Yeah, the same reason your tire pressure gauge 
reads zero at the very bottom instead of 14.
Edward

At 07:17 PM 7/5/2009, David Beierl wrote:
>At 05:45 PM 7/5/2009, John Rodgers wrote:
>>What IS the correct pressure for the cooling system at operating
>>temperature? I seem to remember seeing somewhere that it is 15 psi. As
>>the 'Lil Boy sez "That ain't much!"  Standard temperature and pressure
>>at sea live is  60 �F and 14.73 psi. If the operating pressure of the
>>van cooling is 15 psi, and you get this big burgle - something is
>>plugged up or there is air in the system.
>
>That's 15 (13?) psi *in addition* to whatever 
>the local atmospheric pressure is...
>
>:-)
>d
>
>
>  --
>David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
>'89 Po' White Star "Scamp"