of test cells 20 and 30 according to sensors and methods well known in the art light sensors 40 and 50 convert the light intensity to a proportional output voltage which is monitored by a central microprocessor 135 central processor 135 for use in the invention are readily purchased or may be constructed according to well known methods and use of suitable central processors is well known by those of skill in the art central microprocessor 135 then compares the voltage output with a stored calibration voltage by well known methods at which time a u201c pass u201d or u201c fail u201d status may be assigned by microprocessor 135 the frequency of the led 80 is chosen such that it is absorbed by the dialdehyde reaction product formed by reaction between a dialdehyde analytical reagent and the dialdehyde germicide present in test solution 25 preferably led 80 emits light at a frequency in the range of about 420 nm to about 620 nm more preferably at about 502 nm which frequency is absorbed by the dialdehyde reaction product of ortho phthalaldehyde and 4 amino 3 hydrazino 5 mercapto 1 2 4 triazole the frequency of led 90 is chosen such that it is absorbed by a ph indicator for example.