Each hole in our portafilter baskets can have an error of +/-.05mm. Over the many holes in a basket, the average error mostly cancels out (some ever so slightly larger, some ever so slightly smaller).
However, if a basket only has 1 hole, and we want to calibrate against it, we're looking for 4x better tolerance. This is quite hard to achieve, which is why calibrated orifices are expensive
http://www.theleeco.com/products/pre...s/restrictors/ and made by very few companies. The best ones, that we use, are made for the US military, for jet fuel injection systems.
As to measuring "the actual hole size" with a microscope, I don't know of any practical way to do that. Every basket would have a different labelled hole size, it wouldn't be repeatable, and even this assumes that the hole is perfectly circular, which at these dimensions, isn't usually the case. Here's a photo example I took of a competitor's basket some time ago.
Attachment 34806
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