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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.
||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.|
When the unit is running well, the low side and high side pressures will be as follows:
- Pressure will normally be in the range of 40 to 100 pounds on an r-22 system. Whether this is correct or incorrect can only be determined by the measurements of the Superheat and comparison of it to conditions of the indoor air (conditioned space) and the outside air (ambient).
||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:||OUTSIDE TEMP
|75||30 to 35
|80||25 to 30
|85||20 to 25
|90||15 to 20
|95||10 to 15
|100||5 to 10
|105||5 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.|