Sensor options for measuring PH of a fluid in process?

I need to be able to monitor PH of a pumped fluid. The fluid will typically have contaminates in it so I have concerns with some available sensors without using extreme filtration. What options do I have? PH will range from about 6.5-9.5 typically.

What's the fluid (presumably water based)?
What are the contaminants?
Your comment about "extreme filtration" makes me wonder whether the contaminants are solids and you are worried about breaking a glass electrode.

Yes, water based fluid. Contaminates would usually be brass or stainless steel fines. Nothing of any real mass and should not break anything, but I am curious if these would cause accuracy issues or foul the sensor.

Everything I have read leads me only to the glass bulb type sensors? They seem to need frequent calibration, need special storing procedures, etc. Everything that is not liked in a process type environment. I was hoping there were other options?

Once this is installed, it just needs to work with very little maintenance. Maybe that is only a wish?

If there are cutting oils present, the sensor can be fouled, but laboratories do millions of pH tests of blood and other biological fluids, and the sensors are not particularly troublesome.

You can also divert a small amount from the stream, add a colorimetric pH indicator and do the test spectrophotometrically -- particularly if you do not need very high accuracy. That could all be continuous flow and would probably require less maintenance than a pH electrode.

But could it be done cheap? Yes, oils most certainly would be present in the water from multiple sources.

Oils will affect any color indicator for pH, particularly if there is an emulsion. Light scatter would be a problem. Your best bet sounds like a standard electrode pH meter with periodic cleaning. Some electrodes are much more durable than others.

I am not familiar with the photometric method of testing PH. Can you briefly explain that? Is this similar to to refractive testing?

Not at all like refractive index. Basically, you use a dye that changes color with pH, then measure that color with a meter instead of the eye. Litmus paper is one example. pH test strips used by diabetics and many others is another. pHydrion is a brand. Those dyes can also be used in solids (rarely), gels, and liquids. Their main limitation (like all pH measurements) is that the liquid must be mostly (>>90%) water for them to be accurate.

I once witnessed a "professor" at a small college use the papers to test the pH of some bio-diesel he had made. The pH was always around neutral and he wondered why. I suggested taking some of his diesel, some water, mix them, and then measure the water. It came out quite alkaline. The dyes will not give you the precision of an electrode. At best, you will be +/- 0.5 pH or so. Maybe better with a spectrophotometer. (PS: His Mercedes also suffered engine damage.)

John