Measurement - Dew Point

What is Dew Point?

The Dew Point is the lowest temperature that allows water vapour to remain in a gas without condensing to a liquid state. As the air or gas temperature drops, its ability to absorb water vapour drops until it becomes completely saturated and below this dew point temperature, water droplets will start to form.

In pressurised systems such as compressed air distribution networks, the dew point is directly related to the temperature AND system pressure. As pressure increases, the dew point temperature also rises meaning that the potential for vapour to condense out occurs at a higher temperature. In practice this can mean that instead of the condensing temperature being quite low, the dew point could be at or above ambient temperature.

Why do I need Dew Point Measurement?

Industrial compressed air and gas systems can suffer damage from water contamination directly or through the water subsequently freezing and expanding. Having water vapour in the air or gas can also affect process or product quality. Removing water contamination through filters and drying systems is common practice but as the dew point (and potential for damaging condensation) changes with pressure, the risk of damage will vary throughout a plant.

ISO 8573-1 defines a range of purity classes for compressed air including water with the measurement expressed in terms of pressure dew point. ISO 8573-3 is the section that defines measurement methods for humidity and ISO 8573-9 for liquid water.

Measuring the dew point can be a simple process and should be continuously monitored according to the plant risk analysis recommendations. This needs to occur at the drying plant before the distribution network and also at critical points of use. By measuring the dew point, efficient control of the drying/filtering system can be achieved to optimise the running costs of the compressed air/gas system.

It is common to have a secondary dew point sensor installed downstream of the dryer to offer redundancy should the dryer start to fail and wet air enters the distribution system. This provides an alarm point and  with prompt action can save huge costs for damages and lost production due to water ingress.

Particular emphasis should be placed on any part of the system where the pressure increases or the ambient temperature decreases as these are the situations where problems can occur quickly. For example, the air ring main leaves one building to go to another and the ambient outside air is or can be significantly lower than the indoor ambient. Additional drying capability may be needed and monitored in order to prevent condensation as the pipe leaves the building.

Monitoring the dew point is a key indicator of system health that can be used to guide maintenance tasks as well as maintain plant compliance with the purity classes set in ISO 8573-1. 

Refrigerant dryers are used for non-critical applications and are tested to a limit of <=+3degC for ISO 8573-1 Class 4 compliance. A dew point sensor with a range from -20degC to +50degC would be an ideal solution for this requirement.

Desiccant dryers provide much dryer air and are commonly tested to a limit of <=-40degC for ISO 8573-1 Class 2 compliance. A dew point sensor with a range from -50degC or -60degC to +20degC would be an ideal solution for this requirement. 

Advanced drying solutions to target the ISO 8573-1 Class 1 compliance limit of <=-70degC would suggest a dew point sensor with a range from -100degC for accurate readings at such low humidity.

Dew Point Sensors

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