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Cycle of Operation of Refrigeration system
This section will deal with the use of 
refrigeration gauges and manifold set so 
we can check the pressures of the refrigerant 
inside the system. Some systems may contain 
liquid refrigerant with pressures as high as 
500 pounds.  Use caution as you install your 
manifold gauges. Hose connectors are designed 
to spray the spillage away from your hand, 
but it can still be dangerous.  Be careful.
Cycle of Operation of Refrigeration system This picture shows the gauges, the refrigerant recharging cylinder and the vacuum pump hooked to an outside unit at the access ports. Note that the low side gauge (usually blue) will attach to the suction side of the system which is the lower pressure side. The high pressure gauge is usually red and will attach to the service port of the high side liquid refrigerant valve. Both of the service valves will likely have plunger pins inside them and you will not have to turn any valves or valve stems to access the pressure inside.
Suction Pressure cycle of operation jpg

When the unit is running well, the low side and high side pressures will be as follows:
Checking SuperHeat Drawing Tightly strap a good thermometer to a flat clean (sand paper or metal brush it if necessary) surface of the suction line tubing. Look at the low side gauge and see what the temperature of the refrigerant is at that pressure or use the chart provided in The Diagnostics Chain to convert that pressure to temperature.
Subtract the gauge conversion temperature from the line measurement and that is the current SUPERHEAT of the system.

Here is a chart to determine the CORRECT SUPERHEAT of a capillary tube expansion device system.

Assuming you have an 80°F. dry bulb (at 67°F. wet bulb) indoor temperature and the system has been operating for twenty minutes or more:
Deg. F. Deg. F.
7530 to 35
8025 to 30
8520 to 25
9015 to 20
9510 to 15
1005 to 10
1055 to 7
If the indoor temperature at the time of measurements is above the 80 deg. F. then the superheat will be HIGHER. If the temperature indoors is below the 80°F. then the superheat will be LOWER. A 70°F. indoor with a 100°F. outside may be as low as 1 to 3°F superheat, for example. Additionally, you probably will not be measuring the wet bulb temperature and knowing the moisture level anyway. Use good judgment here and hope for the best guess. You won't be too far off if you are using this superheat information. That is our aim.

If your system uses the thermostatic expansion valve (TXV) device versus the capillary tubes, then you have less difficulty determining the superheat, because they usually will be automatic in nature and will maintain a very consistent 5 to 15°F. superheat. Only when they are failing or other things like dirty evaporator coils or reduced airflow over the cooling coils exist do they allow incorrect or out-of-scale readings.

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