How to calculate pressure dew points (PDP) accurately

Avoiding condensation is vital for many processes. It can damage equipment, make its way into sensitive processes, shorten the life of air-driven tools or reduce product quality. Many applications use gas or air at pressure so calculating pressure dew-point (PDP) temperature accurately is essential. This article covers the basic theory behind calculating dew point temperatures at higher than atmospheric pressure and suggests best practices for sampling and measurement techniques.

What is a pressure dew point?

In industrial applications, dew point is used as a measurement of humidity in either a process gas or within a controlled environment. The term ‘pressure dew point’ (PDP) is used when the gas is at a pressure higher than normal atmospheric pressure. Atmospheric dew point is often abbreviated to (ADP).

Unlike moisture content which is a fixed value no matter the temperature or the pressure, the dew point temperature of a gas is relative to the pressure. The pressure difference does not need to be high to affect the dew point temperature: even a change of 1 bar above atmospheric pressure counts as a pressure dew point.

How does pressure affect dew point?

Dew-point temperature is a key parameter for avoiding condensation. Condensation occurs when the ambient temperature of the process drops below the dew point temperature of measured gas. For example, if the dew point temperature of a compressed air line is +7 °C dew point, and the ambient temperature is +20 °C, there will be no condensation. If the ambient temperature drops to +6 °C, then moisture will condense in the line.

However, dew point temperature is not fixed – it's related to both the absolute moisture content and the pressure of the gas. Change either one of these parameters and the dew point temperature will alter too.

As the total pressure of the gas rises, the partial pressures also rise. Eventually, as the pressure increases, the water vapor in the gas will reach its saturation point and will start to condense, despite the temperature remaining the same. The diagram below illustrates this effect.

What are the main applications for measuring pressure dew points?

The three main applications for measuring PDP are:

How to get the best results when measuring water dew point at pressure

Using the right sample system is essential for accurate moisture measurements. Dead volumes, trapped moisture and choice of materials are all common pitfalls to watch out for – see our article ‘8 common pitfalls in moisture measurement’ for more information on choosing the right moisture sampling components. When sampling at high line pressures, ensuring that the system has been pressure-tested is also important for safety.

Making sure that the dew-point transmitter used is suitable for use at pressure is also important. All Michell Dew-Point Transmitters are capable of operating in pressures up to 450 barg (6570 psig). The Michell MDM300 , MDM50, Easidew Portables, and Compressed Gas Dew Point Meter - Easidew PDP dew point meters are also suitable for use at line pressure. MDM50 and Easidew Portable include integral sampling systems, while the MDM300 has a separate, optional sampling system.

The Michell Easidew Advanced Online Hygrometer is capable of displaying and compensating for live pressure changes with an optional external pressure sensor.

Get expert advice for measuring PDP in your application

Our sales engineers are happy to discuss your application and help you find the best solution. As well as our standard sampling systems, we can also help with customised products. Get in touch today with your question.

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