Its bulk modulus at 1000 ☏ (538 ☌) is 310,000 psi (2.14 GPa), higher than of a hydraulic oil at room temperature. NaK-77, a eutectic alloy of sodium and potassium, can be used as a hydraulic fluid in high-temperature and high-radiation environments, for temperature ranges of 10 to 1400 ☏ (-12 to 760 ☌). Some examples include: glycol ethers, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils. Other base stocks are used for specialty applications, such as for fire resistance and extreme temperature applications. Natural oils such as rapeseed are used as base stocks for fluids where biodegradability and renewable sources are considered important. Today most hydraulic fluids are based on mineral oil base stocks. Beginning in the 1920s, mineral oil began to be used more than water as a base stock due to its inherent lubrication properties and ability to be used at temperatures above the boiling point of water. The original hydraulics fluid, dating back to the time of ancient Egypt, was water.
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