I know this is some measuring tool, but I'm not sure what scale it's measuring.

[GregG]

I purchased this tool on Dave's Watch Parts a few weeks ago.  He advertised it as a Douzieme gauge (12 Douzieme = 1 ligne = approx 2.25 mm).  However, the numbers don't tie out.  I have something that measures 4.6 mm.  That should equal about 24.5 Douzieme, but the scale on this tool measures about 41.5 units.  Does anyone know what system it could be measuring in?

[RichardHarris123]

I don't know but could it be mm? The number representing 1 mm, 2 mm etc and the devisions 0.1 mm. If so the measurement is out by  0.45 mm.

I was correct, see photo. 

[GregG]

When the jaws are empty, the scale on the bottom is pretty much perfectly zeroed.  I suppose if I measure something else and it's also off by 0.45, then it would be millimeters.

EDIT: Now that I think about it.  The bottom scale has a hard stop on it.  When the jaws are empty, if it's not perfectly closed (a tiny 0.45 mm gap), then the zero would be at the hard stop, and the scale would be off.

Edited July 16, 2023 by GregG

[RichardHarris123]

Let me know your findings. 

[GregG]

Unfortunately, I think this is just a regular old millimeter gauge.  At rest, the bottom gauge points just under the 0, less than half a millimeter, so there is some error.  But the larger the object I measured, the greater the deviation from its true value.  I think the small error in the jaws is being amplified by the large lever arm of the bottom scale.

Edited July 17, 2023 by GregG

[HectorLooi]

Such calipers were common in many trades. In dentistry, we used the Iwanson gauge, which measured in 0.1 mm increments. 

There is quite a bit of spring tension in the jaws when measuring an object. So when there is nothing between the jaws and the indicator arm is hitting its backstop, it's impossible to tell if the zero point is accurate. Hence we test for zero by measuring an object of known thickness. For example, a 1.0 mm feeler gauge. The indicator arm can be bent to adjust to the correct reading.

Hope this makes sense.