Dew point
The temperature at which air becomes saturated and its water vapor starts to condense, which makes it a direct measure of how much moisture the air holds. Unlike relative humidity, dew point does not change when air is heated or cooled without adding or removing moisture.
Also called dew point temperature, dewpoint
What is dew point in HVAC design?
Dew point is the temperature at which air's moisture starts to condense, so it measures moisture content directly. Air at 68°F and 45 percent relative humidity has a dew point of about 46°F, and any surface colder than that collects condensation.
Dew point: how it works
Air with a given moisture content has one temperature at which that moisture would saturate it. Cool the air, or a surface it touches, below that temperature and water condenses. Heating the air leaves its dew point unchanged while relative humidity falls, because warmer air could hold more water yet holds the same amount, which is why dew point is a stable variable for moisture control and relative humidity is not.
Where you meet the term
- In a cleanroom basis of design, where the supply dew point is set from the room humidity target and occupant moisture.
- In the ASHRAE data center thermal guidelines, which state the recommended humidity range as dew point limits.
- On a chilled water pipe or duct, which sweats wherever its surface sits below the surrounding dew point.
How to check it
- Read dew point on the room monitor. Environmental monitors that display dew point beside temperature and relative humidity show it holding steady while room temperature swings. Owner check
- Find dew point from temperature and humidity. A technician enters dry bulb temperature and relative humidity on a psychrometric chart or calculator, or measures dew point directly with a chilled-mirror or capacitive dew point sensor. Technician
Figures you will see
| Condition | Figure | What it tells you |
|---|---|---|
| Room at 68°F and 45 percent relative humidity | Dew point about 46°F | Read against the model documentation |
| Room at 68°F and 60 percent relative humidity | Dew point about 54°F | Read against the model documentation |
| ASHRAE recommended range for data center server inlets | Minus 9 to 15°C dew point | Read against the model documentation |
Dew point: what it is not
- Dew point is not the same as relative humidity. A room can read 40 percent relative humidity at 72°F and 50 percent at 66°F with the same dew point.
- A coil cannot dry air below its own surface temperature. The dew point of air leaving a coil is limited by the coolant temperature.
Dew point questions
Why do data center guidelines use dew point instead of relative humidity?
Dew point describes the moisture present regardless of how hot the air is, and server inlet and exhaust air differ widely in temperature. The fifth edition of ASHRAE's thermal guidelines moved its lower humidity limit to a dew point basis.
How do you stop condensation on cold ducts and pipes?
Keep the surface above the dew point of the space around it by insulating ducts and pipes that carry air or water colder than that dew point, and seal the insulation against vapor.
What supply dew point does a cleanroom need?
The supply must be drier than the room by enough to absorb the room's moisture load at the supplied airflow. The HVAC Bench humidity example needs about 37°F supply dew point to hold 40 percent relative humidity at 68°F.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, the bin and degree-day methods used for seasonal energy estimation, the table of heat gain from occupants by activity including light bench work at 275 BTU per hour sensible and 475 latent, the standard-air sensible and latent heat factors, and the orifice flow relationship used to estimate airflow through openings and gaps from pressure difference.
- Thermal Guidelines for Data Processing Environments, Fifth EditionASHRAE Datacom Series book 1 defining the recommended IT equipment inlet envelope of 18 to 27 degrees Celsius with a dew point range of minus 9 to 15 degrees Celsius and an upper relative humidity limit, the wider allowable envelopes for equipment classes A1 to A4 with A1 at 15 to 32 degrees Celsius, and the change in the fifth edition to a dew point basis for the lower humidity limit.
- 2011 Gaseous and Particulate Contamination Guidelines for Data CentersASHRAE TC 9.9 recommendation that data centers be kept clean to ISO 14644-1 Class 8, achievable by continuously filtering room air with MERV 8 filters and filtering air entering the data center with MERV 11 or preferably MERV 13 filters; that relative humidity be kept below 60 percent so settled dust with a low deliquescent relative humidity does not become conductive; and that gaseous contamination stay within modified ISA-71.04 severity level G1.