Dew point
Dew point turns up in 3 fields: corrosion, heat treating, HVAC. Here is what it means to the people in each one.
Dew point in corrosion
Temperature at which moisture condenses from air; specs keep steel at least 3 C above it for blasting and painting.
The temperature at which water vapor in the air condenses to liquid. If steel is at or near the dew point, an invisible film of moisture forms on it, causing flash rust on blasted steel and poor adhesion under coatings. Specs nearly always require the surface to be at least 3 C (5 F) above dew point, often with relative humidity below 85 percent, and inspectors check it with a psychrometer or digital gauge before and during the work.
"Steel's at 11 C and dew point is 9, that's under the 3 degree spread, nobody sprays till it warms up."
Dew point in heat treating
The temperature at which water vapor in a furnace atmosphere condenses, used as a gauge of carbon potential.
Measure of the water vapor in a protective atmosphere. In endothermic gas, water vapor and carbon dioxide rise and fall together, opposite to carbon potential, so a lower dew point means a richer, higher-carbon atmosphere. A rising dew point points to water leaks, air infiltration or a tired generator. Bright annealing of stainless in hydrogen needs dew points of minus 40 °C or drier.
"Dew point's up to 45 on the IQ, that's why the case is lean, check the generator and the door seals."
Dew point in HVAC
The temperature at which air becomes saturated and water starts condensing out when it is cooled.
Dew point depends only on how much moisture the air holds, so it is a direct measure of absolute humidity. Any surface colder than the air's dew point gets wet: windows, cold water pipes, ducts in a humid attic, and a cooling coil. That last case is the point: a coil must run below the entering dew point to dehumidify. Dew points above about 60 degrees F feel muggy indoors.
"Attic dew point was 72 and those supply ducts run at 58, of course they're sweating through the ceiling."