I am trying to create an air tight cable exit system for a fuel tank.

Basically, an instrument is to placed in the tank, with its cable exiting through the top of the tank. This exit point needs to be air tight.

However, most of the cable glands i see on the market are specified as being water tight.

Can i assume this to mean it is air tight as well ??

  • $\begingroup$ Any of the materials may be air permeable... $\endgroup$
    – Solar Mike
    Jul 12, 2019 at 19:53
  • $\begingroup$ Helium is used to test the leak rates for critical seals. $\endgroup$ Jul 12, 2019 at 22:53

1 Answer 1


In general water tight does not mean air tight, while air tight does mean water tight assuming the material is not water soluble. Air contains some smaller molecules than water. Either can diffuse through many different materials if given enough time though. As a side note, if the fuel tank is supplying a motor then fresh air should be allowed in as the fuel level decreases or the motor will have a hard time pulling fuel out as a vacuum builds.

  • 1
    $\begingroup$ Seen a mechanical fuel pump "suck" a metal fuel tank just about flat before - customer noticed when they only needed 3 gallons and it was full... they had fitted a non-vented (cheap aftermarket) locking filler cap. The cost of the new tank far outweighed the saving they made on the cheap cap. $\endgroup$
    – Solar Mike
    Jul 12, 2019 at 20:40
  • $\begingroup$ We did in the gas tank in my mom's 1968 Plymouth station wagon that way. It wasn't "get the cheap cap", though -- it was just when the pollution control stuff was changing every which way, and the parts guy handed us the wrong locking gas cap. Fortunately it was in the mid 1970's, so there were cars in the junk yards with replacement tanks. $\endgroup$
    – TimWescott
    Jul 12, 2019 at 20:45
  • $\begingroup$ Is it because water molecules are bigger, or because water has surface tension? I suspect the latter, in most cases -- I suspect that a sufficiently hydrophobic, sufficiently fine mesh would hold water and let air pass through easily. $\endgroup$
    – TimWescott
    Jul 12, 2019 at 20:47
  • $\begingroup$ In that situation the air would slowly pass through the water as well. $\endgroup$ Jul 12, 2019 at 21:28
  • $\begingroup$ The reason is surface tension, not molecule size. The rest is correct and heed the side note! $\endgroup$
    – mart
    Jul 12, 2019 at 22:20

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