The glass membrane
of a pH sensor is constructed of different types of glass, each with
different useful properties. As pH is measured in a wide variety of
solutions and at varying temperatures it is impossible to create a
single membrane glass which can give accurate and reliable pH
measurements in all conditions.
• General-purpose electrode glass:
Various pH ranges, mainly between 1-12. Typical temperature range: -5 to +80°C (20 to 180 ºF)
• Low temperature glass:
Provide the best speed of response in applications with temperatures close to and below the freezing point (0ºC, 32ºF). Low temperature electrodes are ideal for use in municipal and industrial wastewater applications, particularly in cold climates.
• High temperature glass:
Typical temperature range: 0 to +135°C (32 to 275ºF). High temperature sensors are ideal where the process temperature is 90ºC (190ºF) or higher, making them suitable for heavy process applications in the pulp and paper, pharmaceutical and chemical industries.
There is also a version of the high temperature glass that's called sterilizable glass. The typical temperature range is -5 to +80°C (20 to 180 ºF). However, it can withstand a temperature of about 135°C (275ºF) for usually 20 minutes.
• High alkaline glass:
For highly alkaline media (above pH 12). Its resistance to alkaline solutions reduces the alkali error at high pH values.
There are also high temperature (> 100ºC, 212ºF) versions of this type of glass electrode.
• Hydrofluoric acid (HF) resistant glass:
HF is commonly used to etch glass and chemically attacks the glass membrane and prevents, even in small concentrations, the development of a gel layer. The result is unstable measuring pH values. HF also reduces the life span of your pH sensor.
thickness of the glass will have an impact on the behavior of the pH
probe. A thicker membrane will be more durable, but also has more
resistance and a longer response time. A thinner glass will have less
resistance and a quicker response time, but will be more fragile.
As the glass membrane ages the speed of response reduces until it becomes unacceptably slow.
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