A cylinder holding both liquid and vapor R-134a sits in a 75 °F shop. What mainly determines the pressure inside it?
Its temperature. As long as both liquid and vapor are present, the refrigerant is saturated and the pressure matches the pressure-temperature chart, however much is in the cylinder.
- How full the cylinder is
- The refrigerant's temperatureCorrect answer
- The size of the cylinder
- How long ago it was last filled
Why “The refrigerant's temperature” is correct
When liquid and vapor are both present, the refrigerant is saturated and its pressure is set by its temperature, not by how much is in the cylinder. A nearly empty cylinder and a full one at the same temperature show about the same pressure, which is why weight, not pressure, tells how full a cylinder is.
Source: EPA: Test Topics
Why the other answers are wrong
- How full the cylinder is — As long as some liquid remains, the amount does not set the pressure.
- The size of the cylinder — Cylinder size does not change saturation pressure.
- How long ago it was last filled — Elapsed time matters only because the temperature may have changed.
The source, word for word
Pressure-temperature relationships of common high-pressure refrigerants
This one rests partly on trade knowledge. EPA's outline lists pressure-temperature relationships as a topic and says a standard chart may be used, but it doesn't state this rule in words. The explanation is standard refrigeration physics.
What a reading off the chart tells you
This only works once the cylinder or appliance has settled at a known temperature.
| Pressure reading | What it most likely means |
|---|---|
| Matches the chart | Liquid is present and the refrigerant is what the label says |
| Well above the chart | Noncondensables such as air are in the container |
| Well below the chart | So little refrigerant is left that no liquid remains |
What it means in practice
Because pressure does not show how full a cylinder is, weigh it to stay under 80 percent. Gauges read psig, so add 14.7 where a chart or definition uses psia.
Keep practicing
- Pressure-temperature chart basics
- Why recovery cylinders stop at 80 percent
- Why pressure rises after reaching recovery vacuum
- EPA 608 Core practice test
Unofficial, not affiliated with the EPA or any EPA-approved certifying organization. Questions are original practice written from public EPA material, not real exam questions.