Dwyer 641 Air Velocity Transmitter

Fits clean-air duct velocity measurement where you need a heated mass-flow probe, 16 field-selectable FPM ranges, and an isolated 4–20 mA output — not a dirty-air / exhaust variant.

Key advantages

16 FPM ranges — field-selectable 250…15,000 FPM (1.25…75 m/s) for clean-air ducts.
Isolated 4–20 mA — transmitter output for BMS/PLC velocity loops.
Heated mass-flow probe — clean-air service vs dirty-air Series 641B.
NEMA 4X (IP66) intent — published for non-LED models; ½″ NPT insertion; 100 psi max.

Typical applications

Clean-air HVAC and process duct velocity monitoring.
Retrofits needing isolated 4–20 mA from a ½″ NPT insertion probe.
Sites comparing 641 (clean air, to 15,000 FPM) vs 641B (dirty air, to 2,000 FPM).

Selection guide

Measuring range — Field-selectable 16 ranges: 250…15,000 FPM (1.25…75 m/s).
Accuracy — 3% FS process gas 32…122 °F (0…50 °C); 4% FS process gas −40…32 °F and 122…212 °F (−40…0 °C and 50…100 °C).
Output — Isolated 4–20 mA.
Supply — 12–35 VDC or 10–16 VAC.
Process connection — ½″ male NPT (duct insertion).
Electrical — 3- or 4-wire terminal connections.
Filter — 0.5–15 s selectable .
Pressure limit — 100 psi (6.89 bar) maximum.
IP / enclosure — Designed to meet NEMA 4X (IP66) for non-LED models; optional LED models (enclosure rating differs).
Media — Clean air velocity service (see operating temperature table in OEM documentation).

Common mistakes

Specifying 641 for dirty-air / exhaust duty — that duty maps to 641B (lower range, different accuracy). Confirm media cleanliness first.
Ordering an LED display model and assuming NEMA 4X / IP66 — OEM documentation states that rating for non-LED models; LED enclosure rating differs.
Ignoring the accuracy step-change outside 32…122 °F process gas (3% FS → 4% FS).
Exceeding 100 psi (6.89 bar) duct/static pressure limit.
Wiring as 2-wire loop-powered without confirming 3- or 4-wire terminal need and supply (12–35 VDC or 10–16 VAC).
Flow instruments · Flow buying guide

When to RFQ / what to tell us

Expected velocity (FPM or m/s) and which of the 16 ranges you will lock.
Process gas temperature band (drives 3% vs 4% FS accuracy).
Clean-air confirmation (vs dirty-air → consider 641B instead).
Supply available (12–35 VDC or 10–16 VAC); 3- vs 4-wire preference.
LED display yes/no; duct pressure; ½ NPT insertion depth / probe length if known.
Quantity, ship-to country, filter setting preference (0.5–15 s).

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Specs

Spec	Value
SKU	40991
Brand	Dwyer Instruments
Model	641
Product type	transmitter (air velocity, heated mass-flow)
Measuring range	Field-selectable 16 ranges: 250…15,000 FPM (1.25…75 m/s)
Accuracy	3% FS process gas 32…122 °F (0…50 °C); 4% FS process gas −40…32 °F and 122…212 °F (−40…0 °C and 50…100 °C)
Output signal	Isolated 4–20 mA
Supply voltage	12–35 VDC or 10–16 VAC
Process connection	½″ male NPT
Electrical connection	3- or 4-wire terminal connections
IP rating	Designed to meet NEMA 4X (IP66) for non-LED models
Overpressure / pressure limit	100 psi (6.89 bar) maximum
Media	Clean air velocity service (see operating temperature table)
Display	Optional LED models (enclosure rating differs)
Mounting	Duct insertion with ½ NPT
Approvals	CE
Notes	Filter 0.5–15 s selectable. Values aligned with Dwyer Series 641 documentation.

Documents

Dwyer 641 datasheet (PDF)

FAQ

Q: What velocity range can I select? A: Sixteen field-selectable ranges spanning 250…15,000 FPM (1.25…75 m/s). Lock the range to the expected duct velocity in the RFQ.

Q: Clean air or dirty air? A: 641 is positioned for clean-air velocity service. For dirty-air / exhaust, see the 641B variant (SKU 40994) with a lower range and different accuracy.

Q: What output and supply? A: Isolated 4–20 mA; supply 12–35 VDC or 10–16 VAC; 3- or 4-wire terminals.

Q: What should I send for a quote? A: Velocity band, gas temperature, clean vs dirty air, supply, LED yes/no, quantity. Available for systems specifying Dwyer Series 641 — we do not claim authorized-dealer status.