In what unit is the required final recovery vacuum for a low-pressure appliance normally expressed?
Correct Answer
D) Millimeters of mercury absolute
Low-pressure appliances have to be evacuated far deeper than high-pressure equipment, and a scale based on inches of vacuum is too coarse to describe that level. The recovery requirement for these machines is therefore written in millimeters of mercury measured absolute, meaning measured up from a perfect vacuum rather than down from atmospheric pressure. A technician needs an electronic vacuum gauge to read it; the ordinary compound gauge on a manifold set cannot resolve a vacuum that deep with any accuracy.
Why This Is the Correct Answer
Because a low-pressure machine must be pulled down into a very deep vacuum, the requirement is stated in millimeters of mercury absolute. Absolute means the reading starts at zero for a perfect vacuum and counts upward, which lets a small number describe a deep vacuum precisely. Reaching and verifying it requires an electronic micron or vacuum gauge.
Why the Other Options Are Wrong
Option A: Pounds per square inch gauge
Gauge pressure is measured relative to atmospheric pressure and reads zero at atmosphere. It cannot describe how deep a vacuum is, so it is used for the high-pressure and very high-pressure recovery requirements instead.
Option B: Inches of mercury below atmospheric pressure
Inches of mercury below atmosphere is the scale used for high-pressure appliance recovery, and it only has about thirty divisions from atmosphere down to a perfect vacuum. That is far too coarse for a low-pressure machine.
Option C: Feet of water column
Water column is used for very low air pressures in duct and combustion work, not for refrigerant recovery. No EPA recovery requirement is written in feet of water.
Memory Technique
Little machine pressure, little unit: low-pressure machines get the small unit, millimeters, and they get it absolute.
Reference Hint
Recovery requirements are tabulated in 40 CFR 82.156; the low-pressure appliance row is expressed in mm Hg absolute, not inches of vacuum.
More epa-608-type-3 Questions
Which piece of equipment requires a technician to hold Type III certification before it may be serviced?
What property of the refrigerant is what actually places an appliance into the low-pressure category?
During normal operation, what condition exists inside the evaporator of a running low-pressure centrifugal chiller?
What should a technician do about the water in the chiller tubes before pulling a deep vacuum on the machine?
What happens if the internal pressure of a low-pressure chiller is driven past the rating of its rupture disc?
Which refrigerant was the original factory charge in most low-pressure centrifugal chillers built during the nineteen sixties and seventies?
A running low-pressure chiller develops an opening in a section of shell that is below atmospheric pressure. What passes through that opening?
A technician is assigned to recover the charge from a large water chiller holding nine hundred pounds of R-134a. Which certification does that job require?
Into which appliance category does a machine charged with R-13 for very low temperature service fall?
On a low-pressure centrifugal chiller running at load, how do the evaporator and condenser sit relative to atmospheric pressure?
People Also Study
Business & Financial Management
120 questions · 70% to pass
Contract Administration
60 questions · 70% to pass
Project Management
60 questions · 70% to pass