a fourth preferred embodiment the block comprises the first second or third preferred embodiments just described together with at least one of pathways e f and g more preferably together with at least two of pathways e f and g even more preferably together with each of pathways e f and g pathway e is a so called u201c dark zone u201d in which the sensor element may be placed for a 4 ma i e nil signal check preferably the first supplementary radiation source in pathway f is a light emitting diode such as a uv c light emitting diode in essence this pathway is similar to pathway e with the addition of a uv c light emitting diode preferred embodiment in a portion of the dark zone providing for the ability to conduct a saturation signal check on the sensor element an advantage of pathway f is that it allows for removal of the sensor element from the fluid treatment system from the sensor check preferably the second supplementary radiation source in pathway g is a light emitting diode such as a uv c light emitting diode since pathway f represents a dark zone for the sensor element a second or the same pathway with output from a calibrated uv c light emitting diode preferred embodiment could be used to check one set point on the operational curve of the sensor for example if at the time of calibration the set point light emitting diode were known to generate a signal of 11 ma in the sensor element sensor operation could be.