Application for Rheotherm and RheoVac Instruments
Low Flow Gas Meters (RFQ)

Low Flow Gas Meters (RFQ)

Rheotherm low-flow gas flow meters provide an easy-to-use technology for measuring gas flow Rheotherm Model 210 Gas Flow Metersin small lines—typically 1” or smaller at flow rates from a few hundred SCFM down to 25 SCCM. (For flow measurement in larger pipes and ducts, check out our Duct Flow Meters.) These flow meters give a repeatable reading in mass or standard volume with no external temperature or pressure correction needed. With no mechanical parts, they are reliable and easy to maintain. For hazardous environments, intrinsically safe and NEMA 7 explosion-proof options are available (to learn more, go to our Instruments for Hazardous Locations page).

What Can Our Gas Flow Meters Measure?

Continuous flow of most gases, including corrosive and explosive gases. These low-volume flow meters have been used to measure everything from hydrogen to uranium hexafluoride. A partial listing of the gases that are being measured by our flow instruments is presented here:

  • Ammonia
  • Argon
  • Butane
  • Carbon Dioxide
  • Carbon Monoxide
  • Chlorine Gas (Dry)
  • Coal Gas
  • Coke Gas
  • Diesel Exhaust
  • Digester Gas
  • Ethane
  • Ethanol Vapor
  • Ethylene
  • Flare Gas
  • Flue Gas
  • Fluorine Gas
  • Fuel Gas
  • HC Fuel Gas
  • Helium
  • Hydrocarbon Gas
  • Methane
  • Methanol Formaldehyde Vapor
  • Natural Gas
  • Oxygen
  • Propane Vapor
  • Propylene Gas
  • Sulfur Dioxide
  • Tungsten Hexafluoride (WF6)

Electropolished sensors are available for ultrahigh purity gases. Our applications section has a complete list of fluids we have measured with our mass flow meters.

Rheotherm flow meters have been used in literally thousands of installations since we began manufacturing them in 1978. Industries that currently use our fluid flow measurement meters include:

  • Chemical and Petrochemical
  • Aerospace
  • Wastewater Treatment
  • Natural Gas
  • Oil and Gas Production
  • Nuclear
  • Pulp and Paper

Contact Us about any application you have or use the form on this page to tell us about your application.

How Are They Installed?

These are inline air flow sensors that are typically installed with standard tube fittings, but also available with optional flanges, sanitary fittings, etc. They can be installed in any orientation except vertical with downward flow.

With some models the electronics are integral to the sensor, while other models have a separate electronics enclosure that can be installed as far as 200 feet from the sensor. The separate enclosure is especially useful for locations with limited space where it is impossible or impractical to place the electronics in the same spot as the sensor, or in hazardous environments where the electronics can be located in a non-hazardous area.

What Is the Turndown Ratio?

Typical calibration ranges for our inline flow meters are 10:1 (options up to 200:1), with excellent accuracy over the entire range. The sensors cannot be damaged by over-ranging or liquid carryover. Four calibration channels are available.

How Do They Work?

Bionetics’ low flow meters use either a straight-through or looped flow tube with a heated and an unheated Resistance Temperature Detector (RTD) attached to the outside of the tube.

Rheotherm sensor

The temperature differential between the two RTDs generates the primary flow signal. At high flow rates the differential is low because heat is easily removed from the fluid. At low flow rates the differential is higher, because less heat is removed. Nothing touches the fluid except the flow tube wall, and there are no mechanical parts that can wear, stick, or break.

Due to the absence of moving parts, maintenance requirements are minimal. Volume flow and mass flow of gases can be measured without the need for pressure or temperature correction. All these features and capabilities add up to a thermal low flow measurement meter that cannot be surpassed for accuracy and ease of use.

Our optional 2 x 16 backlit LCD display shows mass/volume flow rate, temperature (Celsius or Fahrenheit), and total accumulated flow stats. Our flow instruments also come with a self-diagnostic feature that detects system problems—from damaged sensors to corrupted data—and generates appropriate status codes which can be viewed by the user. Flow output data can be transmitted in analog or digital form.

How Do I Get One?

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
  • Exclusive inline TU sensor
  • Advanced signal processing electronics
  • Field adjustable
  • Maintenance-free
  • Easy installation
  • Choice of wetted materials
  • Long-term reliability
  Standard Options
Temperature Process: 0 – 140°F
(-18 – 60°C)
Environment: 0 – 120°F
(-18 – 50°C)
 -20 – 350°F
(-30 – 175°C)
Pressure 0 – 1,000 PSI Up to 30,000 PSI
Calibration Ranges

10:1 Turndown;
Flow rate: 0.001 to 320 SCFM
(.03 to 9060 SLPM)

– Up to 200:1 turndown
– Four calibration channels available (for multiple flow ranges or different fluids using the same sensor)
Calibration Accuracy  ± 1% of reading typical
Response Time  1 sec. ; 3 – 5 sec. typ.
(63% of range);
Time constant/delay option
Repeatability ± 0.5% of reading
  Standard Options
Line Sizes
(flow range dependent)
1/16”; 1/8”; 3/16”; 1/4”
3/8”; 1/2”; 5/8”; 3/4”; 1”
Other sizes via fittings/adapters
Process Connection

Plain tube stubs suitable for compression fittings

– Tri-Clover (Sanitary);
– SS Flange (1/2” – 2”):
Most other fittings available
Wetted Material 316 SS – Hastelloy C-276®;
– Monel®;
– Alloy 20;
– Titanium;
– Tantalum;
– Inconel
Display 2 X16 backlit LCD with selectable display of:
– Mass/volume flow rate
– Temperature
– Total accumulated flow
  Standard Options
Enclosure Sensor:  PVC
Integrated Electronics: NEMA 4X Cast Aluminum
Remote Electronics: NEMA 4
Sensor: Stainless steel
Remote electronics:
– Stainless steel
– NEMA 7 Explosion-proof
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
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
Input Power 24 VDC ±4V
(200 mA; 300 mA w/display option)
110/230 VAC ±10V

Inline Sensor Flow Capacities for Gases

FLOW RATE LIMITS C 1/16 1/8 3/16 1/4 3/8 1/2 5/8 3/4 1
Extended Maximum 0.027 1.25 5 11 20 45 80 125 180 320
Normal Maximum 0.017 0.75 3 7 15 34 70 109 160 280
Normal Minimum 0.0017 0.02 0.1 0.2 0.4 0.9 1.6 2.5 3.6 6.4
Extended Minimum 0.0009 0.0017 0.05 0.1 0.2 0.5 0.8 1.2 2 3.2
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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.)