Thermo Pride MODEL OPA - Manual (Page 44)

All installations and services must be performed by qualified service personnel.
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Startup of a new cooling system and checkout of an existing system should
always include superheat measurements. Before measuring superheat, allow
the system to operate under a load for approximately 15 minutes. A conditioned
space load will usually provide enough heat for the evaporator to remove for at
least this period of time. Operating the cooling system for 15 minutes allows the
refrigeration system to stabilize, providing good operating pressures and
temperatures.
NOTICE: Do not attempt a superheat measurement when the ambient
temperature is less than 60
° F. Low loading on the cooling system will
result in low or erroneous superheat measurements.
NOTICE: When adding or removing charge to fine tune the cooling system
performance, allow 15 to 20 minutes of system operating time after each
adjustment of the charge. After 15 to minutes of operation, a reliable
superheat reading may be taken.
3. Evaporator Superheat:
To measure superheat at the evaporator, a suction pressure reading at the outlet
of the evaporator would be desirable. Since most residential cooling systems
(including Thermo Pride evaporators) do not provide a test port (sometimes
referred to as a Schraeder port) at the evaporator, the suction pressure must be
read at the suction inlet to the compressor. Typically, it would be necessary to
compensate for the pressure drop through the line connecting the evaporator and
the compressor. However, due to the close proximity of these components, the
pressure drop will be small and can be neglected. A good resistive-type
thermometer or thermocouple-based sensor should be used to measure the
suction line temperature exiting the evaporator. On evaporators using thermal
expansion valves, read the temperature directly under the expansion bulb,
located on the side of the suction line exiting the evaporator.
Measure refrigerant temperature at the 2:00 or 10:00 positions on the outer
surface of the evaporator suction tube. Wrap the thermometer or thermocouple
with a rag, or insulation, to insure system airflow does not influence the
temperature readings.