DwyerOmega Series 641

Heated mass flow air velocity transmitter for duct and process air monitoring.

Product Overview

Manufacturer: DwyerOmega
Product: Series 641
Type: Installed measurement transmitter
Range / measured parameters: 250–15,000 fpm range options
Communication / outputs: 4–20 mA

Engineering and Selection Considerations

Confirm the exact manufacturer’s ordering code, current availability, calibrated measurement range, output protocol, power supply, operating environment and mounting requirements. Technical parameters can change between models, ordered options or product generations.

Official Technical Documentation

Official manufacturer product information →

Independent AirAlign product reference. Source reviewed 13 October 2026. The manufacturer’s technical documents remain authoritative.

Series 641 — heated mass flow measurement in FPM

The 641 is a heated mass-flow air-velocity transmitter with ordered ranges within the published 250–15,000 feet-per-minute portfolio. Select full scale for the expected duct speed: an instrument with an excessively high span may offer poorer absolute resolution in a low-velocity ventilation application. Verify the configured 4–20 mA mapping, operating temperature and NIST-traceable calibration option for the exact model. Heating/thermal response depends on gas composition and mounting, so use an appropriate traverse/installation point rather than treating one duct reading as total volumetric airflow.

DwyerOmega Series 641 — heated-mass duct air velocity transmitter — verified technical selection

Source-specific engineering data for the model and its defined variant. All specifications are maker statements and are not an independent AirAlign test or approval.

Parameter	Manufacturer specification	Practical limitation
Sensing principle	Heated-mass / thermal dispersion flow probe for clean, non-combustible gases	Do not use as a certified explosive-atmosphere or hydrogen leak detection instrument.
Selectable airspeed	16 field-selectable full-scale ranges from 250 to 15,000 FPM (1.25–75 m/s)	Use correct full-scale setting before interpreting 4–20 mA BAS data.
Accuracy at 0–50 °C	±3% of full scale (FS)	At low velocity relative error can greatly exceed 3% of reading.
Accuracy at −40 to 0 °C and 50–100 °C	±4% of full scale (FS) for process gas	Temperature regime changes specified accuracy; not a single universal ±3%.
Dynamic response / output filtering	95% response in 1.5 s at minimum digital filter; filter adjustable 0.5–15 s	Smooth output reduces noise but delays rapid damper/fan decisions.
Supply / startup current	12–35 V DC or 10–16 V AC; PSU must accommodate 1.5 A initial surge; typical max 300 mA consumption	Do not undersize PSU just to steady-state draw.
Analog output	Isolated 4–20 mA source, 3/4-wire connection; max 600 Ω loop resistance	This differs from a passive 2-wire loop-powered transmitter.
Process connection / protection	1/2-inch NPT male; process up to 100 psi; −40 to 100 °C process air; NEMA 4X/IP66 non-display variants only	Display option may have different enclosure rating.

System integration and commissioning

Dwyer lists fixed probe options 641-6, 641-12, 641-18 and 641-24 with approximately 6/12/18/24-inch insertion probes, and optional LED display or -NIST certificate. Select insertion depth and velocity range based on duct geometry, not merely product family. During BAS acceptance, test 4 mA low-end, 20 mA full-scale, loop burden and startup current. A local probe reading does not equal the duct volume flow without velocity averaging or a verified area/profile model. For industrial exhaust, identify dust, humidity and chemically aggressive gases outside the instrument’s clean-air specification.

Original technical reference: Manufacturer model page, specification or data sheet. Verify the exact purchased suffix and revision, certificates and calibration assumptions.

AirAlign technical editorial enrichment · 25 October 2026. Published specs are not independent laboratory, hazardous-area certificate or active procurement signoff.