Model 641B Air Velocity Transmitter

Dirty Air Flow Applications

The Series 641B Air Velocity Transmitter uses a heated mass flow sensor suitable for dirty air flow applications. It has user-selectable ranges from 250 FPM (1.25 MPS) to 2000 FPM (10 MPS).

Product Applications

  • Exhaust stack flow monitoring
  • Air control in drying processes
  • HVAC air velocity measurements
  • Fan supply and exhaust tracking
  • Clean room ventilation monitoring

Models & Ordering

Shipping Information

All shipping times are dependent on product availability at time of shipment.

Although we make every effort to ensure a continuous supply of our products, occasional circumstances may force us to temporarily run out of a product, or have shipping delays. Should this occur, customers will be notified soon after placement of any orders for such products and, if applicable, items will be put on backorder. For orders requiring expedited shipment, our customer service team can be contacted to confirm product availability

Model Description Usually Ships* Price Order
Ships: Next Day
Price: $683.00
Air velocity transmitter. Next Day* $683.00 Order
Ships: 1-2 Weeks
Price: $880.00
Air velocity transmitter with LED display. 1-2 Weeks* $880.00 Order


Air and compatible, non-combustible gases.
5% FS process gas: 32 to 122°F (0 to 50°C); 6% FS process gas: -40 to 32°F & 122 to 212°F (-40 to 0°C & 50 to 100°C).
Response Time:
Flow: 1.5 s to 95% of final value (output filter set to min).
Temperature Limits:
Process: -40 to 176°F (-40 to 80°C). Ambient: 32 to 140°F (0 to 60°C).
Humidity Limit:
Power Requirements:
12 to 35 VDC, 10 to 16 VAC. 1.5 A rating required on supply due to initial power surge drawn by transmitter.
Output Signal:
4 to 20 mA, isolated 24 V source, 3 or 4-wire connection.
Output Filter:
Selectable 0.5-15 s.
Loop Resistance:
600 Ω max.
Current Consumption:
300 mA max*.
Electrical Connections:
Screw terminal.
Enclosure Rating:
Designed to meet NEMA 4X (IP66) for non-LED models only.
Mounting Orientation:
Unit not position sensitive.
12.6 oz (357.2 g).

*A brief current transient exceeding 300 mA may be seen on startup.


  • SS sensor suitable for dirty air flow measurement
  • Ranges from 250 FPM (1.25 MPS) to 2000 FPM (10 MPS)
  • 4 to 20 mA output
  • Digital filter for signal damping


Model Number Description Usually Ships* Price Order
Cable gland with 1/2 A-155 Cable gland with 1/2" male NPT fitting. 1-2 Weeks $20.75 Order
Flange mounting plate with 1/2 A-156 Flange mounting plate with 1/2" female NPT. 1-2 Weeks $21.00 Order


Product Literature


Catalog Pages




I need to monitor air velocity within a supply and exhaust duct, both are connected to a Class I Division 1 rated gashouse. Is this possible with the 641B in conjunction with an isolated barrier/repeater? The 641B will be duct mounted outside of the gashouse. I was interested in the AVU series as well but our supply contains high humidity levels which led me to the 641B. Thanks for the help.
Unfortunately, you cannot use the 641b in a classified area as it is not intrinsically safe
We have recently replaced a Dwyer air velocity transmitter (PFS1300204) with a Dwyer Series 641. The new sensor works fine for only a few days and then starts reading almost no flow. These devices are used in an Air Scrubber application (exhaust stack). Is this the right application for the Series 641? I suspect that moisture might be the issue, but I cannot confirm.
I do not think you are using the 641 series air velocity transmitter in an appropriate application. Any moisture or particulates in the exhaust could be causing damage to the hot wire sensor, leading to the failure. The difference in the sensors is probably why you got longer life out of the PFS series, which unfortunately, is no longer available. The PFS series used a glass filled polyester sensor, which is a little more robust and can with stand the elements a little better. In your specific application, you might take a look at our 641B series which uses a heated mass flow sensor which can be used in dirty air environments.

Photo Downloads

You must be logged in to view this content.
Click here to login to your account.

Product Reviews

Only registered users can write reviews. Please Sign in or create an account