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Does anybody know what kind of sensors are used in these scales?

I've found lots of S-shaped tension sensors like this one.

I've Googled quite thoroughly for few hours and can not find anything matching my requirements.

So they are:

  • Small;
  • Precise;
  • Sub 100g scale, ideally sub 50g;
  • Relatively cheap (under $70).

Of course I have an option to just buy and tear down that hook scale I mentioned, but I hope you guys know what can be used there.

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  • \$\begingroup\$ Times like this is when I wish I still had my set of Omega catalogs around... \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 25 '14 at 3:22
  • \$\begingroup\$ Thanks all, problem was solved by ripping the guts out of cheap chinese pocket scales. \$\endgroup\$ – user37741 Mar 25 '14 at 15:41
  • \$\begingroup\$ Would you be able to link to a photo of the actual guts? For posterity. \$\endgroup\$ – Spehro Pefhany Mar 25 '14 at 15:51
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They probably use strain gauges - see this wiki page.

enter image description here

There are thousands if not millions of variants and they work by altering their electrical resistance when subjected to mechanical force.

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It's almost surely some kind of load cell, which uses strain gauges in a bridge configuration as in Andy's answer.

It's not obvious to me what the most appropriate shape would be to fit into that case, have axial load attachment points, and have appropriate strength and sensitivity (it has to bend for there to be strain to measure, but not so much there is danger of permanent deformation (past the yield point in ME terms).

The S shape would work in tension as well as compression (opposite polarity output) but that type you linked too seems too insensitive for your application. It's possible the construction is similar inside, just much thinner metal.

Try search terms such as tension micro load cell (and pull load cell for the ESL results). Also "axial load cell".

The main disadvantage with tearing down the cheap sensor is that the output voltage won't be calibrated, which does not strike me as that big a deal if you have some reference weights to hang off of it.

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