Concepts and their limits
EPA lists 25 mm Hg absolute for the general low-pressure appliance evacuation target, in both recovery-equipment date columns. For these learning conversions 1 mm=1000 microns, so pressure units must be converted before comparing readings. The regulatory low-pressure target is 25 mm Hg absolute. For absolute-pressure readings expressed in the same units, a lower number means a deeper vacuum; meeting a maximum pressure is not the same as exceeding it. This original learning model defines gauge vacuum as local atmospheric pressure minus absolute system pressure. It teaches reference interpretation, not a new regulatory target.
Worked decisions
A low-pressure chiller is opened for service under the general evacuation rule. What listed absolute-pressure target applies? The decision is 25 mm Hg absolute. EPA lists 25 mm Hg absolute for the general low-pressure appliance evacuation target, in both recovery-equipment date columns. A learning worksheet uses 1 mm Hg=1000 microns Hg. A pressure reading is 25 mm Hg absolute. What is the equivalent reading? The decision is 25,000 microns Hg absolute. Multiply the supplied 25 mm reading by 1000 microns/mm. Preserve the absolute-pressure reference. A low-pressure learning comparison uses the 25 mm Hg-absolute maximum target. A gauge reports 20 mm Hg absolute. Which comparison is correct? The decision is It is below the target pressure. For absolute-pressure readings expressed in the same units, a lower number means a deeper vacuum; meeting a maximum pressure is not the same as exceeding it. In a supplied instrument model, local atmospheric pressure is 760 mm Hg and system absolute pressure is 25 mm Hg. What is the gauge-vacuum difference? The decision is 735 mm Hg. Use the supplied model 760−25. The result is a pressure difference; do not label it as absolute pressure.
Contrasts that change the decision
Pre-November-15-1993 recovery equipment is used on a low-pressure appliance under the general table. Which target is listed? Here the supported answer is 25 mm Hg absolute. EPA lists 25 mm Hg absolute for the general low-pressure appliance evacuation target, in both recovery-equipment date columns. A learning worksheet uses 1 mm Hg=1000 microns Hg. A pressure reading is 20 mm Hg absolute. What is the equivalent reading? Here the supported answer is 20,000 microns Hg absolute. Multiply the supplied 20 mm reading by 1000 microns/mm. Preserve the absolute-pressure reference. A low-pressure learning comparison uses the 25 mm Hg-absolute maximum target. A gauge reports 25 mm Hg absolute. Which comparison is correct? Here the supported answer is It is exactly the target pressure. For absolute-pressure readings expressed in the same units, a lower number means a deeper vacuum; meeting a maximum pressure is not the same as exceeding it. In a supplied instrument model, local atmospheric pressure is 760 mm Hg and system absolute pressure is 40 mm Hg. What is the gauge-vacuum difference? Here the supported answer is 720 mm Hg. Use the supplied model 760−40. The result is a pressure difference; do not label it as absolute pressure.
Transfer and self-check
Convert 25 mm Hg absolute into microns Hg absolute, using 1 mm=1000 microns.25×1000=25000 microns. The conversion does not change absolutepressure into gaugepressure.
A low-pressure chiller is opened for service under the general evacuation rule. What listed absolute-pressure target applies?
- The relevant stated condition is: A low-pressure chiller is opened for service under the general evacuation rule.
- Apply the relevant source rule. EPA lists 25 mm Hg absolute for the general low-pressure appliance evacuation target, in both recovery-equipment date columns.
- This supports “25 mm Hg absolute”. The alternative “25 inches Hg vacuum” uses a different condition or conclusion;the applicable rule above explains why it does not answer this case.
Compare the other answer choices
This choice (25 inches Hg vacuum) does not fit the stated conditions. EPA lists 25 mm Hg absolute for the general low-pressure appliance evacuation target, in both recovery-equipment date columns.
This choice (25 psig) does not fit the stated conditions. EPA lists 25 mm Hg absolute for the general low-pressure appliance evacuation target, in both recovery-equipment date columns.
This choice (0 inches Hg vacuum) does not fit the stated conditions. EPA lists 25 mm Hg absolute for the general low-pressure appliance evacuation target, in both recovery-equipment date columns.
Try it, then explain it.
Complete a short set without looking at the example. After each answer, explain which evidence or rule supports your decision and why the alternatives do not fit.
Sources & authorship
Original Slope explanations and examples. References support the concepts; they are not the source of copied questions. Independent educator review is pending.
Epa Practices