[EricH]

Well, there is one. IIRC, it's included in the KOP. It's definitely required if you're going to use pneumatics (<R72> in the 2011 rules). It goes into the code, and if the pressure is too low, the compressor turns on.

If you need something more than that, you can refer to the same sensor. Note that it is on the high-pressure side of the circuit, so you may also want one for the low-pressure side, but that's your choice (and you could reasonably expect a more detailed inspection due to having two sensors--which one really runs the compressor and all that sort of thing).