Choosing the best dewpoint measurement instrument

Paul Boughton

Dewpoint (Td) is defined as the temperature to which air must be cooled for water condensation to begin.

A practical example of dewpoint is a glass of cold drink on a warm summer day. Since the glass conducts heat fairly wellthat and also a thin layer of surrounding air are cooled to nearly the same temperature as the drink.

If the temperature of the drink is below the dewpoint temperature of the surrounding airthe air close to the glass surface becomes saturated with water and the excess water condenses on the cold glass surface. These small water droplets are called dew.

Frostpoint

In dewpoint temperatures below 0˚Cwater vapour condenses into frost instead of dew droplets. This is called the frostpoint (Tf) of the gas.

Water vapour saturation pressure over ice is slightly different than that over water. Getting dew to form below zero is not totally impossible eitherbut in practice the industry standard is measuring Td/fie dewpoint above and frostpoint below 0°C. Both are commonly referred to as dewpoint.

Dewpoint is a commonly used parameter to represent the amount of water vapour in dry applications.

In dry conditionschanges in dewpoint values are larger compared to very small changes in relative humidity valueswhich gives dewpoint measurements an advantage in process control.

For exampleat room temperature a change in frostpoint from -40°C to -45°C would correspond to relative humidity values 0.5percent RH and 0.3percent RH.

Dewpoint is related to the partial pressure of water vapour of the gas. Dewpoint is not temperature dependentwhich is a useful feature in many casesas it enables taking a sample from a hot process and making the actual measurement at a more convenient temperature after cooling.

This is a major advantage in eg metal heat treatment processeswhere the process temperatures may be close to 1000°C.

Dewpoint ishowevera pressure dependent parameter. As pressure is raisedthe dewpoint of the gas rises. This is due to the fact that along with the overall system pressurethe partial pressure of water vapour rises.


Common applications

One of the most common dewpoint applications is compressed air systemswhere humidity control is essential. Even though the intake air may be reasonably drysay 15percent RHcompressing it to a pressure offor example7barwhich is a typical value for compressed airwould lead to water condensing in the system.

Obviously liquid water is an unwelcome phenomena in compressed air linesequipment and end-use applications.

A compressed air system can incorporate some kind of drying equipmentmost commonly desiccant or refrigerant dryers.

The performance of this drying equipment in turn can be monitored and controlled through dewpoint measurement.

In the production and transfer of natural gasdewpoint is an important parameter describing the quality and energy value of the gas.

One field of compressed air is its use as a drying agent in purging pipelines and vesselswhere observing the dewpoint of outlet versus inlet gas gives a clear indication on the progress of drying. Also plastics production benefits from dewpoint measurement ensuring the quality and optimising the process of plastic pellet drying.


Dewpoint measurement technologies

Dewpoint instruments fall roughly into two categories: the capacitive instruments providing typically ±2°C accuracy and the fundamental condensing hygrometers providing typically ±0.2°C accuracy.

Capacitive sensors have been developed for the industrial dewpoint measurements. The environments may be rather harsh and pose great demands on the instruments’ long term stability.

The old aluminum oxide based capacitive sensor has been challenged by Vaisala DRYCAP polymer technology featuring excellent long term stability and fast response time.

Alsothe polymer sensor withstands getting wetan important feature in dryer malfunction situationsfor example.

For high accuracy demands such as laboratory reference instrumentsthe traditional condensing hygrometer has been a chilled mirror instrument where a mirror surface is cooled down to dewpoint and the droplets or frost are then observed optically.

To overcome the error sources related to for example impuritiesVaisala has developed the DM500 utilising a new technology where the dew/frost detection is performed by a surface acoustic wave (SAW) instead of a light beam.

What to look for

Common specifications of dewpoint instruments include the measurement rangeaccuracy and operating environment of the instrument.

Naturally all these have to fall into the requirements of the user.

A wide measurement range is not of interest in itselfwhat is important is the performance in the actual measurement range of the user.

Often the most important things for a user are the ones related to the reliability of the instrument in all situations.

A feature that is of major importance in the long run is the long-term stability of the instrument. Dewpoint instruments do require calibration from time to timethe frequency of which depends on the measurement technology and that of the end-use application.

The long-term stability is of course somewhat tricky to know at the time of purchasing the instrumentbut one indication is the specified calibration interval of the instrument.

Life-time costs

Considering the life-time cost of the instrumentnot only the estimated frequency but also the price of calibration is of interest.

And to keep your process running smoothlythe promptness of service from the manufacturer is critical too.

Liisa Åström is with Vaisala OyjVantaaFinland. www.vaisala.com

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