Optimal interaction with dial (knob) button taking discrete values for use on touch device

What would be an intuitive way for users to interact with a dial button of 2" of diameter on a touch device taking discrete values from 1.0 to 8.0? The dial is a representation of a real one featured on an industrial turbine. The basic interaction should allow to increment and decrement the value by 0.1.

The dial cannot be made larger and will be used on a web application, hence ruling out finger rotation as a candidate.

Tapping would be easy enough to be intuitive but how would distinguish between a tap to increment and one to decrement?

We're by no means UX masters but we'd like to offer a smooth experience.