Proper Sensor Selection

中航国际

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.