A dissolved oxygen sensor ought to be simple to understand. Whether it is membrane or optically based, it gives a signal that is proportional to the concentration of oxygen concentration 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. Ordinary, pure 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.
But wait… it gets even better. 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?
If you know the answer then go do something fun. You definitely earned it. But, if you're like me a few years ago, then you’re scratching your head. It’s one of those maddening paradoxes, for which I’ve never read an adequate explanation. So I wrote this little white paper. I hope it helps.