Dew point

WHAT IS DEW POINT?

As the term implies, it is the climate point at which the relative air humidity equals 100% and begins to condense. The dew point is given in °C Td. The dew point temperature is a measurement of the water vapor content in a gas. If air is compressed or expanded, its dew point temperature changes. If air is compressed, it is able to absorb less water and the dew point rises until the air is saturated and begins to condense. In this connection, the term pressure dew point measurement is also used to describe measurement of the dew point in gases above the ambient temperature. 

WHAT DOES LOW DEW POINT MEASUREMENT MEAN?

A low dew point is usually when the dew point temperature falls below -30 °C Td. This means that the air is extremely dry and contains almost no water molecules. A dew point of -38 °C Td corresponds at 23 °C to a humidity value of 0.8 %RH, which corresponds to the accuracy of a Rotronic HygroClip2 probe. This shows why low dew point measurement is very demanding. Very complex electronics and a highly sensitive sensor are needed to deliver high-quality results in the measurement of residual moisture.

WHAT IS IMPORTANT IN LOW DEW POINT MEASUREMENT?

The measurement of such small numbers of water molecules places high demands on the measuring point. For example, it is important that there is always good airflow across the sensor so that representative measured values can be obtained. Rotronic offers a special measurement chamber for this that was developed specifically for the mechanical design of the dew point probe. Excessive flow can lead to a local pressure drop, which influences the measurement, while insufficient flow can result in measurement of a local microclimate. The constant airflow of the measurement chamber of 1 l/min. thus guarantees stable and reliable measurement results.

The equilibrium times in dew point measurements can be considerably longer than those for humidity measurements. All the materials in the system and around the sensor must be dried out. Under certain circumstances it can take hours before a low dew point system has balanced out and the residual moisture has escaped from all the materials.

WHY IS LOW DEW POINT MEASURED?

There can be many reasons for monitoring the dew point. Compressed air systems with an excessively high dew point can condense, thereby causing valves to become blocked or corroded. In addition to this, dry compressed air systems require less maintenance, which saves costs. Equipment connected to the system places high demands on dryness and require a low dew point of the compressed air. Further, there are sensitive processes such as the drying of injection molding granulate and the compressed air for spray painting systems, which place particularly high demands on the dew point of the system. Compressed air systems can further be classified according to ISO 8573. Depending on the classification of the system, there are different dew points that need to be monitored and controlled.