Automotive cooling system pressures

[sscmondial]

I'll start by saying I'm new here, but have found many help full tips over the last two years when searching the net. thanks.

the other day I got to thinking about something that seems simple on the outset but didn't solve out that way. cooling system pressures and rad caps.

rad caps are rated with a pressure relieve, domestic cars are usually in 18~24lbs and imports in bar from 0.9~1.3. now to the question, the cap vents pressure at its listed rating, but is that rating PSIG or PSIA? also once the system is closed is it still subject to external atmospheric pressure?

the reason is I noticed the 0.9 cap on the car and wondered if that's is below atmosphere of 1.0 how does it maintain any pressure to raise the boiling point? goes back to PSIG/PSIA, does it rely on the external air pressure in addition to the spring rating?

oddly I got no answer from Stant when I called the tech dept. it seems like a straight physics question, but with the system closed you can run a vacuum or pressure independent of the external pressure.

 

[MikeHalloran]

They're both gage pressures.

The 18..24 is psig. The 0.9..1.3 is barg.



Mike Halloran
Pembroke Pines, FL, USA

 

[TheBlacksmith]

The spring is imposed between the system and the atmosphere, therefore the reference is atmosphere - PSIG.

 

[reidh]

If the .9 bar was absolute, your coolant would boil at a much lower temperature. That is the main reason for maintaining a positive pressure in the cooling system: it raises the boiling temperature of the coolant.

Reidh

 

[sscmondial]

if it was gauge pressure wouldn't it be higher by two atmospheres. the for eg. 0.9+1.0 for a barg of 1.9 + external atmosphere of 1.0 for a 2.9, and that would be way to high for any system to handle. seems it would then be a bara cap rating against the external elevation pressure.

 

[reidh]

A pressure referenced to the external elevation pressure is gage pressure.

Reidh

 

[MintJulep]

Actually, the rating is a delta pressure.

The inside can be this much higher than the outside before the relief opens.

It doesn't matter if you measure in g or a, as long as you measure both the same.