Application for Rheotherm and RheoVac Instruments

Low Flow Meters for Gas

Rheotherm low gas flow meters provide an easy-to-use technology for measuring gas flow in small lines – typically 1” or smaller at flow Rheotherm Model 210rates from a few hundred SCFM down to 25 SCCM. (For flow measurement in larger pipes and ducts, click here). These 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. Options for hazardous locations (intrinsically safe/explosion-proof) are available.

What Can We Measure?
Continuous flow of most gases, including corrosive and explosive gases. Our flow meters have been used for everything from hydrogen to uranium hexafluoride. Electropolished sensors are available for ultrahigh purity gases. Our applications section has a complete list of fluids we have measured.  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 in-line flow sensors typically installed with standard tube fittings, but optionally available with flanges, sanitary fittings, etc. Any orientation is acceptable, except vertical with downward flow. 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.

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.

How Do They Work?
Our low flow meters use either a straight-through or looped flow tube with a heated and an unheated RTD attached to the outside ofRheotherm sensor the tube. The temperature differential between the two RTDs provides the primary flow signal. At high flow rates the differential is lower because heat is removed more readily. 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 to wear, stick or break.

How Do I Get One?
Submit your Rheotherm inquiry today using the form on this page so we can review your application and prepare your quotation. 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
Request a Quote

Required fields are indicated with an ‘*’Name*




City, State/Prov.



Density/Specific Gravity

Homogeneous? YN

Coats pipe?YN

Flow Range* (incl. units)

Fluid Temp.* (indicate °F/°C)

Max. Temperature*

Oper. Pressure (incl. units)

Max. Pressure* (incl. units)

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

Pipe/Duct Size*

Straightrun (# dia. up/down)

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.)