Fundamentals, Safety, and Tools
The physics that gets tested
Heat is measured in Btu; one Btu raises one pound of water 1°F. Sensible heat changes the temperature of air and is read with a thermometer; latent heat changes state and shows up as moisture removed. Memorize the two air-side equations: Btu/h sensible = 1.08 × CFM × ΔT and Btu/h latent = 0.68 × CFM × Δgrains. The 1.08 constant assumes standard air at sea level; at altitude the constant changes, which is a favorite trick answer.
Pressure is the other language of this trade. Gauges read psig, and psia = psig + 14.7 at sea level. A compound gauge belongs on the low side; the high side uses a pressure gauge only. Deep vacuum is read in microns (25,400 microns = 1 in. Hg; 29.92 in. Hg = one atmosphere).
Psychrometrics matter: dry bulb is ordinary temperature, wet bulb combines heat and moisture, dew point is the temperature at which condensation begins, and relative humidity is a ratio, not a moisture quantity. Enthalpy differences across a coil tell you total capacity.
Safety and tool traps
Never braze without a nitrogen purge (prevents internal scale), never use oxygen as a test gas, never heat a cylinder above roughly 125°F, and always secure cylinders upright. Venting refrigerant is prohibited under EPA Section 608. Lockout/tagout before opening a disconnect, and discharge capacitors before testing.
Expect questions that swap tools: micron gauge for evacuation, manometer for static pressure, anemometer for velocity, clamp meter for amps, megohmmeter for insulation resistance.
Worked example: a system moving 400 CFM with a 20°F temperature rise delivers 1.08 × 400 × 20 = 8,640 Btu/h of sensible heat. Likewise, if suction saturation is 40°F and the suction line measures 52°F, superheat is 12°F.