Why would one need a two foot long micrometer?

Factual Questions
1

My dad loves and collects micrometers. He got one a while ago that’s almost 2 feet long! I’ll assume that most of you know what a micrometer is used for, why then might someone need to know something that specific over such a distance?

2

Even large things sometimes need to be machined and measured to tight tolerances. For example, the cylinders in the huge diesel engines in railroad locomotives. A few mils either way can mean the difference between seizing, running smooth or leaking fluids.

3

I’m thinkin’ rocket parts, or something like that. An application requiring large pieces with very tight tolerances.

4

Sidelight.

The use of a micrometer that size must require some interesting precautions. For example, the total length of the frame must be at least 40 inches and with the temperature coefficient of expansion for steel, a change of about 1[sup]o[/sup]C would change the space between the jaws by 1/10 of a thousandth of an inch. Any measurement standards people on the board to tell us how accuracy was maintained? Of course, the frame could be Invar or some other low coeffient metal, but the problem for really accurate measurements is still there.

5

And the NIST has had its budget cut to such an extent that they will eliminate 2K IIRC jobs. First time in their history that they have had to lay people off. I can’t find the cite for the story, but I will keep looking.

6

This?

7

You think that’s overly precise? The LISA spacecraft will be tracking changes in lengths of picometers, over a baseline of five million kilometers. Admittedly, the length itself is not measured that precisely, but it’s still damned impressive.

8

…waaaiit. If these big micrometers have to be accurate to the nearest something, then they’d better be accurate. Better check the scale with another big micrometer. Then, though, you’d want to make sure that that one was right too…

:stuck_out_tongue:

9

Micrometers with a span greater than 1" come with a secondary standard included so that they can be recalibrated from time to time. I suppose that for the this one the standard could be used during every measurement to reset to zero after letting the micrometer come to the ambient temperature. The standard is low temperature coeffient material and the longer ones are supposed to be used when they are at some pre-determined temperature.