Relative Humidity

Relative humidity (RH) is the amount of moisture in a given sample of air at a given temperature in relation to the maximum amount of moisture that the sample could contain at the same temperature. If the air is holding all the moisture it can for a specific set of conditions, it is said to be saturated (100 percent RH). Since air is a gas, it expands as it is heated, and as it gets warmer the amount of moisture it can hold increases , causing its relative humidity to decrease. Therefore, in a system using subfloor air distribution, the ambient relative humidity will always be lower than in the subfloor.

Ambient levels between 45 and 50 percent RH are optimal for system reliability. Most data processing equipment can operate within a fairly wide RH range (20 to 80 percent), but the 45 to 50 percent range is preferred for several reasons:

• Corrosion. High humidity levels can cause condensation within computer equipment which can cause corrosion to components . For more information, refer to "Corrosion" on page 103.

• Electrostatic Discharge (ESD). ESD can cause intermittent interference in equipment. It is easily generated and less easily dissipated when the RH is below 35 percent and becomes critical at lower ranges. For more information, refer to "Electrostatic Discharge" on page 103.

• Operating time buffer. This humidity range provides the longest operating time buffer in the event of environmental control system failure.

Although the temperature and humidity ranges for most hardware are wide, conditions should always be maintained near the optimal levels. The reliability and the life expectancy of the data center equipment can be enhanced by keeping RH levels within the optimal ranges.

Certain extremes (swings) within this range can be damaging to equipment. If, for example, very high temperatures are maintained along with very high percentages of RH, moisture condensation can occur. Or, if very low temperatures are maintained along with very low percentages of RH, even a slight rise in temperature can lead to unacceptably low RH levels.

The following table shows ranges for temperatures, relative humidity, and altitude.

Table 8-1. Environmental Requirements

Environmental Factor	Optimal	Operating	Non-Operating
Temperature	70 ° to 74 ° F (21 ° to 23 ° C)	50 ° to 90 ° F (10 ° to 32 ° C)	-4 ° to 140 ° F (-20 ° to 60 ° C)
Relative humidity	45% to 50% RH	20% to 80% RH (noncondensing)	Up to 93% RH
Altitude	Up to 10,000 ft (3,048 m)	Up to 10,000 ft (3,048 m)	Up to 40,000 ft (12,192 m)

Note

Severe temperature or RH swings should be avoided. Conditions should not be allowed to change by more than 10 ° F (5.5 ° C) or 10 percent RH in any 60-minute period of operation.

Corrosion

Excessive humidity in the air increases the corrosive potential of gases and should be avoided in the data center environment. Gases can be carried in the moisture in the air and transferred to equipment in the data center.

Drastic temperature changes should also be avoided. These can cause latent heat changes leading to the formation of condensation. This usually happens in areas where hot and cold air meet, and this can cause a number of hardware problems.

• Water can react with metals to form corrosion.

• Water can electrochemically form conductive solutions and cause short circuits.

• If there are electrical potential differences between two dissimilar metals in a component, electrolytic or galvanic corrosion can occur.

• Water can form a reactive combination with gases present in the air, and the resultant compounds can corrode hardware.

Keep relative humidity levels at the appropriate percentage.