How to read a PT chart
A PT chart lists, for one refrigerant, the pressure at which it boils or condenses at each temperature. Find your gauge reading in the pressure column and read across to the temperature. It only holds where liquid and vapor sit together, so it describes the evaporator and condenser, not the lines between them.
From the lesson Pressure-Temperature Relationship and Vacuum Units
Why it matters on the exam
The Core section asks you to turn a pressure into a temperature with a PT chart, and to tell gauge pressure from absolute. The same chart comes back in recovery: a cylinder whose pressure sits above the chart value for its temperature holds noncondensables, such as air.
Key points
- Saturation. Where liquid and vapor share a space, pressure fixes temperature. Warm it and the pressure climbs until evaporation and condensation balance again.
- psig and psia. A gauge reads zero at the air around it. Add 14.7 for absolute pressure, the number the gas laws use.
- Below zero. Left of zero on the gauge is a vacuum, read in inches of mercury. Deep vacuums are read in microns.
- Above the chart. A recovery cylinder or idle system whose pressure sits above the chart value for its temperature holds noncondensables, such as air.
| Temperature | R-134a | R-410A |
|---|---|---|
| 20°F | 18 psig | 78 psig |
| 35°F | 30 psig | 108 psig |
| 70°F | 71 psig | 201 psig |
| 100°F | 124 psig | 318 psig |
Watch the pressure follow the temperature
Common trap
Treating a gauge reading as absolute. 10 psig is 24.7 psia. Drop the 14.7 and every gas-law answer comes out wrong, and a vacuum, which is negative gauge pressure, gets read as negative absolute pressure, which can’t exist.
Question 1 of 5
The low-side gauge on an R-134a system reads 30 psig. What temperature is the refrigerant boiling at in the evaporator?
Tap an answer and I’ll tell you why.
Question 1 is written for this page and checked by a person. The others are practice questions from the MindMax app. None are from a past exam.
Where it comes from
Lesson 2 in Refrigeration Fundamentals · Core Section
Pressure-Temperature Relationship and Vacuum Units
- Use a pressure-temperature chart to convert a saturation pressure to a temperature
- Convert between gauge pressure and absolute pressure
- Relate inches of mercury vacuum, millimetres of mercury absolute and microns to one another
- Explain why a system operating in a vacuum draws air and moisture inward instead of leaking refrigerant out
Key terms in this lesson
- Micron
- A thousandth of a millimetre of mercury, the unit for reading a deep vacuum.
- psia
- Pounds per square inch absolute: pressure counted from a perfect vacuum, 14.7 more than psig at sea level.
- psig
- Pounds per square inch gauge: pressure counted from the air around you, so an open gauge reads zero.
- Saturation temperature
- The temperature at which a refrigerant boils or condenses at a given pressure.
Learn more
FAQ
psig is gauge pressure, counted from the air around you, so an open gauge reads zero. psia is absolute pressure, counted from a perfect vacuum. At sea level, psia is psig plus 14.7.
Pressure fixes temperature only at saturation, where liquid and vapor share the space: in the evaporator and the condenser. In the suction line the refrigerant is all vapor and in the liquid line all liquid, so their temperatures can sit above or below the chart. Those gaps are superheat and subcooling.
Noncondensables, such as air. A recovery cylinder or idle system should sit at the chart pressure for its temperature. A higher reading means something that won’t condense is adding pressure.