Aquaprobe
®
AP-700, AP-800 & AP-2000 Instruction Manual
10401-00870 Rev R
22.2. Sensor Cap Life
All optical dissolved oxygen sensors work on the same principle, and all must have the
sensor cap containing the luminophore replaced periodically due to a phenomenon known
as photo bleaching.
When a sensor cap is new, the luminophore will return a large number of red photons when
excited. As time goes on, a bleaching effect takes place and the number of red photons
returned reduces to a point where they are no longer detectable.
The amount of photo bleaching that the luminophore suffers is in direct proportion to the
amount of time it is excited by the sensor’s blue light source. It therefore follows that the
faster a reading can be taken, the less time the luminophore needs to be excited and the
longer it will last.
The high-speed circuitry within the AquaPlus™ module requires just eleven milliseconds to
take a reading! This incredibly fast reading time increases the useful life of the luminophore
considerably.
Another technique used to prolong the life of the luminophore in the AquaPlus™ module is
variable excitation brightness. When the luminophore is new, the brightness of the
excitation is reduced to a minimum in order to prevent unnecessary photo bleaching. As the
output from the luminophore gradually reduces, the brightness of the excitation is increased
in order to squeeze the maximum possible life from the sensor cap.
The combination of low duty cycle and variable excitation brightness can stretch the useful
life of a sensor cap as far as several years.
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