Thermal meters measure gas mass flow directly without the need for additional need pressure, temperature and/or density monitoring.  They operate either by introducing a known amount of heat into the flowing stream and measuring an associated temperature change, or by maintaining a probe at a constant temperature and measuring the energy required to do so.  Thermal mass flowmeters include two temperature sensors and an electric heater between them. The heater can protrude into the fluid stream or may be external to the pipe. By eliminating temperature changes in the process, any heat transfer within the flow stream is directly proportional to its mass flow.  For a more complete description of thermal mass flow technologies please go to; https://smartmeasurement.com/flow-meters/thermal/measuring-principle

Thermal meters have no moving parts, wide turn-downs (up to 1000:1), a range from 0.02 meters per second (4 feet per minute) to over 200 mps (40,000 fpm), and very low pressure drop.  SmartmeasurementTM, s ATMF series of thermal mass flowmeters can be used with pipe diameters ranging from 8mm (¼”) to up to several meters using our multi-sensor configurations.  Process connections can be flanged, threaded, or hot-tapped ball valve.  SmartmeasurementTM’ s ATMF Series Thermal Mass Flowmeters feature available integral style or remote mount electronics, an easy-to-read OLED display, and a unique calibration self-check technique that allows for field verification of the factory calibration. Please see the Thermal Product Matrix below to select the best configuration for your specific application.
 

ALTM and ALTS Family
Technology	Model	Styles	Diameters	Temperature	Special Features	Outputs	Approvals
SP (industrial)	ATMF8000IL-SP ATMF8000IS-SP	Insertion (IS) Inline (IL)	> ¼ 2 < 40" (50-1000mm)(IS)	IL <392°F(200°C) IS 158°F (70°C) remote	Displays numerical and graphical Calibration milliwatts (mw) for ongoing diagnostics, limited Ex approvals	Analog, Modbus Compliant RS485 RTU communications	CE, DC Powered: CSA C22.2, UL1604, Atex Ex nA IIC T4, and Class I, Div 2, Grps BCD T4"; "DC and AC
SD (portable)	ATMF8000IL-SD ATMF8000IS-SD	Insertion (SD)	> 1 < 40"(25-1000mm) (IS)	200°F(93°C), >158°F(70°C) remote	Datalogger Keypad configuration, 10 hour rechargeable battery	Up to 3800 data points in .csv format (Excel® file), via USB cable	CE
SX (ATEX/CSA/FM)	ATMF8000IL-SX ATMF8000IS-SX	Insertion (IS) Inline (IL)	> ¼ 2 < 40" (50-1000mm)(IS)	IL <392°F(200°C) IS 158°F (70°C) remote	Displays numerical and graphical Calibration milliwatts (mw) for ongoing diagnostics, Europe/North American Ex approvals	Analog, Modbus Compliant RS485 RTU communications	CE and ATEX Zone I,II 2 G Ex d IIB+H2 T6 Gb
SC (Nonhazardous)	ATMF8000IL-SC ATMF8000IS-SC	Insertion (IS) Inline (IL)	> ¼ 2 < 40" (50-1000mm)(IS)	IL <392°F(200°C) IS 158°F (70°C) remote	Low cost, no display, Nema 4X Enclosure	RS232, 4-20mA for Flow, 0 - 5 VDC for Temperature, Pulse for Total	CE
ALTS (liquid/gas flow/level switch)	ALTS	Insertion (IS) Inline (IL)	> ¼ 2 <40" (50-1000mm)(IS)	IL <392°F(200°C) IS 158°F (70°C) remote	Used for both for signal or dual flow or level switch for either gases and liquids	Single or dual contact switch	CE/FM/CSA