On the gages which I have, are three scales... One in Black PSI), one in Green for R-22, and one in red for R-12 refrigerant. It was suggested that I add refrigerant till the suction side (LP) is about 55-60 PSI and the Head pressure is 150-175 psi. I did this and the unit is 'cooling' however is only cools to about 72 degrees F. and the unit continues to run but does not cool further. The unit had line pressure prior to my adding the refrigerant (about 30 psi LP and 100 psi HP.) I am thinking that the unit may have a 'dirty coil'. I set the pressures using the black scale on the gages which is much less than the green R-22 scale. Should I have used the green scale for R-22? The reason that I used the black scale is that I checked the pressures on a properly operating system, and on the black scale the readings were 65psi and 225 psi respectively. The good working unit is a bit bigger than the one which I recharged. THe LP line was cool to the touch on the unit which I recharged, but after running for a while was wet with condensation. I did not want to overcharge the unit, hence my questions.

Thanks!
W.

You should look at the information plate on the AC unit outside and see what type of freon it uses and then use the scale that applies to that freon. I do not know where you are from and what the humidity might be there so I will give you an average reading that we get here on a very hot day Temps in he 95 to 98 degrees F. On the low pressure we will have a reading somewhere around 75#'s and the high side will be in the upper 200's depending on the S.E.E.R. of the unit. The suction line should be very cool to the touch and should have lots of moisture on it. Do all of the charging after changing the filter and if you are going to clean the coil do that also before you do the charging because each on of these things will affect the pressure readings. It never hurts to wash out the outdoor coil also but let all the moisture dry outside before charging.

Actually you need to have a certification to use R-22 refrigerant. Yoe also need the correct tools to check super heat and sub-cooling. You also need a way to tell the temps into the coils and out of the coils both inside and out to do a proper job.