Catheter apparatuses, systems, and methods for achieving renal neuromodulation by intravascular access are disclosed herein. In one embodiment, the invention relates to a sensor apparatus for sensing a chemical in a vapor emitted by a sub-surface fluid sample. A pressure control valve controls a pressure supplied to the second working piston surface. The voltage Vramp will vary with the input voltage Vin, output voltage Vout, and duty, therefore it will reduce the influence of the input voltage Vin or output voltage Vout on the modulation gain and loop gain, even to keep the modulation gain and loop gain in constant value.