Radiator cap venting ? Physics expert question perhaps ?

[MIG Wielder]

You know how it is when you discuss items in the pub ?
Does an "18lb radiator cap" vent at 18 psi above atmospheric pressure ?
So in a vacuum would it actually vent at (18 + 14.7 ) psi ?

The follow-on question is ... since the vapour pressure of water at 100 deg C is 14.7 psi, does this mean that an 18lb pressure cap will only vent at temperatures above 125 deg C ?

Sorry, physics is not my bag.
Thanks all,
Tony.

[Mahesh]

Does this help,

water-pressure-boiling-temperature-2.png (104.88 KiB) Viewed 1089 times

[matt of the vivas]

Hi Tony
As i understand it, a standard PSI reading is taken as pressure ABOVE normal atmospheric pressure. So yes, in a vacumn an 18psi cap will actually open at 14.7 (atmospheric) plus 18 - 32.7psi.
A reading taken like this though would be be referenced as "psia" (pounds per square inch absolute) to indicate that it was taken in a vacumn.
When Tilson has finished polishing his Acclaim he will be able to tell you more - this is a question right up his street.
Matt.

[MIG Wielder]

Great ! Thanks guys. I had seen something similar to that table Mahesh and it says the same thing. And that's the good thing about this forum; we can enjoy a good laugh.
I shall await young Jonner's comments.
Cheers All,
Tony.

[Jon Tilson]

Lol...

I thought our caps would vent at 13 psi ( above atmospheric )

The table is very useful...

But if you are venting you have other problems to which upping the cap pressure is not the solution.

And I would never polish an acclaim as I would just get my hands covered in excrement...


Jonners