Environmental monitoring and control in the data center is usually assumed to be synonymous with temperature control: how to make sure the air is at the optimal temperature at each place. Temperature, however, is only one of the two key parameters that determine the quality of air. Its twin, humidity, is much less understood.
When humidity is too high, water condenses leading to water damage and an unpleasant, sticky environment. When humidity is too low, static charge buildup increases posing risk to electronics.
Most of the confusion related to humidity comes from how it is (not) measured. In many data centers there is at least one humidity monitor that reports relative humidity. The problem lies in what it reports: relative humidity. The word “relative” refers to the humidity (i.e. water content) of the air relative to what it could be given its temperature; hotter air is capable of containing more water. Reporting or controlling relative humidity without referring to temperature at the same time is meaningless. The same air has different relative humidity depending on its temperature.
As air is cooled, its relative humidity increases until it reaches 100%, at which point water will start condensing. That temperature is known as the dew point. If you cooled your data center below the dew point temperature, it would get foggy! The dew point is a great parameter to track because it is absolute – a given mass of air always has the same dew point, no matter what temperature it’s measured at. The dew point of air is the same when it’s entering your server on the cold side or leaving it on the hot side (unless, of course, you have a sprinkler system in your server…). The only way the dew point of an air mass can change is if water is extracted (e.g. by cooling it below the dew point, which is why there is condensation within the HVAC system) or added (e.g. due to humidification or more humid outside air being mixed in).
Given the dew point of a given air mass, its relative humidity at any point can be calculated by knowing the temperature at that point. Packet Power Environmental Monitors allow you to map relative humidity by combining a precise measurement of the dew point and up to 12 distinct temperature points, providing a 3D map of temperature and relative humidity distribution within your cabinets.
Differences in dew point temperatures reported by multiple dew-point monitors can be used to detect potential air mass mixing or (de)humidification, indicating opportunities for potential energy optimization.
Screen display fragment from the Packet Power EMX system, displaying dewpoint temperature and spatial temperature and relative humidity distribution within the cabinet. All data on this screen has been obtained from a single E312-H000 monitoring device.
Packet Power E312-H000 wireless 12-channel temperature, relative humidity and dewpoint monitor. It can be powered externally for from batteries.
Summary:
- Relative humidity is RELATIVE – it is only valid for a given temperature.
- Dew point characterizes the absolute water content of air, regardless of temperature.
- ASHRAE recommendations for data centers and server rooms include:
a) lower dew point limit of 5.5°C or 41.9°F
b) upper dew point limit of 15°C or 59°F
c) maximum relative humidity level of 60%
Alan Katz
Packet Power Article