would be to sink down a microcontroller (battery powered) which will send the measured water pressure via fiber-wire (I think, the fishing wire is like a optical fiber isn't it? :] ) And the bottom is reached if:

  1. The pressure stops rising
  2. A switch hits the bottom
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  • \$\begingroup\$ Cool -- we were a bit hesitant to drop too much equipment into the depths (worried about snags) but this makes sense and might work, thanks! \$\endgroup\$ – Matt Mar 29 '13 at 19:54
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I'm thinking simpler... like a clapper on a narrow bell... hang it off a spring, a platform, a bendable rod that the line feeds thru... as long as the line is bearing the weight of the sinker, it pulls it off center, and the clapper touches the side of the bell... when it hits bottom, the platform or spring or whatever returns to true vertical, and doesn't touch the side of the bell (opening the circuit)... might look at old "tilt" switches on pinball machines...

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