Tools, Equipment, and Shop Practices
The Tools, Equipment, and Shop Practices domain checks that you can select the right instrument for a measurement, use it safely, and keep a shop that does not hurt people. Items cluster around measurement accuracy, vacuum and pressure equipment, brazing and leak detection, and compressed-gas cylinder handling.
Measurement instruments
- Manifold gauges are for rough readings; a micron gauge or electronic vacuum gauge is the only instrument that resolves deep vacuum (500 microns = 0.5 mm Hg = 67 Pa).
- Superheat and subcooling need a saturated-temperature reference plus a pipe temperature; clamp-on thermistors read pipe surface, not refrigerant, so insulate them against ambient.
- Airside work needs a manometer (static pressure in in. w.c. or Pa), an anemometer for velocity, and a digital or sling psychrometer for dry bulb, wet bulb and dew point.
- Electrical: true-RMS clamp meter, megohmmeter for insulation resistance, and a capacitor meter for microfarads.
Pressure, vacuum and gas equipment
- Two-stage vacuum pump with gas ballast; change the oil when it turns milky.
- Nitrogen regulator for pressure testing and purging. Never use oxygen or compressed air to pressure test a refrigeration circuit - oil plus oxygen equals ignition.
- Recovery machine and TDG-approved recovery cylinders, never overfilled.
- Oxy-acetylene set: cylinders upright and secured, oxygen and fuel gas separated by 6 m or a non-combustible barrier, caps fitted, flashback arrestors installed, no oil on oxygen fittings. Keep a fire extinguisher and a heat shield at the work area, and cap or plug tubing left open.
Worked example: temperature-corrected pressure test
A system is charged with dry nitrogen to 150 psig at 21°C (70°F). Absolute pressure: 150 + 14.7 = 164.7 psia. Overnight the shop cools to 18°C (65°F), which is 0.988 of the original absolute temperature, so the expected reading is 164.7 × 0.988 = 162.8 psia = 148.1 psig. A gauge reading of 148 psig is therefore not a leak; 130 psig is.
Traps
- Judging a vacuum by manifold gauge needles.
- Testing with compressed air or oxygen.
- Reading gross cylinder weight without subtracting tare.
- Ignoring hydrostatic test dates, damaged hose covers, or a missing regulator gauge guard.