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Algae Lab Systems
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Accelerating Algae
Accelerating Algae

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Algae Lab Systems on site in Africa
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2016-07-29
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Optical and Electrochemical Dissolved Oxygen Sensors

In this post, we discuss the relative merits of optical sensors and electrochemical sensors for dissolved oxygen meters.  Oxygen sensors, also known as oxygen meters are used to measure the concentration of oxygen (O2) in a variety of liquids and gases.  If you are trying to monitor the growth of algae in water, DO can be a useful indicator since DO is produced as a direct byproduct of photosynthesis.    The two main types of oxygen sensors that are available are optical sensors and electrochemical sensors.  

Electrochemical sensors are of two types, galvanic and polarographic.  The parts of an electrochemical sensor are the anode, the cathode, an electrolyte solution, and a semipermeable membrane cap.  Once the voltage difference is established between the anode and the cathode is established, electrons from the cathode react with the oxygen in it, producing a current through the meter.  In order for an electrochemical sensor to give an accurate reading, the fluid containing the dissolved oxygen you are measuring must keep moving, because the dissolved oxygen concentration is depleted in the vicinity of the electrode during the measurement.

A difference between galvanic and polarographic sensors is that galvanic sensors do not require an external source of voltage.  Another advantage of galvanic sensors is that they can give accurate readings in less than ten seconds.  Meanwhile, polarographic sensors are sometimes slightly less expensive than galvanic sensors.  Before using a polarographic sensor, you must polarize the electrodes for anywhere from five minutes to half an hour before you use it to take the reading.

An optical sensor consists of an emitter, a detector, and a sensor cap.  Dissoloved oxygen permeates the porous material on the sensor cap.  When you operate an optical sensor, the emitter transmits blue light into the sensor cap membrane.  Luminescent dye in the membrane then emits light back and is then perceived by a detector.  The brightness of the light produced by the dye changes according to the concentration of oxygen in the liquid being tested.  The luminescent dye in the sensor cap membrane degrades over time, so you must replace the sensor cap approximately every 18 months in order for the optical sensor to provide accurate readings.

An advantage that optical sensors have over electrochemical sensors is that they require far less maintenance than electrochemical sensors.  Virtually the only maintenance an optical sensor every needs is replacement of the sensor cap every 18 months and perhaps calibration once or twice per year.  With optical sensors, you do not need to replace electrolyte solution or clean electrodes, like you do with electrochemical sensors.

A new galvanic oxygen sensor costs only about $400, whereas an optical sensor can cost as little as $500 or as much as $2,000 depending on the vendor.  Electrochemical sensors need more frequent maintenance and membrane cap replacements than optical sensors do, so even though an electrochemical sensor may seem much more cost-effective in the beginning, the costs can easily even out.  Analysis of typical annual cost of ownership (including labor) for Algae Lab Systems DO sensors comes out at about $500 for galvanic vs. $450 for optical. Visit our site to find out more about optical and electrochemical oxygen sensors and their parts.
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If you haven't already checked out the Algae Biomass Summit, get your ticket now, because this is the biggest algae event around, and what people are doing with algae nowadays will blow your mind! Sep 29- Oct 2 in Washington, DC.
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