30.3 Using Amprobes, Velometers, Pitot Tubes, Manometers & Gauges

30.3 Using Amprobes, Velometers, Pitot Tubes, Manometers & Gauges

Trade E’s instrument bullet is using test equipment: the Amprobe (clamp-on ammeter), velometer, pitot tube, manometer, and gauges. Chapter 22.4 already used pitot math in a TAB install. Chapter 29.3 used refrigerant leak detectors and refrigeration gauges. This section is the diagnostic use of the same kit on a Florida service call: current without breaking the circuit, air velocity at a face versus velocity pressure in a duct, and inches of water for static. The CBT books are Refrigeration & Air Conditioning Technology, 9th Edition (2021), the Troubleshooting Handbook, 2nd Edition (2003), and the Trane Ductulator when you translate velocity into cfm. OSHA 1926 still splits the work: LOTO before internals; rated live meters for clamp-on and voltage.

Quick Answer: Clamp-on (Amprobe): one conductor, compare to RLA/FLA. Velometer: face fpm at a grille or coil. Pitot + manometer: duct VP in inches of water, (V \approx 4005 \sqrt{\text{VP}}) for standard air. Static is also in. w.c., not psig. TP = SP + VP.

Amprobe / clamp-on ammeter — current without a break

Amprobe is the brand that became the field word for a clamp-on AC ammeter. The jaw reads current from the magnetic field around a single conductor. That is how you get the 19.8 A versus 14.1 RLA comparison in 30.1 without opening the circuit.

One conductor. If you clamp the entire SO cord — hot and neutral — the fields cancel and the meter reads near zero on a healthy circuit. On three-phase, clamp one leg at a time; do not swallow all three. A ground conductor in the jaw with the hot will also lie. Many techs still use a line-splitter or a plug adapter that separates the conductors at a receptacle for fractional-horsepower indoor units.

Range and inrush. Set a range above FLA, not above LRA, for running load. Use an inrush (peak/hold) function if you actually need LRA; a slow analog needle will not catch it. A compressor that starts at ~70 A and runs at 14 A is doing what LRA and RLA said. A compressor that runs at 22 A on a 14 A RLA is the 30.1 overload story.

True RMS meters belong on VFD and SCR loads; an averaging meter can mis-read electronic fan speeds. CAT rating and insulated jaws still matter on a 480-volt RTU. Clamp-on is a live test: covers as listed, PPE as Chapter 33, no ohm probes on those same terminals while they are hot. Some clamp-ons also read volts and watts; watts on a motor still want the PF discussion from 30.1 — a clamp that multiplies E × I without PF is reporting VA.

Worked clamp-on. Indoor blower nameplate FLA 6.8 A, 230 V, 1φ. You read 9.4 A, supply static +0.55 in. w.c., return −0.40 in. w.c. External static is 0.95 in. w.c. against a 0.50 in. w.c. nameplate ESP. High static, high amps, low airflow — dirty filter, crushed flex, wet coil, closed dampers — not a failed blower motor until the air path is proven. That is how the Amprobe and the manometer talk to each other.

Velometer versus pitot tube — two different velocities

A velometer (the classic Alnor swinging-vane instrument, or a rotating-vane / hot-wire anemometer doing the same job) reads air speed at a plane in feet per minute. Hold it in the plane of the grille, the coil face, or an outdoor-air hood. Average a grid. Cfm ≈ V × Ak, where Ak is the effective (not geometric) area of the grille — the manufacturer’s Ak, not the painted opening you measured with a tape. A rotating vane laid on a filter rack reads face velocity; a 24×24 filter (4 ft²) at 300 fpm is about 1,200 cfm through that filter if the velocity is representative.

A pitot tube is a dual-port probe: the tip faces the flow and sees total pressure; the side taps see static pressure. A manometer between those ports reads velocity pressure (VP). You use a pitot inside a duct, not at a grille face. Traverse (ASHRAE / AABC / NEBB grid), average √VP (average the velocities, not the raw VP if the item is picky), then

(V \approx 4005 \times \sqrt{\text{VP}})

for standard air (0.075 lb/ft³), with V in fpm and VP in inches of water. Cfm = V × inside area (ft²). Hot, wet, or high-altitude air changes density and the constant; Florida comfort ducts are treated as standard air unless the item says otherwise.

Worked pitot (same arithmetic as 22.4, diagnostic use). VP = 0.16 in. w.c.

(V = 4005 \times \sqrt{0.16} = 4005 \times 0.40 = 1{,}602\ \text{fpm})

A 24×12 duct (2.0 ft² inside) is about 3,200 cfm. If the schedule was 2,400 cfm, you are high. If you wrongly compute (4005 \times 0.16 = 641) fpm, you will starve every other branch trying to “fix” a number that was never low.

When to grab which. Velometer: grilles, coils, OA intakes, hood faces — places a pitot cannot sit in developed duct flow. Pitot: duct traverses, fan inlet/outlet where you need VP, SP, and TP. A flow hood (balometer) is a velometer family instrument that integrates face velocity into cfm at a diffuser. None of these replace a tachometer if the fan rpm is wrong, and none of them replace the Amprobe if the motor is over FLA.

Manometers, inches of water, and gauges

A manometer measures low air (and sometimes gas) pressure. Forms: U-tube, inclined, digital, and Magnehelic (diaphragm gauge). The HVAC air unit is inches of water column (in. w.c. or in. WG). Total pressure = static pressure + velocity pressure:

(\text{TP} = \text{SP} + \text{VP})

Static pressure is the push on the duct wall. Velocity pressure is the directed energy of the moving air. A pitot’s total port minus atmosphere is TP; side port minus atmosphere is SP; total minus static is VP.

Inches of water are not pounds per square inch. 1 psi ≈ 27.7 in. w.c. A blower nameplated 0.50 in. w.c. ESP is about 0.018 psi. Calling that “half a pound of static” is how you talk yourself into a hydronic gauge on a duct. Typical comfort-duct static sits in tenths to about 2 in. w.c. Kitchen grease ducts and high-pressure VAV boxes run higher, still in inches of water, not refrigerant psig. Natural-gas manifold pressure is often 3.5 in. w.c. (inches of water again). Hydronic TAB uses psig or feet of head on a differential gauge across a circuit setter — different instrument, different unit.

Worked static. Blower nameplate 0.50 in. w.c. ESP. Return static −0.25, supply +0.30. Total ESP = 0.55 in. w.c. — close to nameplate, amps should be near FLA if the motor is healthy. Same blower, return −0.45, supply +0.55, ESP 1.00 in. w.c.: high static, high amps, low cfm. Open the air path before you replace the motor you just clamp-on tested.

Gauges on this Trade E bullet include the air gauges above and the refrigerant manifold from Chapter 29 (compound suction in psig/in. Hg, high-side psig). Do not put a 500-psig refrigerant gauge on a 1-inch Magnehelic and expect a duct reading, and do not put a Magnehelic on a R-410A liquid line. Vacuum gauges (microns) are a recovery/evacuation instrument (Chapter 31), not a velometer.

Florida HVAC scenario

Sunrise Comfort, certified Class B in Orange County, is balancing a 5-ton changeout (well under 25 tons). The helper clamps the entire outdoor whip, reads 0.2 A, and condemns the compressor as open. Separately he holds a velometer in a 16×8 trunk, treats 800 fpm as if it were a pitot VP of 800 in. w.c., and reports the duct is “800 inches of water — the fan is in surge.” Correct kit: clamp one leg of the whip and compare to RLA; use the pitot and manometer in that trunk (VP will be a few tenths of an inch of water); use the velometer at the supply grilles with Ak. A Magnehelic on the return shows −0.35 in. w.c., not −0.35 psi. The owner’s 30-ton office RTU next door is Class A work; Class B still has to traverse that 30-ton supply duct with a pitot and read inches of water on the exam.

Traps: (1) Clamping hot and neutral together. (2) Comparing running amps to LRA. (3) Using a velometer as if it read VP. (4) (4005 \times \text{VP}) without the square root. (5) Duct static quoted in psig. (6) Refrigerant gauges on ducts. (7) Ohm tests without LOTO, or live clamp-on treated as an excuse to skip LOTO on internals.