Tool for measuring inner dimensions of a tube from an offset position

swooshfactory

Hi,

I'm trying to measure how far a rod that passes through a tube is offset from the center. Assume that the axis of the tube runs parallel to that of the rod. I can only access the interior of the tube from above (it hangs vertically), and the access hole I have is smaller than the radius of the tube. The tube is glass, and can survive moderate amounts of contact.

I have a primitive idea, where I tape an elbow brace to a rod, with a flexible material on top of the brace and at a right angle to the rod. When I place this setup into the tube, I can spin the rod and wait and see at what length of material the material makes contact with the inner wall of the tube. This would tell me the closest point to the offset axis.

This would work, and to fair accuracy if I'm careful about placing the material flush against the rod, but there must be a tool for this, or a better way.

Ideas welcome...

berkeman

I'm not sure that I'm visualizing the setup correctly, but if you had a mark on the center of the top of the rod, would that mark always be visible through your inspection hole?

swooshfactory

The very top of the rod would be sticking out the top of the tube, not inside of the tube.

berkeman

So if it sticks out of the tube, why is it hard to measure it's position relative to the tube?