Dwyer 641B Air Velocity Transmitter

Fits dirty-air, exhaust, or clean-room ventilation velocity measurement where a heated mass-flow probe and isolated 4–20 mA are required — at a lower FPM ceiling than Series 641.

Key advantages

Dirty-air FPM band — user-selectable 250…2000 FPM (1.25…10 m/s) for exhaust and dusty ducts.
Isolated 4–20 mA — transmitter output for BMS/PLC velocity loops.
Heated mass-flow probe — designed for dirty-air / exhaust service vs clean-air Series 641.
NEMA 4X (IP66) intent — published enclosure class for non-LED models; CE listed.

Typical applications

Exhaust and dirty-air duct velocity monitoring where clean-air probes foul.
Clean-room and HVAC ventilation points within the 2000 FPM ceiling.
Retrofits needing isolated 4–20 mA from a ½″ NPT insertion probe.

Selection guide

Measuring range — User-selectable 250…2000 FPM (1.25…10 m/s) for dirty-air service.
Accuracy — 5% FS process gas 32…122 °F (0…50 °C); 6% FS process gas −40…32 °F and 122…140 °F (−40…0 °C and 50…60 °C).
Output — Isolated 4–20 mA.
Supply — 12–35 VDC or 10–16 VAC.
Process connection — ½″ NPT (duct/exhaust insertion).
Electrical — Terminal connections; max loop resistance 600 Ω.
Filter — 0.5–15 s selectable .
IP / enclosure — Designed to meet NEMA 4X (IP66) for non-LED models.
Display — none (per OEM extract).
Media — Dirty air / exhaust / clean-room ventilation air.

Common mistakes

Ordering 641B when velocity exceeds 2000 FPM — use Series 641 (to 15,000 FPM) for higher clean-air ranges.
Applying 641 clean-air accuracy (3%/4% FS) to 641B — 641B is 5%/6% FS with a tighter high-temp band (to 140 °F / 60 °C).
Ignoring 600 Ω max loop resistance when long cable runs or high supply drop are planned.
Assuming an LED local display — OEM extract lists display as none.
Treating NEMA 4X / IP66 as universal — stated for non-LED models.
Flow instruments · Flow buying guide

When to RFQ / what to tell us

Expected velocity (must fit 250…2000 FPM) and dirty-air / exhaust confirmation.
Process gas temperature band (drives 5% vs 6% FS accuracy).
Supply (12–35 VDC or 10–16 VAC) and estimated loop resistance.
Insertion depth / duct size for ½ NPT mount.
Filter setting preference (0.5–15 s).
Quantity, ship-to country.

Request a Quote · Get selection help

Specs

Spec	Value
SKU	40994
Brand	Dwyer Instruments
Model	641B
Product type	transmitter (air velocity, dirty-air heated mass-flow)
Measuring range	User-selectable 250…2000 FPM (1.25…10 m/s) for dirty-air service
Accuracy	5% FS process gas 32…122 °F (0…50 °C); 6% FS process gas −40…32 °F and 122…140 °F (−40…0 °C and 50…60 °C)
Output signal	Isolated 4–20 mA
Supply voltage	12–35 VDC or 10–16 VAC
Process connection	½″ NPT
Electrical connection	Terminal connections; max loop resistance 600 Ω
IP rating	Designed to meet NEMA 4X (IP66) for non-LED models
Media	Dirty air / exhaust / clean-room ventilation air
Display	none
Mounting	Duct/exhaust insertion ½ NPT
Approvals	CE
Notes	Dirty-air variant of 641 family. Filter 0.5–15 s selectable. Specs from Dwyer 641B.pdf.

Documents

Dwyer 641B datasheet (PDF)

FAQ

Q: When do I pick 641B over 641? A: When the duty is dirty air / exhaust (or similar) and velocity stays within 250…2000 FPM. Higher clean-air ranges map to Series 641 (SKU 40991).

Q: What accuracy applies? A: 5% FS at 32…122 °F process gas; 6% FS at −40…32 °F and 122…140 °F. State your gas temperature in the RFQ.

Q: What output and loop limit? A: Isolated 4–20 mA; max loop resistance 600 Ω; supply 12–35 VDC or 10–16 VAC.

Q: What should I send for a quote? A: Velocity, media (dirty/exhaust), gas temperature, supply, quantity. Available for systems specifying Dwyer 641B — we do not claim authorized-dealer status.