Application for Rheotherm and RheoVac Instruments
Duct Flow Meters for Air & Gas (RFQ)

Duct Flow Meters for Air & Gas (RFQ)

Rheotherm flow sensing probes for duct gases are easy to install and provide long-term service with little or no maintenance. The output can be in mass, standard volume or velocity units. Options for hazardous locations (intrinsically safe/explosion-proof) are available.

Various line connections for air and gas flow sensorsWhat Can We Measure?
Air and most other non-condensing gases, including corrosive and explosive gases. The stainless steel construction and lack of moving parts make it ideal for rugged industrial environments. Common uses include vent air, stack gas and digester gas. The application section has a complete list of fluids.  Contact Us about any application you have or use the form on this page.

How Are They Installed?
These are insertion probe flow sensors that install through the pipe or duct wall. Typical line connections are 1” NPT, but flange, hot tap and sanitary fittings are available.  The probes can be installed in horizontal or vertical pipes.  With some models the electronics are integral to the sensor, and other models have a separate electronics enclosure to install, up to 200 feet from the sensor. Electronics Options

What Is the Turndown Ratio?
Typical calibration ranges are 10:1 (options up to 200:1), with good accuracy over the entire range. The sensors cannot be damaged by overranging.

Duct flow meterHow Do They Work?
Our insertion flow sensors have a shaft with two stainless steel flow sensing tips. One tip has a heated RTD in it and the other an unheated RTD. The temperature differential between the two RTDs provides the primary flow signal. At high flow rates, flow removes more heat resulting in a lower differential. At low flow rates, flow removes less heat so the differential is higher. Nothing touches the gas but the metal probe. More on Method of Operation (PDF)

What If There Is Little to No Straight Run Available?
The historically reliable Rheotherm flow meter for duct flow measurement is available with a revolutionary sensor head which eliminates off-axis error common to other sensors.  The system measures true mass flow rate in turbulent ducts and pipes using our standard gas flow probe design with the Rheovec sensor head added. Rheovec literature (PDF)

How Do I Get A Duct Flow Meter?
There are two ways to submit your application information to us online:

  1. Use the form on this page to provide the details of your application online.
  2. Download and complete the Flow Application Data Sheet and email it to us at

You can also call or visit our Contact Us page for telephone, fax, and email information.

  • No moving parts
  • Advanced signal processing electronics
  • Field adjustable
  • Maintenance-free
  • Easy installation
  • Choice of wetted materials
  • Long-term reliability

  Standard Options
  Service Duct & pipe gases
  Pipe Sizes  1” and larger pipes
Wetted Surface 316 SS Hastelloy C-276®;
Flow Velocity Limits 25-25,000 SFPM
(0.127-127 meter/sec.)
Consult Factory
Turndown Ratio 10:1 Up to 200:1
Accuracy   +/- 1% of reading
 Repeatability   +/- 0.5%
Max. Temperature 175 ºF (80 ºC) Up to 500 ºF (260 ºC)
Max. Pressure Depends on Fitting Up to 4,000 PSI
  Standard Options
Process Connection 1” MNPT – Tri-Clover (Sanitary);
– SS Flange;
– Ball valve/hot tap;
Most other fittings available
Outputs 4 – 20 mA (flow rate) or Serial Burst with Flow Rate, Totalizer, and Fluid Temperature – 0 – 10 or 0 – 5 VDC;
– Pulse (5 VDC open collector);
– HART (4-20 mA);
– SPDT relay;
– 4 to 20 mA temp
Enclosure Integrated – NEMA 4X Cast Aluminum
Remote – NEMA 4
Remote electronics:
– 304 Stainless steel;
– NEMA 7 Explosion-proof
  Standard Options
Display 2 X16 backlit LCD with selectable display of:
– Mass/volume flow rate;
– Temperature;
– Total accumulated flow
Hazardous Environment Integrated electronics: FM-Approved for:
– Class I, Div. 1, Groups B,C,D;
– Class I, Zone 1, IIB+H2 (US);
– Class II, Div. 1, Groups E, F, G;
– Class III, Div. 1
Remote electronics:
Same as above except addition of Group A using ISB
Input Power 24 VDC ±4V
(200 mA; 300 mA w/display option)
110/230 VAC ±10V

Probe-type Sensor Flow Capacities for Gases

feet/min meter/sec
Maximum 25,000 127
Minimum 25 0.127
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Required fields indicated with *




City, State/Prov.



Density/Specific Gravity

Homogeneous? YN

Coats pipe?YN

Flow Range* (incl. units)

Fluid Temp.* (operating)

Max. Temperature*

Oper. Pressure (incl. units)

Max. Pressure* (incl. units)

Continuous or Batch Flow*
Batch/Pulse Time (secs)

Pipe/Duct Size*

Straightrun (up/downstream)

Preferred Wetted Material

Process Fittings

Flange Size/Rating

Local Display* YN
Input Power*

Flow Output* (ctrl-click for multiple)
Additional Requirements
(Hazardous environment, certificates, etc.)