For most algae applications, DO is a key parameter: high DO is a sign of growth, but too much DO inhibits growth. Generally the oxygen is measured dissolved in the liquid, but in some applications head space O2 is also useful.
There are two forms of DO sensor: electrochemical, and optical. We’ll describe highlights of how each works, their pros and cons, and then estimate total cost of ownership. Feel free to skip to the end for a quick bottom line.
How they Work:
Electrochemical sensors, either galvanic or polarographic, consist of an anode, a cathode, an electrolyte solution, and a semipermeable membrane cap. If a potential difference of at least 0.4 volts exists between the anode and cathode, the cathode provides electrons that react with the dissolved oxygen to reduce the oxygen. This means that a current flows which is proportional to the concentration of oxygen. The process consumes the oxygen in the vicinity of the membrane cap, and therefore one needs to insure flow of the liquid or the readings will drop and incorrectly report a lower dissolved concentration.
In optical sensors, an emitter inside the sensor sends blue light towards the sensor cap that has a porous membrane. The membrane contains a luminescent dye that emits the light back to a detector. The dye is chosen such that its luminescence is reduced when O2 is present in the membrane. Therefore the detected illumination is related to the concentration of O2.
Pros, cons, and maintenance
In optical sensors, the main cause of maintenance is that the membrane dye eventually degrades. One or two calibrations per year and a replacement cap every 18 months addresses this. Optical sensors are the longest lasting and most accurate sensors, require the least maintenance, do not require flow of the liquid, and are not very affected by other gasses. They are also, predictably, the most expensive with regard to initial acquisition costs, between $500 and $1900.
For polarographic sensors, the cost is lower, between $250 and $750. An external voltage is required to polarize the electrodes before use, which means a 5-30 minute delay to initiate readings. Next, the electrolyte is eventually consumed (faster than the dye degrades in an optical sensor), the electrodes oxidize and require cleaning, and the membranes need to be replaced. On the other hand, these are the least expensive DO sensors, and the maintenance routines are easy once learned.
Galvanic sensors do not require an external voltage, and produce readings the quickest, in about 5-10 seconds. They do have a similar calibration, membrane replacement, and electrode cleaning schedule as polarographic sensors. Their cost is similar or slightly higher than the polarographic sensors.
Bottom line: annual cost of ownership
The below numbers are just examples, and what you will find in your process will likely be different, but they are a fair example of what to expect. We will estimate that a calibration or maintenance of a sensor takes 30 minutes at $30/hour, although this will vary on the process.
Looking at an optical scenario where the sensor costs $650 and last 2 years, the caps are $90 and last 1 year, and calibration is every 6 months, the annual cost of ownership is $445.
For a galvanic sensor of initial price of $400 lasting 15 months, caps at $10 replaced 3 times per year, calibration performed 12 times per year, the annual cost of ownership is $530.
Based on these estimates, an optical sensor is likely the preferred choice: less expense and perhaps as important, more peace of mind. That being said, the cost for optical sensors is most heavily dependent on the initial cost of the sensor itself, so a $1200 optical sensor may not be worth it.
For information on the Algae Lab Systems DO sensors, visit out Sensors page: http://algaelabsystems.com/sensors/