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Comparing different dissolved oxygen sensor technology!

 
 

Dissolved oxygen (DO) is the level of free oxygen (O2) molecules present in water and is typically measured in mg/L. It is important parameter for monitoring water quality, aquatic biology and other environmental and industrial processes. Dissolved oxygen meters use either an optical or electrochemical sensor to measure DO. In this article we will look at the different types of DO sensors and the advantages and disadvantages of each option

 

Electrochemical sensors

 

 

Electrochemical sensors can be broken into 2 main types of sensors: polarographic and galvanic. Both use electrodes where the dissolved oxygen reacts with the cathode to produce a current. The electrode has a semi-permeable membrane which allows O2 to pass through.
 
Polarographic probes require an external voltage to work, whereas galvanic probes do not require an external voltage. The advantage of a galvanic electrode is that it requires no start-up time, whereas a polarographic probe can take up to 15 minutes to polarise sufficiently. The downside to this is that galvanic sensors are always consuming the anode, even when the unit is turned off which means that the electrode needs to be replaced more frequently. The tradeoff is that polarographic electrodes require more frequent changing of the electrolyte solution as it is consumed during use so they require more maintenance.
 
The main limitation of electrochemical sensors in general is that because they consume oxygen, there needs to be sufficient flow of water across the membrane. So for low DO measurements with small volume samples, the sensors may consume a lot of the oxygen in the sample. As such they’re better suited to flowing or large bodies of water.

Figure 1: A galvanic Dissolved Oxygen probe and how it undertakes measurements

Our Most Popular Polygraphic DO Meter:

DO-5510HA - Digital Oxygen Meter


This Digital Oxygen Meter is supplied with a polarographic type probe with an incorporated temp. sensor which serves for precision Dissolved Oxygen(DO), Oxygen in air(O 2), Temp measurement.
 

Optical DO sensors

 

Optical DO sensors work by measuring the changes in luminescence of a luminescent molecule which gets excited by a blue LED source and then release light in the red wavelength. This red light is measured to determine a reading. The presence of oxygen limits the ability of the luminescent molecule to produce red light, and as such, oxygen levels can be determined. To improve the accuracy and stability of the measurement, the sensor will also emit red light that is reflected by the luminescent layer. This is used as a reference value.

Figure 2: The workings of an Optical Dissolved Oxygen Sensor

Our Most Popular Optical DO Meter:

Starter 400D Portable DO Meter - IC-ST400D-G

The optical probe requires minimal maintenance –frequent membrane replacement
not needed. Also, warm up and stirring of samples when taking measurements is not
required. Equipped with a color LCD display, the ST400D is designed to fit comfortably in the palm of your hands. The automatic/manual endpoints enable easy recall of last
calibration data and stored information with one touch.

If you’re still unsure about which DO Sensor is right for you, contact one of our scientists on 1300 737 871 or at customer-service@instrumentchoice.com.au

  

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  Phone: 1300 737 871
Email:
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