A dissolved oxygen sensor, comprising: a window of optically transparent material disposed in an opening in a fluid flow path, wherein a luminophor is attached to a first side of the window exposed to the fluid flow path; an optical probe opposite the window from the fluid flow path and the luminophor, wherein the optical probe includes: an excitation light source configured to illuminate the luminophor; an optical reception guide routed through the probe and having a longitudinal axis, with a first end of the optical reception guide along the longitudinal axis proximate the window, wherein the optical reception guide is configured to conduct light received at the first end in response to illumination of the luminophor by the excitation light source; a photodiode configured to receive light conducted by the optical reception guide wherein the photodiode is adjacent to a second end of the optical reception guide along the longitudinal axis distal the window and is aligned on the longitudinal axis with the first end and the second end of the optical reception guide; and a printed circuit board having a longitudinal axis aligned with the photodiode and the longitudinal axis of the optical reception guide; wherein the photodiode is physically coupled to the PCB and the PCB includes electronic components configured to determine a measure of oxygen concentration of a fluid in the fluid flow path based on the light received at the photodiode through the optical reception guide. a window of optically transparent material disposed in an opening in a fluid flow path, wherein a luminophor is attached to a first side of the window exposed to the fluid flow path; an optical probe opposite the window from the fluid flow path and the luminophor, wherein the optical probe includes: an excitation light source configured to illuminate the luminophor; an optical reception guide routed through the probe and having a longitudinal axis, with a first end of the optical reception guide along the longitudinal axis proximate the window, wherein the optical reception guide is configured to conduct light received at the first end in response to illumination of the luminophor by the excitation light source; a photodiode configured to receive light conducted by the optical reception guide wherein the photodiode is adjacent to a second end of the optical reception guide along the longitudinal axis distal the window and is aligned on the longitudinal axis with the first end and the second end of the optical reception guide; and a printed circuit board having a longitudinal axis aligned with the photodiode and the longitudinal axis of the optical reception guide; wherein the photodiode is physically coupled to the PCB and the PCB includes electronic components configured to determine a measure of oxygen concentration of a fluid in the fluid flow path based on the light received at the photodiode through the optical reception guide. an excitation light source configured to illuminate the luminophor; an optical reception guide routed through the probe and having a longitudinal axis, with a first end of the optical reception guide along the longitudinal axis proximate the window, wherein the optical reception guide is configured to conduct light received at the first end in response to illumination of the luminophor by the excitation light source; a photodiode configured to receive light conducted by the optical reception guide wherein the photodiode is adjacent to a second end of the optical reception guide along the longitudinal axis distal the window and is aligned on the longitudinal axis with the first end and the second end of the optical reception guide; and a printed circuit board having a longitudinal axis aligned with the photodiode and the longitudinal axis of the optical reception guide; wherein the photodiode is physically coupled to the PCB and the PCB includes electronic components configured to determine a measure of oxygen concentration of a fluid in the fluid flow path based on the light received at the photodiode through the optical reception guide.