An ammonia gas sensing device includes a housing defining a fluid flow path. The fluid flow path includes a fluid inlet, a fluid outlet, and an access port. A gas permeable/liquid impermeable membrane is mounted on and sealed against the housing at the access port such that the membrane is exposed to the fluid flow path but fluid is blocked from flowing outward of the access port around rather than through the membrane. An ammonia sensor is mounted on the housing at the access port in a position outward of the membrane. A system for using the ammonia sensing device includes the ammonia gas sensing device, a light source directed at the ammonia sensor, a photo detector to measure the light reflected off the ammonia sensor from the light source, and a controller for controlling the light source and optical sensor.