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A typical polarographic electrode consists of a silver anode, a gold or platinum cathode and an electrolyte solution (KCl or AgCl). In order to create a sensor, a constant voltage of 0.8 volts is applied to the probe, and a digital meter is installed to read the DO response measured by the sensor.
Figure 67. A simplified diagram of a polarographic sensor. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Limitations
- Response time is described as the time required for the electrode to reach >90% of the output. Typical response time for polarographic sensors are is 30 sec, which makes them not compatible to be used for dynamic measurements.
- Warm-up time for this type is approximately 10 minutes. Wrong readings will occur if measurements are made when the required amount of time has not been attained.
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- Since the electrode consumes oxygen, readings are affected by flow across the sensor tip. Thus enough flow rate at the membrane (or sample renewal rate) must be ensured for accurate results.
Galvanic Electrode
(or sample renewal rate) must be ensured for accurate results.
Galvanic Electrode
A typical galvanic electrode consists of a zinc or lead anode, a gold or silver cathode and an electrolyte solution (KCl or KBr). The principle of the galvanic sensor is that it uses 2 different types of metal and the difference in them with the electrolyte results in an electromotive voltage. This electromotive voltage is around 0.8 V which is enough to operate the sensor. Therefore the biggest advantage of the galvanic sensor is that there is no need for outside voltage source and there is no warm-up time.
Figure 8. A simplified diagram of a galvanic sensor and circuit. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Limitations
- The sensor continuously consumes the anode, even when truned off. Therefore the lifetime of the sensor is much sorther than of the polarographic sensor and the warranty is usually for 6 months only.
- Since the electrode consumes oxygen, readings are affected by flow across the sensor tip. Thus enough flow rate at the membrane (or sample renewal rate) must be ensured for accurate results.
Advantages
- No warm up time, it can be sed immediately after turn-off
- The electrolyte is never used up, in theory it can be used forever.
- Fast response time.Figure 7: A simplified diagram of a galvanic sensor and circuit. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Measuring dissolved oxygen with either sensor type
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The fact that the sensor measures the pressure instead of the concentration for dissolved oxygen is known to be true because a sample of fresh water can dissolve more oxygen than a sample of sea water at the same temperature and at the same altitude (or under the same barometric pressure). However, the sensor’s output signal is identical in both samples since the oxygen pressure is identical in both media. See the following figure for an example of this concept.
Figure 89. DO sensors measure % saturation. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
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