Let me try restating my previous answer regarding suction pressure and add a bit more explanation. There is no exact suction pressure useful in charging any system.
If you have no sight glass you can start by putting enough refrigerant in while the system is ruuning, so that the cooling coils blows out air 15 degrees cooler than the air in the box...
..... for instance if the air in the box is 70F you can charge the system slowly until the air comes out of the cooling coil at 55F.
When the box gets down to 45F and the beer is at 45 F the suction line should be running slifhgtly cool at the compressor but not sweating.
If not, add a little refrigerant until the suction line is cool.
At the same time feel the U bends on the condenser coil... that coil should run with the top U bend HOT, and the middle U bends warmer... and the bottom U bend just 5 degrees warmer than room temperature... so that if the room is 75F... the bottom U bend should be running at 80F.
That will be pretty close to a good charge. When the beer is down to 38F the suction line at the compressor should be very cool or sweating... BUT NOT FROSTING.
I do not run my beer boxes at 49F... that is way too warm.. beer needs to be held at 36 to 38 degrees F, no warmer than 40F.
When the box is cool and beer is between 36 and 38F... check the suction pressure...that is the pressure the box was designed to run at with the refrigerant it is designed for.
DO NOT ADD REFRIGERANT OR CHARGE A SYSTEM WITH REFRIGERANT UNLESS YOU KNOW WHICH REFRIGERANT IT USES.
Each type of refrigerant uses a different *type of oil... if the oils and refrigerant are not properly matched you can cause great damage to the beer box refrigeration system.CLICK HERE FOR THE SPECIFICATION SHEET ON THAT UNIT.
it is for the unit built in 2010 however, if your unit is much older than that it may not use the same refrierant and you will have to verify what it uses by look on the *condensing units* name plate.
Refrigerant used in that model built in 2010 is R-134a.
It is NOT compatible with R-12 and R-22 used in the much older units... we need to be sure of the refrigerant before we do any charging of refrigerant.
CLICK HERE FOR A PRESSURE / TEMPERATURE CHART ON R-134A
Nofice the column under R-134a... notice the column listing temperatures on left side of the chart... find 20F on that chart..go across to the R-134a column and notice that the pressure is 20.6 psig when the coil is at 20F.
When the coil is at 20F it will generally be blowing out air 15 degrees warmer, in this case at 35F.... that will keep the beer between 36 and 38F.
However, if air flow is restricted, or the coil is dirty the coil will have to run colder to cool the box properly so you may have to run the suction pressure a bit lower, closer to 15 or 17 psig...
It is the sweating suction line that tells you when the charge is accurate...NOT the suction pressure.
The college classes on how to read that chart take about 2 days...spend some time studying it, that will help you a lot.
As long as you rate my assistance so far *positively we can continue without any time limit. I hold questions open in that case.