313A brazing, pressure testing and evacuation practice questions.

Five free practice questions on brazing, pressure testing and evacuation: block B of the Red Seal 313A Refrigeration & AC Systems Mechanic exam, about 19 of its 125 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Evacuates systems

   Evacuating a 20-ton rooftop unit through the manifold's 6 mm (1/4 in) hoses is taking hours to reach 1000 µm. Which change speeds the evacuation the most?

   1. Add a second manifold so the pump pulls through both service ports at the same time
   2. Warm the vacuum pump with a heat gun so its oil thins and it pulls a deeper vacuum
   3. Remove the Schrader cores with core-removal tools and connect the pump with short, large-bore hoses
   4. Change to a vacuum pump with double the CFM rating and keep the same hoses and manifold
   Show the answer

   C. Correct. The Schrader core and the small hose are the restrictions; at deep vacuum the gas flow is limited by the smallest opening in the path, not by the pump's rating. Core-removal tools take the cores out while keeping the port sealed, and short 9.5 mm (3/8 in) or larger hoses on both the high and the low side let the pump see the system. The core-removal tools also provide the valve for isolating the pump during the decay test.

   Why not the others

   A. Pulling from both sides helps, but through two cores and two small hoses the restriction is still there. Removing the cores and using large-bore hoses is what makes the difference.

   B. Warming the pump does not change the flow through the hoses, and the system's own moisture, not the pump's ultimate vacuum, is what governs the time. Fresh oil matters; heat does not.

   D. Pump capacity matters little once the flow is choked by a 1/4 in hose and a Schrader core. The bigger pump reaches the same restriction and waits just as long.

2. Critical · Handles materials and supplies

   Four cylinders arrive for an R-410A heat-pump job. The packing slip says R-410A and the cylinders are the rose colour you associate with R-410A, but the printed labels read R-407C. Which action is correct?

   1. Treat the labels as authoritative, quarantine the cylinders and contact the supplier
   2. Charge one cylinder into the system and confirm the refrigerant by its operating pressures
   3. Use them; the cylinder colour and the packing slip agree, so the labels must be a printing error
   4. Use them for the job but record the label discrepancy on the site paperwork
   Show the answer

   A. Correct. The printed label is the legal identification of the contents; cylinder colour is only a convention, and the industry has moved away from colour-coding, so it cannot be relied on. A label that disagrees with the packing slip means the supply chain has a fault: the cylinders are set aside and the supplier resolves it. R-407C in an R-410A system would run at the wrong pressures and capacity.

   Why not the others

   B. Charging an unidentified refrigerant contaminates the system and the recovery cylinder you would then need. Identity is settled before any refrigerant leaves the cylinder, not by experiment on the customer's equipment.

   C. Two pieces of paperwork agreeing does not make the label wrong. Cylinder colour is not a controlled identifier and a mislabelled cylinder is exactly how a wrong refrigerant enters a system. The contents are unknown until the supplier confirms them.

   D. Recording the discrepancy does not resolve it. Using cylinders whose contents are in doubt puts an unknown refrigerant into the system; the correct action is to quarantine them and have the supplier identify the contents.

3. Recall · Uses refrigerants, gases and oils

   Why must a container of POE compressor oil be kept sealed and opened only immediately before use?

   1. POE evaporates quickly, so an open container loses both volume and viscosity
   2. POE absorbs oxygen from the air, which then circulates as a non-condensable gas
   3. POE is hygroscopic; it absorbs moisture from the air that evacuation cannot remove afterwards
   4. POE oxidizes in air within minutes and permanently loses its lubricating film strength
   Show the answer

   C. Correct. Polyolester oil pulls water from the air rapidly, and the water bonds into the oil so that a normal evacuation does not pull it back out. In the system the water and oil hydrolyze into acids that attack windings and copper-plate bearings. POE is bought in small sealed containers, opened once, used quickly and any remainder discarded or resealed for immediate use only.

   Why not the others

   A. Refrigeration oils have very low vapour pressure and do not evaporate at room temperature. What an open container gains is water, which is the problem.

   B. Oil does not carry enough dissolved oxygen to matter, and dissolved gases are removed by evacuation. Absorbed water is what evacuation cannot remove and what makes POE handling strict.

   D. POE is chemically stable in air; it does not oxidize on a time scale of minutes. Its weakness is moisture absorption, not oxidation.

4. Procedural · Prepares work site

   You are preparing to brush-clean the tubes of the water-cooled condenser on a 200-ton chiller. Which isolation points must you locate before the job starts?

   1. The chilled-water valves and the make-up water, because the condenser drains to the floor
   2. The electrical disconnect, the condenser-water supply and return valves, and the refrigerant service valves
   3. The condenser-water valves only, since the building operator keeps the compressor locked out
   4. The chiller's electrical disconnect only, because the refrigerant side of the machine will not be opened
   Show the answer

   B. Correct. The RSOS lists water, electrical shut-offs and other utility services among the isolation points to locate during work-site preparation. Cleaning the condenser tubes means opening the water side, so the water valves must be found and the chiller locked out electrically; the refrigerant service valves are located in case the refrigerant side has to be isolated during the work.

   Why not the others

   A. The chilled-water loop is the evaporator side and is not involved in a condenser tube clean. The isolation points for this job are the condenser-water valves, the electrical disconnect and the refrigerant service valves.

   C. Never rely on someone else's lockout. The mechanic doing the work locates the electrical isolation point and applies their own lock and tag before working on the machine, in addition to isolating the water side.

   D. The refrigerant side stays closed, but the condenser water side is being opened. Without the water supply and return valves located, the tube bundle cannot be drained and isolated safely.

5. Procedural · Performs leak and pressure tests on system

   A chilled-water loop consists of 200 ft (61 m) of pipe with a 4 in (102 mm) inside diameter. Using the formula sheet (volume of a cylinder = π × r² × h; 1 ft³ = 7.48 USG), how much water is needed to fill it for a hydrostatic test?

   1. 131 USG
   2. 42 USG
   3. 17.5 USG
   4. 522 USG
   Show the answer

   A. Correct. Radius = 2 in = 0.1667 ft. Area = π × 0.1667² = 0.0873 ft². Volume = 0.0873 × 200 = 17.45 ft³. Water = 17.45 × 7.48 = 130.6 USG, about 131 USG (roughly 500 L). The RSOS asks the mechanic to calculate the volume of liquid or gas needed for a test, and the sheet supplies the cylinder formula and the conversion.

   Why not the others

   B. This result leaves out π: 0.1667² × 200 × 7.48 = 41.6. The area of a circle is π × r², and π is on the sheet as 3.1416.

   C. This is the volume in cubic feet (17.45 ft³), not gallons. The ft³ to USG conversion on the sheet (× 7.48) was skipped.

   D. This uses the 4 in diameter as the radius (π × 0.333² × 200 × 7.48). Radius is half the diameter; the error inflates the area four times.

02Answer key

03What block B covers

This block covers the hands-on work done on every job. It includes site preparation and hazard assessment, receiving and storing equipment and cylinders, brazing and soldering with a nitrogen purge, nitrogen pressure tests and hydrostatic tests, evacuation and micron-gauge decay tests, refrigerant blends, oils and recovery, field wiring of control circuits, and fire stop, insulation adhesives and thread sealants.

The Red Seal Occupational Standard names this block “Performs routine trade activities”. It carries 15.2% of the exam, about 19 of the 125 questions, split across two tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Brazing under a nitrogen purge

Copper heated in air grows black cupric-oxide scale inside the tube, which breaks loose and plugs the TXV strainer and drier. Flow a gentle stream of dry nitrogen through the line, vented at the far end, while each joint is heated and cools.

Correcting a standing pressure test

Nitrogen pressure follows absolute temperature, so correct for it before calling a leak. A 300 psig test started at 80 °F should read about 288 psig at 60 °F. Test pressure never exceeds the lowest design pressure of anything connected.

Reading the decay test

With the pump valved off and the micron gauge on the system, a rise that keeps climbing steadily is a leak, so go back to a nitrogen leak test. A rise that levels off around 1000 to 2000 microns is moisture still boiling off.

Charging blends as liquid

Zeotropic blends such as R-448A leave the cylinder as liquid so their composition does not shift. Into a running system the liquid is throttled into the suction side so it flashes first; with the system off it can go straight into the high side.

05The other blocks

The 313A exam has six blocks. Each has its own page of free questions:

The 313A Refrigeration & AC Systems Mechanic practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.