Tools, Instruments and Measuring Techniques
Pressure and vacuum
A manifold gauge set has blue (low side), red (high side) and a yellow centre port for the vacuum pump or charging. Use it for pressures, never as a vacuum gauge - evacuate to a micron gauge. Keep hoses dedicated: what touches R-410A should not touch R-22. Electronic leak detectors outperform soap bubbles in awkward places, but soap solution remains the confirmation test.
Electrical and environmental
A clamp meter for current, a digital multimeter for voltage, resistance and continuity, a megger for insulation resistance, an electronic scale for charging by weight, a sling psychrometer or thermohygrometer for DBT and WBT, an anemometer for airflow, an infrared thermometer for coil and line temperatures, plus tube bender, flaring block, swaging tools, torque wrench and nitrogen regulator.
Worked example
A clamp meter shows 6.8 A on a unit with nameplate RLA 7.2 A, but ambient is 46 °C instead of 35 °C and supply voltage is 205 V instead of 230 V. Current rises with ambient temperature and falls with supply voltage, so 6.8 A is acceptable here. Compare like with like before condemning the compressor.
Traps
Reading a compound gauge on the wrong scale, using damaged hoses that admit air during evacuation, and skipping calibration - a gauge reading 0.5 bar high makes every diagnosis wrong.