Reading Analog Gauges & Pressure–Temperature Relationships (R-22 & R-410A)

Pressure–Temperature Relationship

• Direct relationship: when pressure rises, temperature rises; when pressure falls, temperature falls. Saturation temperature reflects this link.
• Saturation temperature = temperature at which refrigerant changes state; occurs in the middle of the coil (evaporator or condenser).

Saturation Temperature

• Describes the point of phase change; used on PT charts for refrigerants.
• Located in the middle of the evaporator coil or condenser coil on a system.

Reading the Pressure–Temperature Chart

• The chart shows pressure (PSIG) versus saturation temperature (°F).
• Evaporator example (41°F):
  • R-22: $41^<br />\ne^{ op}$? Wait, correct: for 41°F on R-22, pressure ≈ $70\ \mathrm{PSIG}$
  • R-410A: pressure ≈ $120\ \mathrm{PSIG}$
• Same evaporator temperature can yield different pressures depending on refrigerant.
• High-side example (condensing side at ~120°F):
  • R-22: pressure ≈ $255\text{ to }265\ \mathrm{PSIG}$
  • R-410A: pressure ≈ > 405\ \mathrm{PSIG}; at around 120°F, slightly higher than 405 PSIG.

Evaporator: 41°F Example

• For evaporator temperature of 41^ e^{ op} (°F):
  • R-22: $70\ \mathrm{PSIG}$
  • R-410A: $120\ \mathrm{PSIG}$

Condensing Side: ~120°F Example

• Condensing temperature near 120^ e^{ op} (°F):
  • R-22: $255\text{ to }265\ \mathrm{PSIG}$
  • R-410A: > 405\ \mathrm{PSIG}; at 120°F, slightly higher than 405 PSIG

Practical Takeaways

• Pressure and temperature move together in a closed refrigeration system.
• Boiling point (saturation temperature) is controlled by system pressure; higher pressure raises saturation temperature, lower pressure lowers it.
• The PT chart is refrigerant-specific; use the chart for the exact refrigerant in the system.
• Ask suppliers for a PT chart for quick reference; many are free.