While filling liquid oxygen into a tank, why do they do open one end? Then what is use of filling?

I don't think people who are filling this truck are beginners. enter image description here

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    $\begingroup$ Don't you agree that in order for something to get in the tank, something should get out (to avoid increasing pressure to up levels where it wouldn't be possible to fill)? $\endgroup$
    – Molx
    Commented Sep 13, 2015 at 4:54
  • 3
    $\begingroup$ I really hope that's nitrogen and not oxygen. $\endgroup$ Commented Sep 13, 2015 at 7:38
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    $\begingroup$ Moreover, you need to allow the vapours to get out. $\endgroup$
    Commented Sep 13, 2015 at 10:16
  • $\begingroup$ Releasing oxygen gas right over the exhaust pipe seems like a wonderful idea. $\endgroup$
    – Marcel
    Commented Nov 5, 2016 at 12:31

1 Answer 1


The end is opened to allow oxygen gas to escape. The flow of liquid oxygen into the tanker will not be pumped, just undergoing the natural tendency to flow from an area of high pressure to low pressure. As such, there would come a point when the residual gas in the tanker would start to provide back pressure on the flow input, slowing it down & potentially stopping it once pressures equalize between the tanker and the source.

By allowing oxygen gas to escape the tanker, the pressure will always be lower in the tanker, and the flow will be faster more adequately fill the tanker. The same technique is used when topping up liquid nitrogen in an NMR spectrometer, for example.

  • $\begingroup$ The liquefied gas is transported without pressure, like you would carry it around in an open Dewar, in contrast to a pressurized gas bottle that has about 200bar, which would be difficult to build on such a scale. $\endgroup$ Commented May 12, 2020 at 12:57

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