Saturation temperature
The temperature at which a refrigerant changes between liquid and vapour at a given pressure. Because the relationship is fixed for each refrigerant, a pressure reading can be converted into a temperature and compared with what the system is actually doing.
Also called sat temp
What is saturation temperature?
Saturation temperature is the temperature at which a refrigerant boils or condenses at a given pressure. The relationship is fixed per refrigerant, so a pressure reading converts directly to a temperature.
Saturation temperature: how it works
While a refrigerant is changing state, its temperature stays fixed for a given pressure regardless of how much heat is going in or out. That fixed relationship is what a pressure temperature chart lists. Measuring a pressure therefore tells you the temperature at which the fluid is boiling or condensing at that point in the circuit, which is the baseline both superheat and subcooling are measured from.
Where you meet the term
- In every superheat and subcooling calculation, as the converted value the actual temperature is compared against.
- On a gauge set, where the scales around the dial or the digital readout do the conversion for the selected refrigerant.
- In a coil condition assessment, where the saturation temperature is compared with the air temperature entering that coil.
How to check it
- Select the correct refrigerant on the gauge. Each refrigerant has its own relationship. A conversion made against the wrong one produces a plausible number and a wrong diagnosis. Technician
- Note whether the blend has glide. Blends boil across a range rather than at one point, so the chart lists bubble and dew values and the manual states which one to use. Technician
Saturation temperature: what it is not
- Saturation temperature is not the temperature of the pipe. It is what the refrigerant would be at while changing state, and the difference between the two is superheat or subcooling.
- One chart does not cover all refrigerants. Each has its own pressure temperature relationship.
Saturation temperature questions
What is a PT chart?
A pressure temperature chart lists, for one refrigerant, the temperature at which it boils or condenses at each pressure. Digital gauge sets hold the same data and apply it once you select the refrigerant.
What is temperature glide?
Some blends do not change state at a single temperature but across a range, because their components boil at different points. Charts for those refrigerants list a bubble point and a dew point rather than one figure.
Why convert pressure to temperature at all?
Because temperature is what the rest of the diagnosis is in. Comparing the saturation temperature with the actual pipe temperature gives superheat and subcooling, and comparing it with air temperature shows how the coil is performing.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE TerminologyIndustry reference definitions for heating, ventilation, air conditioning, and refrigeration vocabulary.