An apparatus for determining spectral absorption by a specific substance in a fluid having an energy source, a sensor an interface device, a support structure and a connector. The energy source directs energy having a predetermined wavelength into the fluid. The sensor produces an electrical output reflecting the energy it senses and indicative of the spectral absorption of the energy by the specific substance. The interface means generates a reference voltage to allow the apparatus to cooperate with prior art devices. A support means secures the other elements in proximity to the fluid. The connector communicates electrical signals to a remote display device.