Versatile moisture measurements in wet and dry processes

Very often the simple solution to a problem is the best one. However, with technology it’s easy to be swayed by the offer of a new idea that apparently offers greater benefits, but also brings with it layers of extra complexity which often outweigh the gains.

Moisture control in process applications is a good example. In recent years many new sensing technologies for trace moisture have been introduced, such as various derivates of laser based spectroscopy, each of which have their place. However, often they don’t perform basic functions as well as the tried and tested aluminium oxide or ceramic sensing technologies.

While the principle has remained simple, the technology benefits from a wealth of knowledge and field experience which means that the latest generations of the ceramic and aluminium oxide sensors have vastly improved since the first versions were released on the market.

Because it is a well-established technology, the cost of installation and maintenance is also significantly lower than the newer technologies. In addition it gives users confidence that, should they encounter a problem, they will easily find someone with the skills and understanding to resolve it.

Ceramic sensors are also the most versatile of the process moisture sensing technologies. The measurement range covers +20 to -100°C dew point, or from 23,000 ppmV to less than 10 parts per billion. This means that a dynamic measurement of over nine orders of magnitude is possible with just one sensor.

For moisture measurements in hydrocarbon liquids they are still the only effective sensing method. They also offer the unique advantage of being able to measure dew point at line pressure, which is an advantage for gas pipelines. Gas company contracts often state a permitted dew-point limit at line pressure, rather than specific moisture content and being able to directly measure this is an advantage for a contract partner.

Some of the common criticisms of ceramic moisture sensors are often irrelevant for many process applications. For example, TDLAS and QCM analyzers are able to measure at a higher accuracy. However for many hydrocarbon processing applications an accuracy of ±1°C or ±2°C dew point is adequate.

Speed of response is also cited as another failing of aluminium oxide sensors, especially as they need to dry down after a moisture event. But again this needs to be considered in the context of the application. Although a TDLAS analyzer is capable of responding in seconds where a ceramic sensor takes minutes, for many applications the speed of response offered by the ceramic sensor is sufficient. It is more important to ensure the correct sample conditioning system is installed, as this will have a greater effect on measurement sped than the analyzer technology.

To summarize – there are applications where the specific features of a TDLAS or QCM analyzer justify the additional costs. However for a significant number of applications in hydrocarbon processing, ceramic moisture sensors provide a cost-effective and perfectly adequate solution. That they have been tried and tested in the field for a number of years also means that users can rely on them with confidence.

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Date: 16 December 2016
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Written & issued by Michell Instruments Ltd.