The low-pressure evacuation level is given as 25 mm Hg absolute. What is "absolute" pressure measured from?
Recovery requirements · EPA 608 Type III. The correct answer, and the reasoning behind it.
Evacuation levels for low-pressure appliances, and major versus non-major repairs.
The question
The low-pressure evacuation level is given as 25 mm Hg absolute. What is "absolute" pressure measured from?
- Local atmospheric pressure
- The refrigerant's saturation pressure
- A perfect vacuumCorrect answer
- The chiller's normal operating pressure
Why “A perfect vacuum” is correct
Absolute pressure counts up from zero, a perfect vacuum, while gauge pressure counts from atmosphere. 25 mm Hg absolute means only 25 millimetres of mercury of pressure remain in the appliance, which is a deep vacuum.
Source: 40 CFR 82.156(a), Table 1; EPA: Test Topics
Why the other answers are wrong
- Local atmospheric pressure — Gauge readings are measured from atmosphere; absolute readings start at a perfect vacuum.
- The refrigerant's saturation pressure — Saturation pressure is a property of the refrigerant, not a zero point for measurement.
- The chiller's normal operating pressure — Operating pressure varies and is not a reference point for absolute measurement.
Practice more of this
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