SmartMeasurement Thermal Mass Flow Meters
SMARTMEASUREMENT™ THERMAL FLOW METERS
Thermal flow meters
The challenges of measuring gases are much greater than measuring any other fluid because a small change in pressure and/or temperature can affect measurement accuracies greatly. These challenges are presented on https://www.smartmeasurement.com/gas-flow-meters/
Thermal Mass flow meters alleviate most of these challenges better than any other gas flow measurement device. With the exception of Coriolis gas mass flowmeters all other gas flowmeters are volumetric flow measurement devices. However, Coriolis gas measurement has proven to be effective in high-pressure applications, but lower-pressure gas poses challenges.
Traditionally gas flow measurement was done with Volumetric gas flow meters. These non-mass meters must be used with temperature, pressure sensors and a flow computer to correct for these changes. Compensated for dynamic changes in pressure and temperature are expensive and inaccurate as one must add the inaccuracies of the volumetric flow meter, pressure and temperature sensors for the gas flowmeters overall accuracy.
Key Applications
The ATMF flowmeters from SmartMeasurement Thermal Mass flowmeters have been successfully installed in a wide variety of industries and applications, including:
- refinery gases
- bio or waste gas in wastewater plants
- landfill gases
- pollution gases in stacks and flares
- combustion air and natural gases in boiler controls
- compressor stations
- gas storage tanks
- carbon dioxide gases in food and beverage applications
- steel plants
- HVAC
Why choose a Thermal Mass flow meter
There are many process gas flow measurements, from biogas or waste gas requiring very low flows, to measuring combustion air and natural gas for boilers or hydrocarbon gases in refineries, to stack gases for pollution monitoring and any other applications that require accurate flow measurement (gas). This can create challenges for older, traditional flow measurement technologies such as turbines, differential pressure, vortex, ultrasonic, positive displacement, etc., because only measure volumetric flow and need other instruments to measure mass flow. Thermal mass flow meters measure mass directly without the need to take into account of changes of process constant temperature differential, pressure and density as volumetric flow meters do. They have excellent low-ow sensitivity, virtually no pressures, easy to install with insertion-style meters. Measuring gas mass flow without other instruments to compensate for dynamic process changes makes SmartMeasurement’ s ATMF series of gas mass flowmeters the instrument of choice.
The ATMF series of thermal mass flow meters feature accuracies of +/-0.5% of Full Scale +/-1% of Reading, greater than 100:1 turndown ratio, ability to measure from the smallest pipes of ¼” (6mm) to large ducts of unlimited sizes with our multipoint mass flow meters or the patented FAT (flow averaging Tube) probes. Flow velocities of 4-40,000 fpm (0.02-200 mps) at temperatures of 932oF (500oC) with operating pressures of up to 1000 psig (69 barg). The ATMF series are most popular in measuring air, natural gas, biogas, and industrial gas measurement applications. For applications with limited straight runs https://smartmeasurement.com/wp-content/uploads/2021/02/Straight-Run-Requirements-of-Various-Flow-Technologies.pdf flow conditioners of various sizes are offered.
Special Cases
The Case for Insertion thermal mass meters the ATMFIS
SmartMeasurementTM offers a wide ranges of insertion thermal mass meters that can be used in pipes of 2’ (50mm) or greater. This makes thermal mass-meter installation less expensive than inline meters because they are installed on the top of the pipe with only one fitting. Moreover, insertion meters can be “hot tapped” with various ball valve retractors without shutting down the process in for maintenance and/or evaluation.
In the Case for thermal mass flow averaging tubes the ATMFFAT
In situation of limited straight runs, SmartMeasurement’ s ATMFFAT can reduce straight-run requirements of up to 3 diameters downstream. The ATMFFAT use a flow averaging tube in conjunction with thermal mass transducer placed inside the tube which has a number of large diameter inlet ports along the length of the upstream section. The pressure at each inlet port is averaged inside the tube to create the axial flow through the tube, providing a stable and conditioned flow to be measured across the flow transducer. The gas returns to the main flow stream through the ports located near the sensing elements. Anomalies in the actual flow profile or installations in non-circular ducts may still some require minor adjustments to achieve the best accuracy. The biggest advantage of this design is that only 3 diameters are required for stable and accurate thermal flow measurement. Please see the diagrams below The ATMFFAT comes both in inline configuration, the ATMFFATIL for sizes of 2-4” and insertion style the ATMFFATISL for larger diameters. The ATMFFAT also can come with an auto-purge fitting using the customer’s own compressed air to periodically clean the sensor and flow averaging tube’s port holes for wet and dirty gases.
The Case for Multi Point thermal mass flow meters the ATMFMT
In application where there are non-circular or large ducts, a single transducer insertion style mass flow meter may not provide stable flows due to uneven flow profiles which poses accuracy flow and stability issues. Insertion meters must be placed at appoint of average flow and would be impossible to determine this area on a pipe/duct in non-circular or large pipes/ducts. In applications such as air ducts, combustion air ducts, flare or stacks, a multi-sensor thermal flow meter is needed to determine the average mass flow rate. SmartMeasurement offers our ATMFMT multipoint Systems, designed with 2 to 5 individual transducers in a flow tube to accurately measure the average flow in these difficult applications. The transducers are field replaceable should one or more sensors become inoperative. In situations where the gases are wet and/or dirty such as air intake ducts or air exhaust and flue stacks, etc. the ATMFMT can come with an Air Purge System (APG) using the customer’s own compressed air system. The ATMFMT with an APG ensures to clean the transducers (sensors) ensuring accurate and stable mass flow measurement in dirty gas applications. The ATMFMT are design for applications as diverse as ambient air flows in HVAC ducts, exhaust gas in large diameter stacks, boiler NOx efficiency systems and municipal waste incinerators. See the diagrams below:
Featured Products
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Standard inline/insertion mass flow meter
- ¼" (6mm) inline up to 4" (100mm), insertion >2" (50mm)
- All gases
- -23-392°F( -5-+200℃) up 977°F (525°C) ATMFIS
- 1% of reading plus 0.5% FS
- 1500PSI (100 bar)
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316SS and HASTELLOY
- 4-20mA, pulse, MODBUS, HART & Profibus DP
- Ball valve, NPT, flanges (ANSI,DN,JIS)
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Direct mass flow of any gas
- Oil & gas or any Ex-proof applications, as well as where ongoing meter diagnostics are necessary
ATMF-MT
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Multipoint mass flow meter
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6" (150mm) or greater up to 6 points
- All gases
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-23-392°F ( -5-+200℃)
- 1% of reading plus 0.5% FS
- 25PSI (1.8 barg)
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316SS
- 4-20mA, pulse, MODBUS, HART & Profibus DP
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Flanged, optional purge unit
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For ducts and stacks
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ALL industries that require CEMS (Continued Emission Monitoring)
ATMF-FAT
- Insertion and inline Flow averaging mass flow meter
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2" (50mm) or greater
- All gases
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40°–150°F (5°–65°C)
- 1% of reading plus 0.5% FS
- 500 PSI (35bar)
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316SS
- 4-20mA, pulse, MODBUS, HART & Profibus DP
- Tube fitting, flanged or ball valve
- For ducts and stacks with relatively clean gases
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For ducts and stacks with little or No straight runs
ATMF-HP
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High purity mass flow meter
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¼-4" (6-100mm)
- High purity gases
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40°–150°F (5°–65°C)
- 1% of reading plus 0.5% FS
- 500 PSI (35bar)
- 316SS Internal finish – Typical <=25Ra Max.; Optional <=10Ra Max
- 4-20mA, pulse, MODBUS, HART, Profibus DP & BACNET
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VCR, Butt Weld and Tri-clamp
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For high purity application
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Mainly semiconductor and Chemical plants, etc. clean room environment
Application and Installation Techniques
The primary application for thermal mass flow meters is measuring direct mass flow of gases without the need for pressure or temperature compensation. Another feature unique to thermal mass flowmeters that it can measure from smallest pipe sizes of ¼” (6mm) to unlimited sizes of ducts of several feet or meters in diameter and non-circular ducts. Inline meters, the ATMFIL come with several connection options such flanges, VCR, electro-polished tubes for semiconductor industries. Where insertion styles the ATMFIS come with flanges, NPT threads, flow averaging tubes ATMFAT or multi-point configurations, the ATMFMT. Flow velocities of 4-40,000 fpm (0.02-200 mps) at temperatures of 932oF (500oC) with operating pressures of up to 1000 psig (69 barg) enables the SmartMeasurement ATMF series of gas mass flowmeters to many challenging gas flow applications. Applications such as very low flow velocities in biogas or waste gas measurement to high velocity compressor stations makes the ATMF the gas flowmeter of choice.
Theories of Operation
Regardless of insertion and/or inline style thermal mass flow meters the probe of the meter supports two sensors that extend into the gas flow. The sensors are resistance temperature detectors (RTDs), consisting of durable reference-grade platinum windings protected in a 316 stainless steel sheath. One RTD called the active RTD or flow sensor and the other temperature sensor. , As gas flows past the heated sensor, gas molecules carry heat away from the heated sensor, while the other compensates for the changing in process temperature. The temperature difference between the two sensors is proportional to its mass flow. Since RTDs are proportional to temperature, hence the changes in the resistance of the two RTDs transforms into a change in resistance or voltage. Thermal mass meters work best in dry gas application. Wet gases can also be monitored but has to be factory adjusted and it is important that users let manufactures know if the gas is wet. However, moisture condensation on either of the sensors will cause errors and it is advised that the sensors are periodically cleaned under these conditions.
The most popular thermal mass principal is called constant overheat or variable current which is used by SmartMeasurementTM. One RTD called the active RTD or flow sensor has a circuit heating it while a second detector acts as the reference-sensor to measure the gas temperature. Refer to Figure 2 (next page). As gas flows past the heated sensor, gas molecules carry heat away from the heated sensor. The circuitry maintains a constant overheat between the flow and reference sensor. A change in the mass (molecular) flow rate changes the amount of cooling, resulting in a temperature change, disrupting the circuit balance. Almost instantly, the circuit reestablishes the lost energy by varying the input current to the flow-sensor to obtain the correct overheat temperature. The electrical power used to maintain this overheat determines the mass flow signal.
Some manufacturers use the constant current technique. It also uses two RTDs, sheathed in thermowells which are slightly separated. Heat is applied internally to one RTD relative to the other, creating a differential temperature between the two. The heated sensor (flow sensor) is heated at a certain fixed temperature. As with variable current techniques the other RTD or temperature RTD measures the changes in process temperature This differential temperature is greatest at no flow conditions and decreases as flow increases, cooling the heated RTD. This differential is electronically converted into an electrical signal which can be linearized across a large turndown to provide a wide range air/gas mass flow meter. As flow goes over the sensors, electronic circuitry measures the changes in temperature which transforms into mass flow rate.
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