Active surface cleaning for a sensor

A sensor for monitoring a species which is a component of a fluid, the sensor including:

• an internal reflection window configured in use for contacting with the fluid;
• a mid-infrared light source configured to direct a beam of mid-infrared radiation into the internal reflection window and produce attenuated internal reflection of the beam at an interface between the internal reflection window and the fluid;
• a first narrow bandpass filter configured to receive from the internal reflection window the attenuated internal reflection of the beam and to filter the received attenuated internal reflection of the beam, wherein the first narrow bandpass filter is configured to preferentially transmit mid-infrared radiation over a band of wavelengths corresponding to an absorbance peak of the species;
• an infrared detector for detecting filtered mid-infrared radiation transmitted through the first narrow bandpass filter;
• a processor arrangement, operably coupled to the infrared detector and configured to measure an intensity of the detected mid-infrared radiation transmitted through the first narrow bandpass filter and determine from the measured intensity an amount of the species in the fluid;
• and
• a heater configured to locally heat the internal reflection window to clean the surface of the internal reflection window in contact with the fluid, wherein the internal reflection window includes a conductive or semiconductive material and the heater comprises an electrical power supply for sending a current through the internal reflection window to induce resistive heating of the internal reflection window.