A technician's electronic vacuum gauge reads in microns rather than in millimeters of mercury. Which reading on that gauge corresponds to the level a low-pressure appliance is required to reach?
Correct Answer
B) 25,000 microns, since a millimeter of mercury works out to one thousand microns
A micron is one thousandth of a millimeter of mercury, so the required level converts directly: twenty-five millimeters of mercury absolute is twenty-five thousand microns. That is a far shallower vacuum than the five hundred micron figure technicians associate with system dehydration, and mixing the two up is the usual error. Recovery is finished at twenty-five thousand microns; drying the machine out afterward with a vacuum pump is a separate job that goes much deeper.
Why This Is the Correct Answer
One millimeter of mercury equals one thousand microns, so the twenty-five millimeter absolute requirement reads as twenty-five thousand microns on a micron gauge. Knowing the conversion lets a technician use whichever instrument is on the truck and still document the same level, and it keeps the recovery target from being confused with the much deeper vacuum used for dehydration.
Why the Other Options Are Wrong
Option A: 250 microns, since a millimeter of mercury works out to ten microns
This uses ten microns to the millimeter, which would put the target a hundred times too deep and would take days of pumping to reach on a chiller.
Option C: 2,500 microns, since a millimeter of mercury works out to one hundred microns
This uses one hundred microns to the millimeter. The answer looks reasonable next to a dehydration reading, which is exactly why it catches candidates who never learned the conversion.
Option D: 100,000 microns, since a millimeter of mercury works out to four thousand microns
This treats the millimeter as four thousand microns and lands four times shallower than the requirement, so the machine would still hold a significant part of its charge.
Memory Technique
Milli and micro are three zeros apart, the same jump as millimeters to nanometers. Twenty-five becomes twenty-five thousand.
Reference Hint
Twenty-five millimeters of mercury absolute equals twenty-five thousand microns; a micron is one thousandth of a millimeter of mercury.
More epa-608-type-3 Questions
Some jobs on a chiller count as a major repair under the refrigerant rules, which changes what a technician is allowed to do with a badly leaking machine. Which job falls into that category?
An owner asks how many pounds of refrigerant the rules allow to be left inside a chiller once the recovery has been signed off. What is the correct answer to give him?
The motor of a hermetic centrifugal chiller sits inside the pressure boundary rather than outside it on a shaft. What follows from that construction?
An oxygen monitor is used to decide whether a chiller machine room is safe to enter after a refrigerant release. Below what oxygen reading is the atmosphere treated as oxygen deficient?
A low-pressure chiller's rupture disc has a discharge line running from it. Where does that line have to terminate?
A refrigerant vapor detector in a chiller machine room reaches its alarm level. Besides sounding, what is that alarm expected to do?
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?
In what unit is the required final recovery vacuum for a low-pressure appliance normally expressed?
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