Optical and Galvanic Dissolved Oxygen Sensors
We looked at the data from couple of dissolved oxygen monitors on an aeration lane at a UK Waste water treatment works. Both systems feature the ATi air blast AutoClean system, the only difference is that one used the tried and tested galvanic sensor with a Teflon (tm) membrane while the other has our optical sensor. Both systems give the same performance so how do they compare from a cost of ownership point of view?
The long term costs of ownership of the optical dissolved monitors are still unknown, they have not been around that long. The membranes are specially impregnated with optically active materials and most optical sensors need a new membrane every 12-24 months, the membrane in the ATi Q45D-ODO?system is expected to last 3 -5 years.
A new optical membrane costs around 75 to 150 depending on the manufacturer. All optical sensors will eventually fail; the question is when. A lifetime of ten years is a good estimate. Once an optical sensor has failed it is not economical to repair it, it will need replacing. Galvanic sensors contain maintainable elements, electrolyte and electrodes. ATi has 100s of galvanic systems working over 15 years after installation.
A galvanic membrane sensor will need a new membrane every 12 months and a new anode every 2-3 years. This gives an annual consumable cost for the ATi Q45D galvanic system of 5/$8. If an optical sensor lasts 10 years and needs a new optical membrane every 2 years, the annual consumable cost will be 100/$150. Costs from other optical sensor suppliers can be higher.
In summary, if a comparison is made between efficiently auto cleaned sensors the cost of ownership for optical systems is around 20 times that of galvanic system for no major performance improvement. ATi can supply both systems so ultimately the choice will be down to the end user.
Both sensors after months of running with zero manual intervention, are still clean and still in calibration.
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