Bob,

 I can't help you on telling when the unit is full, but I may have a
clue as to where the leak could be.
 If the hoses (both High and Low side), but particularly the High side
hasn't been changed lately, that  =20
 could be the source of the leak.  Over the years, the Freon degrades
the rubber in the hoses, and can become
 porous.  When that happens, there is enough room in the rubber
structure for freon molecules to get through=20
 the fabric of the hose.  Over a relatively long period of time, the
freon can all leak out.
 The point being, replace the hoses if they haven't been replaced
recently.

 Regards,

 Jim

> -------- Original Message --------
> Subject: [OT] how to tell when you've filled your auto air conditioner
> From: "Bob Blick" <bobblick@ftml.net>
> Date: Tue, July 05, 2011 11:10 am
> To: piclist@mit.edu
>=20
>=20
> I was "helping" a friend fill his r134a air conditioner yesterday. No,
> the neighbor kids didn't steal the refrigerant so there must be a leak,
> but very slow over several years.
>=20
> I never have understood how to tell when you have the system near the
> perfect level of refrigerant. It is very hard to measure temperature
> accurately and the pressure ranges given in the service manual are so
> broad it's impossible to use those.
>=20
> So I decided that with 90 F air entering the condenser and high plus low
> side pressure total about 260 psi (225+35) at high idle, and 45 F air
> coming out the vent is OK. But it is probably still a little low on
> refrigerant.
>=20
> But other than evacuating and refilling with a measured amount, does
> anyone know a better way with just thermometers and a gauge set?
>=20
> And with very slow leaks(several years) does the oil also leak out?
>=20
> I have found useful resources on the web but nothing that tells me how
> to know when it's "full".
>=20
> Thanks,
>=20
> Bob
>=20
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