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There's an art to leak detection. It is not difficult--unless the installer has done a lousy job (many leaks) and run the plumbing in places that are impossible to reach or eyeball.
Many repairmen will use a sniffer (leak detector) while a system is running and there is airflow in the area. The folks who make sniffers, one and all, say that's the wrong way to use them they can only be used in still air--when the system is pressurized but OFF.
Many ways to screw up an AC repair job, at a certain point it actually pays to scrap it and install something newere, better, with a warrantee from a professional. (Well, if you can find one. Odds are he's too busy making the big bucks off supermarket reefer displays to run out of boats.)
Assuming you can get access to the joints, there are only a few ways to detect leaks:
1-Pressurize the system, run it. Slobber UV-dye soap water over all the joints liberally, they will blow bublbes if they are "grossly" leaking.
2-Buy a sniffer, with or without airflow suction on it. This is a hand-sized box with a wand and a bare diode on the end of the wand. When the bare diode is passed into refrigerant gasses, the box beeps, something like a geiger counter. You've got to adjust the sensitivity (zero it out) and keep the tip unconaminated by oils or dirt, and they only work in STILL AIR. But, they can detect leaks of as little as 1/4oz to 1/2 oz of refrigerant per year--that means real slow leaks.
3-Charge the system with oil that has UV dye in it. Run it at full pressure for a while, shut dow, examine with a black light (and yellow goggles, necessary for the contrast) to see where the dye came out.
4-Most expensive, an ultrasonic sniffer. Like the other sniffer, but it has a tiny microphone and literally hears the ultrasonic "pheeee" of escaping high pressure gas--of any kind. Like any other sniffer, you have to run the business end very close to, but not touching, the suspecting leaks. Sometimes at a rate of 2cm per minute--patient work.
But the leaks CAN be found, unless some genius has made the system so inaccessible that monkeys with three elbows and double joints can't get in there to check it.
Last edited by hellosailor; 10-16-2007 at 08:59 PM.