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Proper Sensor Selection

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发表于 2021-9-8 14:39:31 | 显示全部楼层 |阅读模式



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Selecting the correct pH sensor for each application plays an essential role in optimizing the pH sensor lifetime. The following aspects are important to understand before making a selection:
1.pH sensor glass
2.reference sensors
3.the effects of temperature and pressure

1. pH glass characteristics
First, let’s have a look at standard pH glass. The surface of the glass of a pH sensor has pores caused by the structure of the glass. These tiny pockets are perfect for H+ ions, which get stuck in them. Or as the industry calls it ‘the sensor is selective for H+ ions’ (see figure 1).

Many people ask “Why is the above important to know?” Let’s look at an example: in high alkaline applications (> pH12) the H+ ion concentration is very low. This is often done by dosing with Sodium Hydroxide (NaOH). This means that the Na+ concentration is high. The Na+ ions will block these H+ “pores” in the structure of the glass. Because these pores now have a Na+ instead of an H+ ion it will still see it as an H+ ion and register a lower pH than that it is in reality.
This is often referred to as the “Sodium Error” and occurs above a pH of 12. The reading of the measurement will be typically 0.17 pH lower than it is in reality (see figure 2). This is true for all pH sensors; not just the ones Yokogawa manufactures. It is the nature of pH glass and measuring principle.

FL-P-20210106-05-Proper Sensor Selection.pdf

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pH Sensor Selection Variopin

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