how to make a device to measure string tension?

Choice of classical guitar strings and technical issues connected with their use.
riffmeister
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how to make a device to measure string tension?

Post by riffmeister » Mon Apr 28, 2014 10:26 pm

anyone have some ideas?

OldPotter
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Re: how to make a device to measure string tension?

Post by OldPotter » Mon Apr 28, 2014 10:36 pm

In the early years of biplanes, rigging wire were tensioned with a gauge. It seems that a sprung piston was pushed against the wire and its movement measured on a scale.
I have never seen one but have only read of it. I can imagine how it might work but don't know the physics. It had crossed my mind to try to make one, but something else I haven't got round to.

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Tim
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petermc61
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Re: how to make a device to measure string tension?

Post by petermc61 » Mon Apr 28, 2014 10:48 pm

When you find out Riffy, please forward the information to D'Addario!

JohnPierce

Re: how to make a device to measure string tension?

Post by JohnPierce » Tue Apr 29, 2014 3:59 am

OldPotter wrote:In the early years of biplanes, rigging wire were tensioned with a gauge. It seems that a sprung piston was pushed against the wire and its movement measured on a scale.
In the early days, maybe. There are now (and have been for a long time) electronic tensiometers, but for practical purposes, the actual tension is only of much importance for control cables, and probably for the people specifying rigging cable sizes. When you go to do it, you have cable cut to the manual-specified length using manual-specified attachment mechanisms and what you actually do is adjust the rigging so that all the parts are in the correct relationship to each other. Some people actually find this a fun job. Some people like whacking themselves with paddles, too.

On sailboats, big yards and hard core racers may use tensiometers, but mostly it's done by measuring the stretch of a two meter long part of the cable. Essentially, you mark two points two meters apart and then measure how far apart they become when the cable is tensioned. I never found this much of a fun job, either, but it's not nearly as bad as mucking with a biplane.

None of this is likely to be helpful to the OP. The stretch method could be used in principle, but I think you'd need a lot of information to do the math. If you could get very accurate measurements of the deflection that occurred with a precisely known load applied perpendicular to the string, you could calculate it from that. Good luck.

JohnPierce

Re: how to make a device to measure string tension?

Post by JohnPierce » Tue Apr 29, 2014 4:26 am

A simple (sort of) way just occurred to me if what you want to answer is a question like "How much tension must be applied to this particular string to tune it to E?" and so on.

Make a frame with two uprights. One upright has a fixed attachment point for an end of the string. The other upright is guitar-scale-length away from the first one, and has a smooth bar at the top (or a pulley wheel if you want to get fancy). You lead the string from the fixed point over the bar/wheel and attach it to a container of precisely known weight (the vertical distance between the bar/wheel and the attachment to the container should be quite short). Add weights into your container until the string comes up to pitch. Now the trick is to measure the pitch, but one of the electronic tuners that works off of vibrations should be able to do that. This won't be exact, but given the variance in strings of the same nominal diameter and composition, that isn't likely to matter much.

I wouldn't be surprised to find that Trevor Gore or one of the other luthiers who are seriously interested in the physics of guitars has already made something that works better than the above. I also know that places that deal in yarn, small diameter wire, etc, have small tensiometers, but I don't know if they'd work on strings mounted on a guitar, and they'd probably be expensive.

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rojarosguitar
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Re: how to make a device to measure string tension?

Post by rojarosguitar » Tue Apr 29, 2014 6:12 am

I have made a device that allows to measure the string tension. I'll come back later today, when I have more tome to describe it.
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James Lister
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Re: how to make a device to measure string tension?

Post by James Lister » Tue Apr 29, 2014 7:15 am

jwp wrote:Make a frame with two uprights. One upright has a fixed attachment point for an end of the string. The other upright is guitar-scale-length away from the first one, and has a smooth bar at the top (or a pulley wheel if you want to get fancy). You lead the string from the fixed point over the bar/wheel and attach it to a container of precisely known weight (the vertical distance between the bar/wheel and the attachment to the container should be quite short). Add weights into your container until the string comes up to pitch. Now the trick is to measure the pitch, but one of the electronic tuners that works off of vibrations should be able to do that. This won't be exact, but given the variance in strings of the same nominal diameter and composition, that isn't likely to matter much.
This is what I would do, and I don't see any reason why it shouldn't be accurate if done carefully. The pulley wheel shouldn't be necessary, and would make it difficult to determine the exact string length. You could even do it on your guitar if you wanted - although a standard string might not be long enough. Just tie one end to the tuners as normal, pass it over the saddle, through the tie-block hole, over a pulley at the end of the guitar, and hang the weights on the end off the edge of a bench.

James
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es335
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Re: how to make a device to measure string tension?

Post by es335 » Tue Apr 29, 2014 7:59 am

I suppose the test setup will be the least problem. Universal testing machines are established worldwide for testing of elongation and tensile strength of plastic materials including monofiles cords. They are in principle high precision upright laths for very slow speed operation and equipped with linear and tension/compression measurement. The only special issue would be to attach a guitar scale bench simulation, which will most presumably be horizontal with a deflection pulley in the bottom position of the test space of the universal test machine.

Opposed to this the investigation and definition of the test conditions will be quite elaborate and time-consuming. Test temperature, humidity, conditioning or not of the strings before testing, test speed, how to deal with the elongation of the strings - wait until they stabilize and what is considered as "stabilized", how many samples should be tested and how to evaluate the result (median, geometric or arithmetic mean value) etc. etc.!

This will take quite a lot of time but is the only way to establish a standardized test procedure which is sufficient reproducable to be accepted by all string manufacturers. IMHO this looks like a really nice job for a term paper in physics?!? :wink:

riffmeister
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Re: how to make a device to measure string tension?

Post by riffmeister » Tue Apr 29, 2014 11:46 am

jwp wrote:Make a frame with two uprights. One upright has a fixed attachment point for an end of the string. The other upright is guitar-scale-length away from the first one, and has a smooth bar at the top (or a pulley wheel if you want to get fancy). You lead the string from the fixed point over the bar/wheel and attach it to a container of precisely known weight (the vertical distance between the bar/wheel and the attachment to the container should be quite short). Add weights into your container until the string comes up to pitch.
This is exactly what I was thinking. One could take high E strings from several brands/tensions, line them up on the device, use the same weights for each and then measure the relative pitches. This would sort them relative to one another. I am not so interested in knowing the exact tension for each, more interested in determining their relative tensions.

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rojarosguitar
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Re: how to make a device to measure string tension?

Post by rojarosguitar » Tue Apr 29, 2014 4:13 pm

Here's a sketchy description: I took a piece of wood (a square bar of app. 1m length), an electronic luggage scale that (accidentally) also shows negative values, which is tied to one end of the bar. I drilled a peg hole for a violin peg 1:30 ( but it could be a single guitar tuner) and then I have something that is usable as movable saddle. With that I can adjust the scale length.

I tie the string to the luggage scale at one end and to the tuning peg at the other, tune to pitch with the adjusted scale length and null the luggage scale after having the right pitch and the string settled a bit. then I release the violin quickly. The negative reading of the scale is the tension of the string (in kg or lbs to your preference).
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JohnPierce

Re: how to make a device to measure string tension?

Post by JohnPierce » Tue Apr 29, 2014 8:19 pm

riffmeister wrote:I am not so interested in knowing the exact tension for each, more interested in determining their relative tensions.
Ah... In that case, I think James Lister's correct that using a guitar as the test frame is the simplest way. And you could do six at a time, though there are things you'd need to be careful of, primarily slight differences in nut-slot-contact-point to saddle-contact point between the six positions. Testing a single string in all six positions would probably give you enough information about that to allow for it for practical purposes. I have little feel for what's really needed in the way of accuracy in this arena - 30+ years of dealing with everything from astrophysics to biochemistry problems warps one's sense of what "exact" is supposed to mean.

Ramon Amira
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Re: how to make a device to measure string tension?

Post by Ramon Amira » Tue Apr 29, 2014 9:03 pm

Perhaps D'Addario and the other string companies are already using some analogous device. It's certainly possible. It's also possible and probable that they're using some theoretical formula to calculate what should be the tension given all the factors they input. Since that method of calculation sends rockets to the moon, I don't see why it shouldn't be used - and successfully - for something as relatively simple as measuring the tension of a guitar string. The problem is to get all the string manufacturers to adhere to a uniform standard for the calculation.

Ramon
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Trevor Gore
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Re: how to make a device to measure string tension?

Post by Trevor Gore » Tue Apr 29, 2014 11:10 pm

The "tool" is called a monochord. Pythagoras is reputed to have used one. A board, string fixed at one end, two moveable bridges, weights (adjustable) hanging off the string at the other end, usually after it's gone over a pulley. Frequency measurement using one of any number of free frequency measurement devices. If you're serious, make sure you can calibrate everything.

Google images for monochord to get a few ideas of the implementation. How I made and use mine is, of course, documented where you'd expect, including a neat way of measuring string longitudinal stiffness. Very useful for calculating compensation requirements.
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10ae1203

Re: how to make a device to measure string tension?

Post by 10ae1203 » Tue Apr 29, 2014 11:13 pm

Use a fish weighing scale?

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rojarosguitar
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Re: how to make a device to measure string tension?

Post by rojarosguitar » Wed Apr 30, 2014 8:36 am

10ae1203 wrote:Use a fish weighing scale?
Why not if the reading is precise enough and the range big enough...
Music is a big continent with different landscapes and corners. Some of them I do visit frequently, some from time to time and some I know from hearsay only ...

My Youtube Channel is: TheMusicalEvents

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