DO is measured in three different ways: polarographic, galvanic, and optical. They were each invented in that order, and each have their own pros and cons. This is a great article that goes into way more depth than we will here. Let's start with polarographic.
Polarographic probes have been around for decades. They operate by measuring the voltage between a silver anode and a cathode with a noble metal, such as gold. These are both contained in a potassium chloride electrolyte fluid. Oxygen diffuses through a membrane from the water and the then reduces along the gold cathode's surface. This generates a measurable current between the anode and cathode which is directly proportional to the amount of dissolved oxygen present.
- Pros: relatively inexpensive sensor. Lot's of resource and places to get them.
- Con's: Can give faultily low and inconsistent readings when it needs to be recalibrated. The anode has to be scrubbed and eventually replaced. This takes some technical knowledge and comfort opening the probes.
- Pro's: Industry standard sensor in many applications. Can go for longer without needing recalibration. Can be recalibrated in the air.
- Con's: The anode oxidizes faster than polarographic sensors, meaning the sensor needs to be replaced more often.
- Pro's: Longest-lasting DO sensor. Rarely needs recalibration. Very fast response time. More resistant to biofouling.
- Con's: Tend to be more expensive.
Osmo Systems has developed patent-pending technology surrounding our own optical DO sensor that has significantly lowered the cost. More to come as we roll out our next generation of prototype units.