Compressor Surge Control Flow Measurement

I received this question on one of my blog posts, Surge Control Considerations in Centrifugal Compressors:

dear jim

i have a question? the PV equation of woodward A/S controller have firstly, the flow (orifice flow) from second to third stage . the transferencial function of this flow transmiter is cuadratic or lineal? for the analog output.?

very thanks for your time.

I appreciate your thoughts in how to respond.