Is there a good way to determine nozzle size?

Towards the end if last year I designed and built my hot-end. The nozzle is formed from a 0.6mm MIG tip. Following a tip I saw here on the forums, I used a hammer to reduce the aperture size. (The build details and report can be found on my blog here: foraysinto3d.blogspot.com/2012/01/chapter-24-in-which-i-design-and-build.html )

When my prusa comes back from its Christmas holiday to the garage next week I want to start using slic3r, which requires the nozzle diameter.

Is there a reliable and fairly easy way to determine the nozzle size, especially since it is now mounted?

I have tried free-air extrusion, but I seemed to get varied results.

Extrude very slowly into free air and the die swell should be very little so it should come out almost the same size as the nozzle.

Having said that die swollen size is the important parameter, not the nozzle diameter.

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[www.hydraraptor.blogspot.com]

Just and idea : I'd put the nozzle flat on a photocopier, copy it with the greater size change ratio you can use, measure the copy and do some maths.

Thanks for the suggestions. I'll try the free-air extrusion again once I get back up and running. How slow should I go? Somewhere in the region of 10mm/s, or less?

I would use 1mm/s.

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[www.hydraraptor.blogspot.com]

Hammer it on a bit of hard wire of measered diameter, Piano wire is good if you can get holdof some, very hard and shiny.

I'm not so sure that accurate nozzle size is very important.
Knowing the diameter of the filament being driven into the nozzle and the rate at which it is driven are absolutely critical though because they determine the volume. If the nozzle is too wide then it will attempt to overlap the previous lines, too narrow and you will potentially get some snaking of the extrudate. Volume MUST match though or you either won't get proper inter layer bonding or your filiament will back up, jam and your models will be oversized.

Ages ago i drilled my own nozzle, was not sure what size it was, i just did a whole bunch of test prints with infill to determine the correct or best size, in the end it worked out well.

ive found that aluminum nozzles actually get larger over time because they essentially anneal once they are heated to operating temperature, espescially 7000 series alloys.