Humidity – What is it?

Air is made up of a combination of gases which include Nitrogen, oxygen, carbon dioxide and water vapour. The water vapour in air is known as humidity. Air containing the proper amount of water vapour in the IT environment plays an important role in maximising the availability of computing equipment. Too little humidity causes static electricity to build and potentially damage equipment, too much humidity causes condensation which creates issues with corrosion of metals and shorting of circuits.

There are two terms that are used with measuring humidity, the first is relative humidity (also known as RH) it is always expressed as a percentage from 0% to 100% and represents the percentage of water vapour in the air relative to the maximum amount of water vapour the air can hold at a given temperature. The second term is dew point, which is always expressed as a temperature, is the temperature at which the gas turns to liquid on objects. The two terms are related in that the dew point temperature will rise as the humidity percentage rises.

Monitoring systems for Data Centres such a Schneider Electric’s Struxureware and Ecostruxure IT include combined temperature and humidity sensors to give the IT professional data and create alerts when thresholds are exceeded in the humidity being too low or too high. Acceptable ranges of humidity in the data centre is between 20%RH and 80%RH.

Causes of change in humidity levels

Infiltration, condensation and ventilation cause changes to humidity.

Infiltration – if a high humidity body of air is next to a low humidity of air they would equalise to a level between the high and low points. In our environment, this means that the normal office humidity/temperature and the data centre can be different and if a door is left open, it can cause humidity changes. If colder air is let into the data centre or hot aisle containment system then the humidity could drop to levels outside the thresholds.

Condensation – in some conditions the process of cooling air from an IT environment can remove large amounts of water vapour resulting in low relative humidity levels and condensation on the cold cooling coils. For this reason, it is recommended that the cooling process include pumps to transport the water away from the IT equipment.

Ventilation – Fresh outdoor air must be introduced into buildings to supply oxygen to the people inside. The office air handing systems should not be used in the data centre as it affects the relative humidity. When buildings are designed this is taken into account when the server room/data centre is designed at the build stage. A server room/data centre which is built later in an area originally designated as office space, should be sealed from the normal air handling system to prevent these issues.