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Vane vs Hot-Wire Anemometers: What Is the Difference?

The two field anemometers measure the same thing by different physics, and each is right for the jobs the other does badly. A vane averages a whole grille but stalls at low speed; a hot-wire reads a whisper of air through a 10 mm hole but drifts with temperature.

By the Akermin Editorial Team · 3 min read

A vane anemometer measures air speed by how fast a small propeller spins; a hot-wire anemometer measures it by how much moving air cools a heated element. Vanes are robust, direction-sensitive, average over their own diameter and are best for grilles, ducts and outdoor wind above about 0.4 m/s. Hot-wires read very low velocities, fit through small test holes and respond fast, but are fragile, sensitive to air temperature and read the flow at a single point. Choose by the lowest speed and the smallest opening you need to measure.

How each works

Vane

A lightweight propeller, 25 to 100 mm across, turns on low-friction bearings; a magnet or optical sensor counts revolutions. Rotation speed is nearly proportional to air speed above a starting threshold. Because the propeller spans an area, the reading is an average across it, which is convenient at a grille and misleading in a narrow duct where the head blocks the flow.

Hot-wire (thermal)

A thin wire or film is heated a fixed amount above air temperature. Air passing over it removes heat; the circuit measures the power required to hold the temperature and converts it to velocity. The element is a few millimeters across on a telescopic probe, so it reads a point and fits through a 10 to 15 mm hole. Because the physics depends on the temperature difference, the instrument must measure air temperature and compensate.

Side by side

Vane Hot-wire
Velocity range About 0.4 to 30 m/s About 0.05 to 20 m/s
Typical accuracy ±2 to 3% of reading ±3 to 5% of reading
Low-speed performance Poor; stalls Excellent
Temperature sensitivity Low High; compensated within a range
Direction Must face the flow Nearly omnidirectional
Spatial averaging Over the vane diameter Point measurement
Probe access Needs a clear opening the size of the head Fits through small holes; telescopic
Durability Robust; dust and moisture tolerant Delicate element; keep clean and dry
Response Slow; bearings and inertia Fast; fractions of a second
Cost Lower Higher

Choosing by job

  • Supply and extract grilles: vane. The averaging suits a large face; use a traverse for big grilles.
  • Duct traverses through test holes: hot-wire. It fits the hole and reads the profile point by point.
  • Fume hood and safety cabinet face velocity (0.4 to 0.6 m/s): hot-wire, because low speeds and multi-point grids are involved.
  • Cleanrooms, laminar flow benches, diffuser throw: hot-wire.
  • Outdoor wind, sport, drones, spraying: vane or cup.
  • Bathroom fan check: either; a vane at the grille is simplest.
  • Air leaks and drafts around windows: hot-wire, for the low speeds and the ability to probe a gap.

Errors to avoid

  1. Vane not square to the flow; readings fall with the cosine of the angle.
  2. Vane head blocking a small duct and reading its own obstruction.
  3. Hot-wire used outside its temperature compensation range, or with a dirty element.
  4. Hot-wire in pulsing flow reporting a fluctuating number; use the instrument’s averaging mode.
  5. Reading a single point on a grille and calling it the average.

Background on what the measurement is for: what an anemometer measures. Instruments are covered in the environmental monitoring category.

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

Which anemometer is better for HVAC?

For grilles and diffusers, a vane. For small ducts, fume hoods, low-velocity checks and probing through test holes, a hot-wire. Many technicians carry both.

Why does a hot-wire read differently when the air is warm?

It measures cooling of a heated element. Warm air cools it less, so the instrument must compensate; outside its compensation range it reads low.

What is the lowest speed each can measure?

Vanes stall below roughly 0.2 to 0.4 m/s. Hot-wires read down to about 0.05 to 0.1 m/s, which is why they are used for cleanrooms and hood face velocities.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.