How is Eva foam hardness measured in degrees?

Usually EVA sponge foam is measured in degrees of hardness using an Asker C gauge.

There are two ways to measure the hardness, with skin on, if the EVA has been moulded or skin off if the EVA is not moulded and the surface does not have a skin, i.e it has been scoured.

The difference, usually, between skin on and skin off measurement is about 10 degrees, the skin off being the lower measurement.