Here are the notes I found online that might help me:
Does that sound relevant ?
Recharge Your Refrigerator - Sail Magazine
Kollmann Marine
Things to remember before connecting gauges and servicing:
1. Purge refrigerant through gauge set to remove air and moisture.
2. Stop compressor before connecting gauges, as it may be running in a vacuum.
3. Everything in the system including the ice box needs to be 70˚ F. or above.
After the compressor is started the suction pressure will drop and stabilize this may take 10 minutes. Then warm the box, A setting of 6 to 8 psi would be the correct charge for 134a refrigerant. If at 10 to 20 minutes this pressure is low, add a small amount of refrigerant. Yes small amount, and then wait till it seeks a new stabilized pressure before adding more. If the system wad over charged, and yours is not, the pressure would be higher than 10 psi. The exception to these pressures would be if you have AB's large 15 inch evaporator then the pressures should be 2 psi higher. A small overcharge of ½ ounce or more (15 Grams)of refrigerant will cause an increase of daily amp-hrs used.
If you are unable to adjust the refrigerant level in the first 20 minutes, stop and let the evaporator warm back up to 70˚F. After 20 minutes, the suction pressure will automatically adjust down to lower temperatures and lower flows as the plate and box cools. Use pressures for servicing only in a time window between 10 and 20 minutes with a warm box. Fine tuning later is done by setting frost line (upper heat), confining it to evaporator.
After removing the gauge set be sure to replace the sealing cap with its seal still inside on the service fitting. Common slow refrigerant leaks are caused by poor sealing of these caps.
Answer: am assuming, if you are adding 134a refrigerant, that your compressor is either a
BD3 or BD50 and not a BD2.5. Turn compressor off first. Purge air from
hoses then connect refrigerant hose to suction port. With bottle upright, to
add gas not liquid, allow only 2 ounces of refrigerant to enter system, then
shut off bottle and start compressor. The quickest way to charge system
correctly is to use an ammeter in power wire and very very very slowly add
refrigerant until amperage reaches 5 amps. As evaporator cools amperage
will be less than 5 but never above 5.5 amps. Then from this point on ignore
amperage and run unit for 2 hours checking frost on surface area of
evaporator. Your unit will be charged correctly when the evaporator's surface
area has frost covering 90 to 100% of its surface and no frost on return
line toward compressor. If frost area is less than 90% add refrigerant. Your
unit will hold from 3 to 5 ounces of refrigerant. With 1/2 ounce too much
refrigerant system will deliver poor performance. If you do not have my
12/24 volt refrigeration manual see if one of the other boaters has one you
can use. The above method of adding refrigerant will not harm your unit and the box will get cold.