Tools, Instruments, and Measurement
What the exam expects
Candidates should match the instrument to the measurement and recognize its limits. Manifold gauges read high-side and low-side pressure; a micron gauge reads vacuum depth; an electronic manometer reads duct static in inches of water column; a thermocouple or clamp-on thermometer reads line temperature; an anemometer or velometer reads air velocity; a sling or digital psychrometer reads dry-bulb and wet-bulb; a clamp meter reads current; a multimeter reads voltage, resistance, and continuity.
Know that gauge manifolds are not vacuum instruments, that analog gauges have scale error at low pressure, and that vacuum must be measured at the farthest point from the pump with the gauge isolated from the system when the pump is valved off. Hoses should be vacuum-rated and core depressors used on service ports.
Traps
- Reading superheat at the compressor instead of the evaporator outlet.
- Leaving the manifold connected during a standing vacuum test, which masks a leak or moisture.
- Confusing inches of water column with psi. 1 psi = 27.68 in. w.c.
- Assuming a clamp meter reads accurately on a shared conductor or a VFD output.
- Forgetting that a dirty filter raises the pressure drop and skews any airflow calculation.
Worked example
A manometer reads a supply static of 0.62 in. w.c. and a return static of -0.28 in. w.c. Total external static pressure = 0.62 + 0.28 = 0.90 in. w.c. Compare that to the equipment's rated TESP (commonly 0.5 in. w.c. for residential air handlers) before blaming the blower.