A friend of mine has a reach in to keep flowers. The setpoint of the cooler is 47 degrees. It will maintain this temperature for a few days and then it will not drop below 70 degrees. It uses a cap tube system. It was originally R12 that was converted to R134a. The data plate does not have the weigh in charge and there is no high side port. I didn't have my tools on me when I looked at it a month ago. The condenser seemed to have a gradual decrease in temperature. There was little difference in temperature from the top of the condenser to the bottom. The temperature was higher than ambient temperature. I also could hear flashing in the evap. I initially suspected it to be low on charge. My friend told me there is a large amount of condensate that came out of the unit after it was shut off for a while. I'm assuming the unit is icing up. With no weigh in charge data, I am going to have to charge the unit using a different method. I'm ruling out a restriction based on the fact that there doesn't appear to be refrigerant backing up and the lack of subcooling. I know POE systems with cap tubes have problems with restrictions.
My problem has to do with charging a critically charged system accurately. I was taught to charge a system using a 15 degree SST below desired set point. For example, if you the box temperature was 40 degrees, the SST would be 15 degrees lower. Therefore, a 134a system would be have a low side reading of around 29 psig (25 degree saturated temp). Another way to charge it would be through superheat (20 to 30 degrees). Is this a proper way to check the charge of a unit or was I taught incorrectly?