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From wikipedia:

If a liquid is used as the working fluid, the strength of the vacuum produced is limited by the vapor pressure of the liquid (for water, 3.2 kPa or 32mbar at 25 °C). https://en.wikipedia.org/wiki/Aspirator_(pump)

I'm exploring the possibility of using fully saturated salt water in a closed circuit outside during the winter. The fluid's vapor pressure decreases with decreasing temperature. Please let me know if that would increase or decrease the strength of vacuum, or what the magnitude change would be. Ideally I'd like a function I can plug the vapor pressure into, and see the theoretical best vacuum strength.

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    $\begingroup$ I would think the lowest achievable pressure, corresponding to the "strongest" vacuum, is given by the vapor pressure at that temperature. This makes sense if you consider the fact that if you lower the pressure of the liquid below the vapor pressure, it will vaporize. Raoult's law will give you the vapor pressure as a function of salt concentration, and the Clausius-Clapeyron equation will give you the vapor pressure as a function of temperature. So you can plug in a temperature and concentration and figure out the best theoretical vacuum that way. $\endgroup$
    – Kyle Arean-Raines
    Commented Sep 8, 2015 at 19:52
  • $\begingroup$ A word of warning that has nothing to do with physics: the use of fully saturated salt solutions in a mechanical system sounds like a recipe for disaster for several reasons. For one thing salt solutions tend to be extremely corrosive and they have a tendency to crystalize in all the places where they can do the most mechanical harm. $\endgroup$
    – CuriousOne
    Commented Sep 8, 2015 at 20:33
  • $\begingroup$ @CuriousOne Makes sense, thank you. I liked salt water over a diluted solution of antifreeze due to cost. I'd probably be using plastic/vinyl everywhere. $\endgroup$
    – Andrew Smart
    Commented Sep 9, 2015 at 21:10
  • $\begingroup$ @KyleArean-Raines I saw Raoult's law, and wasn't aware of the Clausius-Clapeyron equation. I was just relying on tables to find the vapor pressure of salt water. Thank you that will be helpful! Seems everywhere people only use tap water with aspirators. Even there I hadn't seen a characterization of resultant vacuum when water temperature or aspirator design varies. $\endgroup$
    – Andrew Smart
    Commented Sep 9, 2015 at 21:24
  • $\begingroup$ @KyleArean-Raines Ha! I didn't realize till last night that the vapor pressure of the fluid is the theoretical strongest vacuum strength (lowest achievable pressure). I wouldn't have realized that without your comment, so thank you. Feel free to make an answer and I'll mark it as such. Intuitively I guess an eddy is created, where the the moving fluid is vaporized and then sucked into the fluid again, preventing any further evacuation of the container. I hope salt won't crystallize there. $\endgroup$
    – Andrew Smart
    Commented Sep 10, 2015 at 17:18

2 Answers 2

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[Moved over from a comment]

I would think the lowest achievable pressure, corresponding to the "strongest" vacuum, is given by the vapor pressure at that temperature. This makes sense if you consider the fact that if you lower the pressure of the liquid below the vapor pressure, it will vaporize. Raoult's law will give you the vapor pressure as a function of salt concentration, and the Clausius-Clapeyron equation will give you the vapor pressure as a function of temperature. So you can plug in a temperature and concentration and figure out the best theoretical vacuum that way.

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NurdRage shows how using colder water allows a greater vacuum to be reached in this video:

https://www.youtube.com/watch?v=tYLlkTDstmo

If the gas being pumped has a large heat content- such as boiling water vapor in his example- the lowest pressure is limited by the temperature of the warmest water in the cycle, after it has absorbed that heat input. A larger aspirator with better profiles and lower boundary layer losses could probably get closer to ideal performance.

Water pumps and icemakers are cheap enough, go big, go cold, and don't bother with annoying liquids if you can.

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