Introduction to dissolved oxygen
Why is oxygen important? For us humans, if we have oxygen, we survive...yay! If not, we don't...boo. So, superficially, this may not seem like a very important parameter to know - you either have oxygen, or you don't. However, for many microorganisms, there are a lot of shades of gray.
For a bacteria or a yeast, different amounts of oxygen produce different results. For instance, starving a yeast cell of oxygen produces ethanol as a metabolite product instead of carbon dioxide. Starving a lake of oxygen not only prevents fishies from living in it, but also promotes the formation of large algae surfaces. Cool, right?
The biggest problem with measuring dissolved oxygen currently is the cost of the equipment available to do it. Typically, dissolved oxygen probes run well into the $400+ range, thus placing them well out of the realm of hobbyists. The cost is not wholly unwarranted - dissolved oxygen meters used a platinum catalyzed reaction with very specific membranes to measure oxygen response. By cutting out the platinum catalyst and the specialized membrane, the cost of a DO meter could drop considerably...enter the optode!
Building a dissolved oxygen probe
How an optode works
In order to reduce cost, we'll be building a dissolved oxygen optode instead of the more common dissolved oxygen electrode.
In an electrode, a small change in a voltage or current is used to detect a change in oxygen concentration. In an optode, a small change in reflected light intensity is used to detect changes in oxygen concentration:
There are advantages and disadvantages to each system.
Properties of commercial dissolved oxygen electrodes:
- Very robust - easily waterproofed
- Very accurate
- Need to recalibrate is rare due to non-reactive nature of the membrane and platinum
- Very small amperages are produced - an amplifier circuit must be built at the amp meter position
- Very, very expensive - $400 and up
Properties of commercial dissolved oxygen optodes:
- Film must be intact for proper sensing - not as robust
- Film must be permeable to oxygen, but impermeable to media (i.e. water)
- Calibration is difficult - more frequent recalibration necessary due to film degradation
- Chemicals in sensing foil respond at visual wavelengths, so background light can interfere with accuracy and precision
- Very cheap components needed
- No need for an amplifier circuit!
- Commercial probes are very, very expensive - $400 and up as well, but based on the design components, could it be cheaper to make?
Although the optode has several drawbacks that make it impractical for some uses, there are enough benefits in its simplistic design to make it a potential probe that can handle many situations for $20 or less!