A sensor assembly for a flame sensor apparatus includes a photodiode that generates a current. The sensor assembly includes a seal assembly supporting the photodiode. The seal assembly includes an inner conductor defining an inner conductor end. The inner conductor includes an inner conductor surface disposed at the inner conductor end. The photodiode is attached to the inner conductor end of the inner conductor and to a middle conductor end of a middle conductor. The photodiode is electrically connected to the inner conductor surface. The seal assembly is triaxial so as to protect the current generated by the photodiode. The seal assembly withstands temperatures up to or greater than about 325° C. The seal assembly forms a hermetic barrier that, with the photodiode supported within a sealed volume, limits the passage of materials/gases through the seal assembly.