Bleed
Bleed turns up in 4 fields: tribology, boilers, waterproofing, vacuum. Here is what it means to the people in each one.
Bleed in tribology
Separation of oil from a grease, either as a storage problem or as the intended release of oil to a bearing.
The release of base oil from the thickener structure. Some bleed is the whole point, because that oil is what actually lubricates the rolling contacts. Too much in storage leaves a puddle on top of the pail or cartridge, and too much in service leaves a dry, hard crust behind. Oil separation is measured with static tests such as ASTM D1742 and D6184 and rises with low-viscosity base oils, heat and soft grades.
"Every cartridge off that pallet has oil bleed pooled on top, they sat in the hot shed all summer."
Bleed in boilers
To let trapped air out of a hot water radiator, baseboard or high point by opening its small vent until water appears.
Hot water radiators and baseboard have small manual vents opened with a key, coin or screwdriver. Bleeding them lets out air that blocks flow and makes a radiator cold at the top. Frequent bleeding means air is getting in somewhere, often through an automatic fill valve feeding a leak or a pump located wrong.
"Bleed the rads on the second floor, the tops are cold and you can hear it gurgling."
Bleed in waterproofing
Staining from fluids or plasticizers that migrate out of a sealant into porous stone or masonry.
Some sealants release plasticizers or unreacted silicone fluids that wick into marble, limestone, granite and concrete, leaving a dark band along both sides of the joint that also attracts dirt. The stain is usually permanent. Stain testing with the actual stone, as in ASTM C1248, and specifying non-staining grades prevent it. Bleed also describes color running from a sealant into a coating applied over it.
"There's a dark halo an inch wide on every limestone joint, that's bleed from the wrong silicone."
Bleed in vacuum
To admit a small, controlled flow of gas into a vacuum system through a needle or leak valve.
To let gas in deliberately and slowly, through a needle valve, a variable leak valve or a mass flow controller. Techs bleed nitrogen into a foreline to keep a rotary pump in viscous flow so it cannot backstream, bleed argon into a chamber to set sputter pressure, or bleed a known gas in to calibrate an RGA. A bleed is also the small gas line or valve used to do it.
"Bleed a little nitrogen into the foreline when the system idles overnight so the mech pump doesn't backstream."