Endress Hauer O2 sensor
Presence of industrial effluents?
The temperature seems high, especially in winter.
Endress has good technical support.
I think you should contact them directly.
Actually, 86-95°F in activated sludge seems hot. Is the bacterial floc forming properly?
Presence of reducers such as H2S, for example?
If you are thinking of replacing the sensor and you still prefer O2 measurement over ORP, which is quite common for the purposes of regulating aeration, I would look into a polarographic sensor.
According to the Endress & Hauser manual for your sensor:
The theoretical service life of an electrolyte filling for use in air-saturated drinking water at (68 °F) is 5 years max. "The electrolyte life is shortened by diffused, dissolved gases such as H2S, NH3 or high concentrations of CO2."
This particular situation occurs with:
• anaerobic phases (e.g., denitrification)
• highly polluted industrial waters, particularly at high temperatures.
That may be the answer to your problem. I suggest you contact Endress with the specific conditions under which your sensor operates and discuss the problem with their support team.
Sorry for the delay in responding:
1) These are indeed industrial effluents. At the moment, the temperature is around 95°F. However, in the summer, the temperature rises even higher and we have had difficulty maintaining 97°F, despite the intake of industrial water.
2) I'm waiting for feedback from Endress technical support. The floc is in good shape and settles well.
3) The O2 sensor is just a guideline. It’s there, so might as well use it. Possibility of polarographic sensor noted. It might be an idea.
4) When the sensor was working well, I had to replace the electrolyte every 6 months. The cartridge was almost dry.
Following a response from Endress, I am thinking of replacing the cartridge and the O-ring and possibly lubricating the seal to ensure it is airtight and will prevent contamination of the electrolyte.
This is where I am at.