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I have owned a dynamometric wrench for two years now, and I got it second-hand in the first place. I fear that relying on its indication could be even less accurate than tightening by intuition. For example, I know that oscilloscopes need to be tuned once per year.

How can I measure the accuracy and precision of my wrench? I have a tape measure and a low-end electronic scale (rated 45kg+-10g, but I have noticed deviations as much as 50g between measurements of 2kg parts), plus a wide range of inserts for the wrench. This is a 20Nm-100Nm piece of equipment.

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    Generally, you need to send it to a service which specializes in calibrating equipment such as torque wrenches. Costs something like 30 USD, normally. – Batman Jun 16 '15 at 17:32
  • You can easily set up a test jig if you understand what you're measuring. 20 Nm is a force of 20 Newtons applied to a lever that is one meter long. (1 Newton is about 102 grams.) – Daniel R Hicks Jul 3 '15 at 20:55
  • @DanielRHicks It becomes a bit more tricky if the lever isn't massless, though! You can calculate the lever's torque if you know its mass distribution. – Will Vousden Apr 6 '17 at 11:49
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    @WillVousden - Use a fish scale. Or a rope and pulley. – Daniel R Hicks Apr 6 '17 at 12:49
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    @WillVousden - If you're really paranoid about it, use a wheel. With a rope wrapped around the wheel you're assured that the pull of the weight is always at the exact radius planned and the force is always tangential to the circle. – Daniel R Hicks Apr 6 '17 at 19:21
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How about this:

  • attach an extension that can be clamped in a vise such that the handle of the torque wrench is horizontal, and its own weight is applying torque that attempts to rotate the extension around its axis

  • using some wire or string, attach an empty 1 liter plastic bottle to the handle of the torque wrench

  • adjust the setting of the wrench such that it's own weight (+ the empty bottle) is just enough to cause it to "click"

  • increase the setting by an amount such as 3Nm (on a wrench that is 30 cm long, this would correspond to a weight of 9N which is slightly under 1kg)

  • add water to the bottle until the wrench clicks again

  • measure the mass of the water added and compare to the expected amount based on the torque setting increase and the length of the handle of the wrench

The only missing part is that we don't know if the initial setting (corresponding to the torque of the wrench's own weight) is accurate. You could find the center of the mass and measure its distance from the point where sockets are attached to the wrench, then calculate the torque from the weight and distance.

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I think the best option is to buy a digital torque wrench adapter (for example, this) and check if your wrench "clicks" when the adapter is showing the desired value. It's a bit pricey, but that's how it is, and you can use it many times.

You can also send it to a specialized place that do that for you, such as this, but although I am not sure how much this costs, it should be (much more) pricier than buying the digital adapter, and you will have to do it again eventually.

You can also do something like @Nik suggested, which is similar to what you can find here, but I think this is difficult to do right and worse, it's very error-prone.

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    This only works if (a) the digital torque wrench adapter is itself accurately calibrated at purchase and (b) retains its calibration. – RobS Apr 6 '17 at 22:09
  • Digital tends to be vulnerable to damage from oils penetrating seals, and from dropping or crush damage. A mechanical torque wrench can take much more abuse and remain accurate enough. Plus there's the knack of reading analogue. – Criggie Apr 6 '17 at 23:40

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