Measuring system with accuracy of three decimals

Measure distance in kilometres. Then an accuracy of 3 decimals is 1 metre.

How about an interferometer? 1 um is dead easy, and 10's of nanometers with a bit of work. (Is nano still sexy?)

George H.

Aren't those pretty tricky to align and keep aligned? Other than on an optical bench, that is.

-- Cheers, James Arthur

Oh sure you've got to have an optical table. The OP mentioned a laser and I kinda assumed the optical bread board. I guess for the novice the alignment can be tricky... though once you've done it a few times it's as easy as riding a bike. Once it's alinged and bolted down things are fine. We sell an interferometer kit. (Don't ask the price.) We ship it around the country to various trade shows. I'm amazed, but it has always arrived in near perfect alingment. Turn on the laser and start counting fringes.

George H.

Start by making sure you understand accuracy vs precision.

Ed

How about a photogate and count sheep by the thousand?

You can measure sheep singly to 3 dp. 1.000, 2.000, 3.000 zzz..

Yeah, you could do it that way, simpler to implement too - great idea.

The benefits of brainstorming.

Between us we have solved the OPs problem.

Maybe, I suspect his problems may be a little deeper than those explored here.

Stays aligned? Cool! That sounds like great fun.

-- Cheers, James Arthur

ase

Here's a picture of a few of the pieces.

The mirror mounts are custom made and designed to be 'rock solid'. (single piece of aluminum with a flexure hinge.) Each mount has a single degree of freedom. I guess the 'cool' thing about it is how small a change you can see. When the bread board is a standard 1/2" aluminum slab... (no stiffening members) you can see the fringe pattern change when a single 1/4-20 screw is placed on the table.

George H

Count the legs & divide by four for better error checking.

Given the OP couldn't define his requirement dividing by four is probably beyond his skill set.

Some people are beyond help. :(