Understanding Humidity Monitoring in the Data Center

Environmental monitoring and control in the data center are 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!

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. The only way the dew point can change is if water is extracted (e.g. by cooling it below the dew point) or added (e.g. due to humidification or more humid outside air being mixed in).

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommendations for data centers and server rooms include: 

Improve Your Humidity Monitoring

Packet Power Environmental Monitors allow you to map relative humidity by combining a precise measurement of the dew point and up to 6 distinct temperature points per monitor and an unlimited number of monitors, providing a 3D map of temperature and relative humidity distribution within your cabinets.

Screen display fragment from the Packet Power EMX system, displaying dew point temperature and spatial temperature, and relative humidity distribution within the cabinet. Each environmental sensor from Packet Power reports temperature, dew point and relative humidity.

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.

Retrofitting a data center (or really any large building) with wireless environmental monitors is simple. This video shows how quickly a Packet Power environmental monitor can be installed and begin giving you valuable humidity data. 

Many companies find that once they have temperature and humidity information, they decide to invest in differential pressure sensors so they can identify how air is flowing through their facility and work to optimize their HVAC efforts.

Resources:

• Case Study: Environmental Monitoring Made Easy for Colocation
• Partners who can help make the most out of your environmental data
• Data center acronyms defined

 Summary:

1. Relative humidity is RELATIVE. It is only valid for a given temperature.
2. Dew point characterizes the absolute water content of air, regardless of temperature.
3. Environmental monitoring devices can help you maintain conditions optimal for your equipment.