code: def __cnc(cls, calib, we_c):
          
          if we_c is None:
              return None
  
          offset_v = calib.pid_elc_mv / 1000.0
  
          response_c = we_c - offset_v                # remove electronic zero
          cnc = response_c / calib.pid_sens_mv        # pid_sens_mv set for ppb or ppm - see PID.init()
  
          return cnc
docs: Compute cnc from weC