Can I tell if there is freon in the AC?

Is it possible to tell if a roof-top AC still has freon in it?
Is there a gauge, or can I push a valve and open it a tiny bit and hear if anything comes out? Or some other way. I figure they might be empty already, and that's one likely reason why they don't work.
I"m cleaning out a building that is about to be razed, and it has 3 big AC's on the roof. Two are broken. The third was fixed 9 months ago and probably still works. We don't want to vent freon, but if the freon has already escaped, a friend and I would like to sell the copper, etc.
Last week we took 1875 pounds of various metals to the scrap yard.
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If it is a large unit, it should have two Schrader type valves where you hook up the gauges. Window AC and refrigerators usually have a pinched off copper tube. Scrap junkies often check by poking a hole, but getting caught can be a big fine.
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Edwin Pawlowski wrote:

If you're not sure, you probably shouldn't be monking around with it. A realease of refrigerant through the schrader valve core can have problems. Like you open it to let a little out and the core sticks, then in your panic, you try to stop it - and since it's attached to the liquid like, liquid refrigerant starts to escape, and now you've frost bit your fingers [ minis 40 degrees]. It's best just not to monkey and let someone who knows do it.
--
Zyp



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and recover the refrigerant if there is any present. The technician should then tag the units to indicate that they are free of refrigerant. This is required by federal law and penalties for venting freon into the atmosphere can be severe. The freon may have residual value, also. There has to be someone in the area who services such units. Call and ask
Any liquid freon in the unit would be at ambient temperature and under pressure. If the freon escapes and the pressure is no longer present, it will spew and boil violently and get extremely cold. It can be very dangerous, especially if there is a large quantity present. Do not risk messing with it, due to the danger and the legal issues.
Don Young
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wrote:

Thank you.
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wrote:

Thank you.
Clicked too soon. Thank you. I'll talk to the other guy, whose idea this was, and we'll do it right.
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