Like a lot of other engineers, I was taught that a full site glass means the unit is charged correctly. I have learnt so much on this forum in the last two weeks about superheat and subcooling and doing it scientifically rather than by feel. So I would like to explain a job I did today on a callout and give you all the figures I measured, and then leave it to the experienced boys to comment on it. CONSTRUCTIVE CRITICISM ONLY PLEASE!!

An NRS Beer Master that runs on R134a had lost its charge due to a capillary wearing through against the discharge pipe. Cut a long story short, replaced the LP switch, put in a new drier, pressure tested to 200 PSI (@55'C SLT), evacuated to 4 Torr and started charging it. The evaporator uses an internally equalised TEV. Condenser fan runs at 1 speed. Cellar setpoint 13'C.

Initially I put in 2 KG and watched the site glass as it started to fill up. Before it was completely full, I stopped and allowed the system to run for 10 minutes to stabilize.
Readings: 20 degrees on condenser
31 degrees off condenser
23'C on evaporator
13'C off evaporator
36 PSI suction pressure
14'C SST
115 PSI liquid pressure
33.5'C SLT
So that gives me 9K superheat and 1.5K subcooling. My understanding of that is that it is still short of refrigerant. I also realise that we should only really be taking these measurements when room is close to design temperature. This will take hours to get there due to all the warm beer kegs though.

I continued to add small quantities of refrigerant in 50g intervals and monitored SH and SC values, which did not change much.

I then blocked up the condenser coil to simulate warmer days and pushed the values up as follows:

20 degrees on condenser
41 degrees off condenser
19'C on evaporator
11'C off evaporator
40 PSI suction pressure
16'C SST
160 PSI liquid pressure
45'C SLT
Obviously the unit has been running for awhile, so the room temperature is dropping slowly.
So now my SH is 9K and my SC is 1.5K. No change????

I have stopped charging at 2.5KG and the best SC I got was 2K and the SH stays at 9K. The cellar temp down to 18'C and off evaporator at 11'C.

So the big question, was I on the right track? My concern is that if I keep charging it while the room is still about 5 to 7'C from design temperature, that I am making a mistake. I was aiming for a SC value of between 4.5K and 8.5K. The SH would be 60%-70% of the evaporator TD.

Thanks in advance for your answers!