Thermal Mass Meter

A Thermal Mass Meter works by measuring the total mass flow rate of a fluid flowing through a pipe or duct.  In a thermal flowmeter’s simplest working configuration, fluid flows past a heated thermal sensor and a temperature sensor.  As the molecules of the fluid flow past the heated thermal sensor, heat is lost to the flowing fluid.

Thermal mass flow meters, also known as thermal dispersion or immersible mass flow meters comprise a family of instruments for the measurement of the total mass flow rate of a fluid, primarily gases, flowing through closed conduits.  A second type is the capillary-tube type of thermal mass flow meter.  Many mass flow controllers (MFC) which combine a mass flow meter, electronics and a valve are based on this design.  Furthermore, a thermal mass flow meter can be built by measuring temperature differential across a silicon-based MEMS chip.


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A Thermal Mass Flow Meter is a type of flow meter used to measure the flow rate of a fluid, such as a gas or a liquid, in a pipe or duct. It measures the mass flow rate of the fluid, rather than the volumetric flow rate, which is more accurate and independent of changes in fluid temperature and pressure.

Thermal mass flow meters work by measuring the heat transfer from a heated element to the fluid. The meter consists of a heated element and two temperature sensors, one upstream and one downstream of the heated element. The heated element raises the temperature of the fluid, and the temperature sensors measure the difference in temperature between the fluid upstream and downstream of the heated element.

As the fluid flows through the pipe, it carries away heat from the heated element, causing a temperature difference between the two temperature sensors. This temperature difference is directly proportional to the mass flow rate of the fluid, which can be calculated by the meter’s electronics.

Thermal mass flow meters are commonly used in a variety of applications, including in HVAC systems, process control, and environmental monitoring. They are particularly well-suited for measuring the flow rate of gases, including natural gas, compressed air, and industrial gases.


Insertion style Thermal Mass Meter include a sensor & probe assembly that is inserted into the process gas flow conduit to allow the process gas to flow across the flow sensing elements.  Our insertion style flow meters are available with 3/4″, or 1″ OD probes.  Tube fittings and ball valve retractor assemblies, with or without a mounting flange, are also available from the factory as options.  The tube length must be specified upon ordering.  For other probe diameters and lengths, please consult the factory.

Integral style Thermal Mass Meter have all of the electrical components and connections located within one enclosure.  The enclosure is mounted directly to the insertion probe assembly at the point of measurement.  The enclosure includes the all of the electrical connections as well as the linearizing electronics and the display/keypad assembly.

Thermal mass meter use the principle of convective heat transfer to directly measure mass flow.  ProFlow’s proprietary thermal mass flow sensors use two ratiometrically-matched, reference-grade platinum Resistance Temperature Detectors (RTDs).  The platinum sensing element wire is encapsulated in a 316 Stainless Steel sheath or, if specified, a Hastelloy C sheath.  Our microcontroller operated smart sensor technology preferentially heats one RTD; the other RTD acts as the temperature reference.  The process gas flow dissipates heat from the first RTD, causing an increase in the power required to maintain a balance between the RTDs.  This increase is directly related to the gas molecular rate of flow. Our sensors are temperature compensated for a wide process gas temperature range and insensitive to pressure changes, so the output signal is a true mass flow rate signal.


Additional information

Weight 9 lbs
Dimensions 24 × 14 × 14 in

3/4", 1", 1-1/2", 2", 3", 4"

Pressure Class

150LB, 300LB, 600LB


NPT, Flanged, Triclamp


Yes, No


24 VDC, 120 VAC

Communication Protocol



4-20 MA, 0-10 VDC


Explosion Proof, Intrinsically Safe



Currently no product agency approvals.

Enclosure Certifications by manufacturer
US – CL. I. GR. A,B,C,D; CL. II. GR. E,F,G; CL. III.;TYPE 4X
IECEx – Ex d IIC Gb, Ex t IIIC Db, Exia (intrinsically safe)
ATEX – II 2G Ex IIC Gb, II 2Dd Ex t 111C Db

Linear signal output
0–5 VDC & 4–20 mA (Flow and Temperature)

Event Relay Rating one with Frequency or two, 1 Amp @ 30Vdc (33W)

Signal Interface
RS232 & RS485 Modbus RTU embedded, HART , LCD (flow rate, flow total, gas temperature)

Accuracy, including linearity (Ref.: 21°C)
±(1% of Reading + 0.5% of Full Scale + GTC)

±0.2% of Full Scale

Sensor response time
1 second to 63% of final value

Turn down ratio
100:1 @ 10 SFPM (.051 NMPS) Minimum Reading

Electronics PCB temperature range
-40° to 158°F (-40° to +70°C)

Environmental temperature range
-40° to 140°F (-40° to +60°C)

Gas temperature range
-40°–392°F (-40°–200°C)

Gas temperature coefficient (GTC)
0.02% Full Scale/°C

Gas pressure effect
Negligible over ± 20% of absolute calibration pressure

Pressure rating maximum
500 PSI Std.

Input power requirement
24VDC @ 250mA, 115 VAC 50/60 Hz optional, 230 VAC 50/60 Hz optional

Flow Transmitter power requirements
5 watts maximum

RAM Back-up
Lithium Battery

Wetted materials
316 Stainless Steel (Hastelloy optional)

Standard temperature & pressure (STP)
70°F & 29.92″ Hg (Air .075 lb./cubic foot)

NIST traceable calibration

NOTE: Specifications subject to change without notice.




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