We can use a known refrigerant's PT chart to determine non-condensables in the refrigerant only if we can measure which quantities?

• A. Only true if we can measure both pressure and temperature
• B. Always true regardless of measurements
• C. Never true
• D. True only for pure refrigerants

Answer: (A)

When using a refrigerant’s pressure–temperature chart, the key idea is that the chart defines the saturated relationship between pressure and temperature for that refrigerant. If non-condensables are present, the gas mixture will not follow that saturated P–T curve. To spot the deviation, you need to know both the actual pressure and the actual temperature and plot that pair on the chart. Only having one of those measurements doesn’t tell you whether the system matches the known P–T relationship, so non-condensables can’t be determined from a single measurement. That’s why you must measure both pressure and temperature. This detection method applies regardless of whether the refrigerant is pure or not, as the chart’s purpose is to reveal deviations from the expected saturated state.