This nifty gauge is a finicky thing to use. . .
Not unexpected, but I didn't get any measurements yet I am confident in.
Nearly all precision instruments used in the machinist trade take time to get a "feel" for the tool. Accurate cutting only happens after repeatable, accurate measuring. That is what sets apart a stellar artisan from the group.
Understand all the forces acting on the instrument and use a technique or opposite force to negate it (note Ken61's spring).
Gravity is the biggest culprit. If using the tool in a horizontal orientation, the tool needs calibrated in that position. Then don't rotate the tool or the calibration will likely change as the direction of force changed. This is why I feel it is best to use this type instrument in the vertical position.
Also, temperature will move metal, especial long slender shapes like these rods. If you grab the tool from one location(car, shop, ect...) and take it to another, it needs to stabilize at the new location before calibrating and taking measurements. Once you are calibrated, a warm hand on one spot on the instrument may again shift the readings. Gloves may be a help?(check Drew's isolation hook on the block).
Play with the new toy on known thicknesses(shim stock/feeler gauges) until you get repeatable readings. Once you have a "feel" for the instrument, measure some tubes.
John
