I finally figured out how a dissolved oxygen sensor works
The title of this blog looks like a self-inflicted wound. It suggests that I am ignorant about how one of the most basic tools in the water quality tool box works.
It’s true.
I have a PhD in physical chemistry. I’ve been in the water business now for 6 years. I am almost finished writing a book on water quality instrumentation that will be published by AWWA. And, until recently, I didn’t know how a dissolved oxygen sensor works.
Okay so that’s a little harsh. I really do know a fair amount about D.O. probes. But I firmly believe that, unless you know the fundamental principles behind a technology—be it an internal combustion engine or a mass spectrometer or a Clark cell—then you really don’t know how the piece of equipment embodying that technology works. I wrote my book because understanding the basic workings of a sensor is absolutely essentially to knowing how to use it—to set it up, calibrate it, maintain it and troubleshoot it. And yet finding thorough explanations on the basics have been almost impossible to come by.
Case in point is the dissolved oxygen (D.O.) sensor. Whether it is a Clark cell, Galvanic probe or luminescent probe it works in a way that I found baffling. Here’s what I mean:
The D.O. sensor gives a signal that is proportional to the concentration of oxygen in water. Like any other sensor, one calibrates it by immersing it in a standard for which the dissolved oxygen concentration is known. Are we in luck or what? We don’t have to buy expensive and degradable standards. In fact for 2% accuracy tap water works just fine. Just stick an air stone in or a really good mixer to make sure the water is saturated with air. We know that pure water at 25 °C and at sea level will hold 8.3 mg/l (or ppm) of D.O. Tap water as a calibration standard? Does it get any better?
Yes it does! You don’t even need water with an air stone. You can hold the sensor in air and get the same result. How can that be? Air is comprised of 210,000 ppm of oxygen. How can 210,000 ppm of oxygen give us the same reading as a sample of water containing 8.3 ppm of dissolved oxygen? This is the paradox that drove me nuts.
The answer is that the D.O. sensor does not measure oxygen concentration. It measures partial pressure of oxygen in solution. Say what? Oxygen dissolved in water exerts pressure? It’s easy to see how oxygen in its gaseous state can be characterized by a pressure but oxygen molecules bouncing off of water molecules should not be exerting anywhere near the same force per unit area. No matter how much I thought about it I couldn’t accept the fact that a probe in air gives the same response as a probe in water.
The dissolved oxygen sensor is one case where our intuition fails us. The real explanation takes about 10 minutes of reading. I wrote a two-page white paper to make the case. If you want to read it just download it here.
Either way the point of this blog is that our intuition is right most of the time—but not always. Even Einstein had trouble accepting quantum mechanics because of its bizarre consequences that flew in the face of common sense. And he was the genius that derived the laws of relativity, in which fast objects do things that are just as strange as tiny objects ruled by quantum mechanics.
Science always gives us the correct answer. It’s up to us to ask the right question.