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Gas Mass Flow Control: Why Thermal Mass Flow Technology is Recommended for Best Accuracy

It’s no secret. Price, but perhaps more importantly, performance, drive every new product acquisition you make. How important is measurement accuracy and control to your lab experiment or industrial process? It’s critical. If your flow readings are off or the flow is not controlled with precise accuracy your research is compromised, product quality suffers, and processes loose inefficiency.

So What Type of Flow Meter Technology Should You Choose for the Highest Accuracy?

Competitive “indirect-type” mass flow measurement technologies—like differential pressure devices that boast speed as the most important factor—can only measure volumetric flow and must calculate mass flow. However, thermal mass flow technology is an industry standard for mass flow control of gases because it measures flow directly, at the molecular level. In essence, counting and controlling every gas molecule flowing through the instrument to achieve unmatched precision.

The molecular mass or weight of the flowing gas is what you really care about to optimize your flow process. Because thermal mass flow controllers measure gas mass flow rate directly, they remain unaffected by temperature or pressure effects. It doesn’t matter how hot or cold the gas is or what pressure fluctuations may be happening upstream. You always get smooth, steady, accurate and extremely reliable gas mass flow rate delivered where you need it, every time.

For over 40 years, Sierra has pioneered the development of precision gas mass flow controllers (MFC). Today, the breadth and depth of our high-performance SmartTrak mass flow controller lineup, with over 100,000 successful installations, proves we’ve never let up. Experience our passion for mass flow control.

Download the User’s Guide to Thermal Mass Flow Controllers

Primary Standard Gas Flow Calibration—The Only Guarantee of Accuracy


Calibration facility

At Sierra, we have a saying: “An instrument’s accuracy is only as good as its calibration.”

The accuracy of your mass flow controller (MFC) is essential in assuring the efficiency, performance, and quality of your flow meter. In many cases, if your instrumentation is not calibrated, then a decline in performance is possible due to sensor drift from the factory calibration. Various things cause drift: dirt buildup, aging of the electronics, physical changes in the sensor, etc.

To make sure your MFC is reading at the accuracy you specified at purchase, many users recalibrate or validate flow meter or MFC  annually. In some industries, assuring your flow meter’s accuracy is required by either corporate policy or government regulations like EPA, FDA, MACT mandates. There are many ways to calibrate MFCs to assure accuracy including transfer standards, but the best way is a primary standard calibration.

Primary Standard = Precision Calibration

Only primary standard gas flow calibration systems deliver world-class levels of accuracy and traceability.

Here is what to look for in mass flow meter primary standard calibration:

  1. Primary standards are characterized by the basic quantities of time and distance, while transfer or secondary standards, such as laminar flow elements, are calibrated against another device, generally another flow meter. Primary standards can also be verified by every national laboratory.
  2. The calibration standard should be NIST traceable standard accuracy, better than 1% of full scale.
  3. The most accurate primary standards adhere to the NIST “rule of four.” This means the gas flow primary standards are required to be four times more accurate than the device under test. This “rule of four” needs to be a requirement for any factory calibration or calibration house.
  4. Flow meter calibration is both a science and an art–look for expertise in flow meter manufacturing or flow meter calibration. In reality, the manufacturing factory not a third party calibration house, will give the very best flow meter calibration due to the flow calibration core competency and working knowledge of the meter.
  5. Facilities should be ISO 9001 certified and/or ISA 17025 & NAVLAP compliant.

Sierra is one of few manufacturers today that performs a detailed 10-point calibration across the entire mass flow range. We strictly adhere to using primary standards and the NIST “rule of four.”  Sierra’s SmartTrak mass flow controller is a prime example of this. SmartTrak’s NIST-traceable standard accuracy is better than 1% of full scale. We offer even better accuracy–as good as half a percent of full scale upon request.

Learn more about utilizing Sierra’s SmartTrak mass flow controller for your next project. Over 100,000 installed SmartTrak mass flow controllers can’t be wrong. And SmartTrak’s unmatched accuracy and performance is guaranteed with a lifetime warranty.

Discover The Swiss Army Knife of Mass Flow Controllers

Direct Mass Flow or Volumetric: The Thermal Flow Meter Technology Advantage!

In today’s world where everyone is watching the bottom line, you need high performance, cost-effective instrumentation—and capillary thermal mass flow meters and controllers have been proven to meet this criteria in a wide range of process applications.

Thermal mass flow technology is an industry standard for mass flow control of gases because it measures flow directly, at the molecular level.  In most processes flowing gases like Air,  Argon, CO2, N2, Etc., it is gas mass, not gas volume, which is the critical variable of most interest.

Volumetric flow measurements are less reliable than mass flow measurements because changes in gas temperature and pressure effect measurement performance.  You lose accuracy and you lose reliability and in many cases even money due to process inefficiency and waste.  Volumetric flow meters need additional temperature and pressure compensation to convert the volumetric flow rate into mass flow rate.

Gas flow measurement and steady control of mass flow rate with capillary thermal technology is the cleanest choice.

No messy calculations or T & P compensation, just pure physics at work for you.

Check out our infographic below to see the thermal mass flow advantage for yourself.

Do You Have the Thermal Mass Flow Advantage?
Capillary Thermal Principal of Operation is the Core Technology for Thermal Mass Flow.
How has capillary thermal mass flow been used? Gas Mixing for laboratory research.
beverage manufacturing
SmartTrak 100 is the Swiss Army knife of MFCs. Watch Video.
Learn More

Need more info? Find out more about Sierra’s gas mass flow meters and controllers with application flexibility and proven performance for lab researcherssystem integrators and more.

The ‘Swiss Army Knife’ of Mass Flow Controllers

swiss-army-controllerDid you ever wish your mass flow controller could do it all? You’re not alone.

End users ranging from machine builders to laboratory researchers are often faced with the challenge of needing flow measurement and control equipment for a wide variety of applications.

Many customers call the SmartTrak 100 mass flow controller the “Swiss Army knife” of MFC’s.

Finding a versatile mass flow controller that can be quickly changed to handle an array of flow measurement challenges can be a daunting task—until now.

What to Look for When Purchasing a Mass Flow Controller

Accurate price comparisons: Did you know 20 standard liters per minute is the typical cut off before you’re forced into a larger, more expensive mass flow controller?

SmartTrak is the only MFC in the industry that controls mass flow rate up to 50 standard liters per minute in a single compact Semi-Spec footprint, just a mere one inch wide by three inches long in size. This means you don’t need to jump to a larger size, more expensive MFC if you are running higher flows. Compare SmartTrak pricing head to head with other MFCs at flow rates from 20 to 50 standard liters per minute, and you’ll find as much as 75% savings.

Compare flow rate spec with meter footprint: Many companies sell mass flow controllers that jump to a much larger body over 50 slpm so the footprint of the meter doubles in size and increases in price to handle the higher flow rate. SmartTrak provides over twice that flow range, controlling flow anywhere from zero to fifty standard liters per minute, in a single, compact, one-inch by three-inch footprint. At no extra charge.

Look at delivery time and extra fees for expedited orders: Sometimes you need equipment in a hurry. How long will it take to receive your flow controller, and what are the fees to expedite your order?  Configure and buy Sierra mass flow controllers at our online store for next-day shipment with no expedited delivery fees. If you need something special, Sierra customizes and provides specialty tuning, often free of charge.

NIST traceable calibration and flexible calibration options:  What calibration options are available, and are they NIST traceable calibration certified? If you prefer to calibrate and maintain your own mass flow controllers, Sierra provides the training and software to do it. If not, we also have certified calibration facilities to service your mass flow meters and controllers.

Ease of adjustment: How easy is it change settings? With a remote or face mounted pilot module display/interface, view and change every aspect of SmartTrak at any time with the push of a button. Simply choose a gas from the menu and it’s ready to go, without losing any accuracy. You can also adjust engineering units, flow range, and set zero, span and full scale independently for each gas.

Accuracy is not only vital in flow measurement and control, but is also vital when deciding which MFC to purchase. Follow the guidelines above to make sure you’re comparing apples to apples and making an informed decision when selecting the best mass flow controller for your high flow rate related application.

How to Supercharge the SmartTrak Mass Flow Controller with Compod

smarttrak-compod_highStreamline, simplify and save time and money by plugging the innovative Compod upgrade into the face of any SmartTrak 100 mass flow meter or controller. SmartTrak is not only a true multi-gas digital mass flow controller, it can control your process too.

If you need more functionality to control a positive shutoff valve, have a pulse output, or read a pressure transmitter, that is no problem with Compod. It is also programmable by the user. Set up simple process control systems driven by SmartTrak without the need for PLCs or computers.

Daisy chain multiple mass flow controllers to network multiple instruments using open-source fully network-enabled multi-drop RS-485 / Modbus RTU. Upgrading your SmartTrak MFC with Compod adds two digital output channels and one analog input channel and a configurable pulse output channel to allow functions of much more complex systems at a fraction of the cost.


Compod acts as a vital link in your more complex process control network. It monitors the operation of instruments and provides potential problem alerts. Send and receive data and even perform data logging and diagnostics that notify users of valve faults or system upsets.

Compod is available with a local LCD display for local monitoring and flow totalization and can be used with new instruments or added to existing models.  Sierra provides free Compod software or you can write your own using open-source MODBUS protocol.

Watch Video “>SmartTrak 100, Master of ALL Flows!

Common applications include:

Compod really puts the “smart” in your SmartTrak mass flow controller. Learn more about Compod and the SmartTrak series.

Complete Guide to Gas Mixing and Blending | Sierra Instruments


Gas mixing sounds simple enough. You simply mix a number of pure gasses to create a new mixture. Unfortunately, it’s not usually as simple as it sounds. There are many variables and factors that play a role. 

In this post, we’ll dissect this challenging application and discuss how to obtain perfect gas mixing and blending with capillary thermal mass flow controller technology.

Why It’s Important

There are many applications and industries that rely on gas mixing and blending. In a laboratory environment, our customers need gas mixtures to test catalysts to see the effect of certain concentrations of pollutants on gasses. The food industry needs a mixture of gasses to prevent oxidation of food. The semiconductor industry needs accurate gas mixing to generate certain atmospheres in their ovens. Hospitals mix gases to create O2 rich air or narcotic gasses. 

There are hundreds of other applications that utilize gas mixing like welding, dilution, chemical reactions, testing of gas analyzers, filling light bulbs, and double glazing windows.  The most common gas blending applications, however, are related to burning and combustion.

Do you remember the basics of combustion? You need fuel, oxygen, and ignition. If the fuel and the oxygen are both gasses, it is ideal to mix them in the perfect ratio to get optimum and clean combustion. 

We can name several applications for this type of gas mixing: Your car, big boilers, small flames used in the glass industry, big power plants, Waste burning plants, etc.

Why Mass Flow Meters?

In gas mixing and blending, direct gas mass flow control provided by a mass flow controller (MFC) not volumetric flow control, is very important. Proper mixing and blending saves money and increases repeatability of the end product and improves quality.

Combustion or burning applications have their own set of challenges. Combustion or burning is a chemical process, a reaction between molecules. To quantify the gas used, then, we should count those gas molecules and not the space between these gas molecules. We do that by measuring the gas mass flow, the weight of the gas (molecules), and not the volume (molecules + space between molecules).

The nice thing about Mass Flow Controllers is that they do just that: They measure and/or control the gas mass flow. They are molecule counters! 

Accuracy in Gas Mixing & Blending

Mass flow controllers are devices that have an accuracy of % of full scale (%FS). That means that the uncertainty of a 1% device is 1% of the maximum flow range of the instrument.

For example, if you purchase a mass flow controller with a range of 0-500 SLPM and the unit has an uncertainty 1% of full scale, the uncertainty is 1% of 500 = 5 SLPM. So, when you control a mass flow rate of 500 SLPM, the flow can be between 495 to 505 SLPM. However, if you control mass flow rate at 10 SLPM using the same 500 SLPM instrument, the uncertainty is still +/-5 SLPM.  As a result, the controlled flow could be anywhere between 5 SLPM up to 15 SLPM. Pretty bad!

The good news is that the newest generation of high-performance Thermal Mass Flow Controllers has drastically improved with the aid of advanced electronics, sensor, and laminar flow element design and linearization mathematics are getting closer to being % of measured value devices. And for a gas mixing and blending application requiring multiple mass flow controllers, the price of the instrument matters.

You’re probably wondering what all this means for gas mixing and blending applications? The lesson learned is that the absolute accuracy is not as good when you operate the mass flow controller at the bottom of the maximum flow range of the instrument, so size your instrument correctly. 

This sizing of mass flow controllers in a gas mixing system is critical. You should be very clear what the purpose and the operational range of each mass flow controller in your gas mix system should be. Ask the manufacturer of the instrument to assist you as they probably have many customers faced with the same challenge.

‘Kwik Kalucations’ with K-Factors

‘Kwik Kalucations’ with K-factors make gas mixing with a single gas mass flow meter simple.

If you’re looking to improve flexibility and save money when it comes to gas mixing and blending applications, consider applying a “K-Factor.” A dimensionless numerical relation of a specific gas vs. air or nitrogen, a gas K-factor never changes. If K-factors will work in your application, you can use one gas mass flow controller calibrated on air or nitrogen for several gasses by simply applying the K-factor.

K-factors, in general, do not change and are used for measurement and control of mass flow rate in units of SCCM and SLPM.  For example, if you calibrate your capillary-sensor gas mass flow controller with air for 0-100 SLPM, but intend to run 100 SLPM of argon through it, you will only get an output of 69 SLPM for the instrument. That’s because the physical properties of argon that affect heat transfer (the standard gas density and specific heat) are different than air.

Furthermore, when you run enough argon through the instrument so that it indicates 100 SLPM, you will actually find the flow of argon is 145 SLPM. Thus, the K-factor of Argon is always 145/100 = 1.45.

Every gas has its own K-factor, but there can be slight differences across various brands of instruments. Is the K-factor a perfect linear function? Well, we assume so, but in reality that is not always true, especially with lower-cost flow sensors and flow meters for gas on the market today. Errors can be up to 7% on very specific light gasses like hydrogen or helium for these lower performing instruments. 

In contrast, the SmartTrak 100 digital mass flow controller is a premium instrument with a high level of reliability with K-factor behavior enabling such technologies as Dial-A-Gas, multi-gas capability. This makes the SmartTrak 100, together with the advanced digital communication possibilities of the Compod, an ideal choice in gas mixing and blending applications.

Dilution in Gas Mixing

Dilution is a special application in the world of gas mixing. Basically, you are diluting a concentration of gas in an already existing gas mixture. Let’s say you have a gas mixture of 10% Ar / 90% N2 and you really need 5 Ar / 95% N2 for your application. The solution is to add more N2 to the mix, but how much? 

To answer the question of ‘how much,’ we first need to know if the gas mix ratio is in % mass or % volume. Let’s assume it is % mass. In that case, we need to generate a new mix of 50% of the original 10% Ar / 90% N2 mix and dilute (or mix in) that with 50% N2. The result is a new mix of 5% Ar / 95% N2 mix.

Most concentrations, however, are much smaller, more in the 0.0001% region. In general, one expresses this in ppm values (ppm = parts per million). This means that 10000 ppm = 1%.

Wouldn’t it be nice to have an automated gas mix system that calculates this all for you? Sierra can provide this for you, contact us to learn more about our SmartTrak® 100 Digital Mass Flow Controllers for Gas Mixing & Blending

Benefits of Capillary Thermal Mass Flow Controllers

  • Direct mass flow with +/- 0.5 percent full-scale accuracy
  • Patented, inherently linear laminar flow element design
  • Mass flow rates up to 1,000 slpm and down to 0 to 0.1 sccm
  • Pressure to 5,000 psig (345 barg) with low pressure drop of 4.5 psid (310 mBard)
  • Provides smooth and flexible valve performance, even at low flows
  • True multigas digital mass flow controller—up to 10 pre-programmed gases
  • 10-point NIST calibration on primary standard


Watch this video to learn more about SmartTrak®: Master of All Flows.

Core Technology Series: Capillary Tube Thermal Mass Flow Meters and Controllers

Understanding Capillary Tube Thermal Mass Flow Meters & Controllers:  

Flow meters and controllers are used every day in general purpose industrial and laboratory applications and in the semiconductor industry.  Have you ever wondered how capillary tube flow meters work?  or How to specify the perfect flow meter for your application? I am excited to present this ongoing core technology series based on excerpts from Sierra’s Founder and Chairman (Also my Father), Dr. John G. Olin’s, white paper entitled, “Capillary Tube Thermal Mass Flow Meters & Controllers- A User’s Guide.”  This “User’s Guide” is designed to educate both flow beginners and experts in the most common types of direct mass flow thermal flow meters, typical applications, principle of operation of capillary tube thermal flow meters, best practices for users, including the selection, installation, and operation of the instruments.   Now let’s start at the beginning.

What is a Capillary Tube Thermal Mass Flow Meter or Controller?

Capillary tube thermal mass flow meters directly measure the mass flow rate of clean gases and gas mixtures in lower flow ranges. A capillary tube thermal mass flow controller adds an integrally mounted flow control valve to the flow body of the mass flow meter and both monitors the mass flow rate and controls it to be equal to a set-point value selected by the user.

History of Capillary Tube Thermal Mass Flow Meter and Controllers

SmartTrak 100 flowmeter for highly accurate gas mass flow control

Capillary tube thermal mass flow meters and controllers were first commercialized in the early 1960’s. The space industry was one of the first users, but before long the industry that fabricated solid state semiconductor devices recognized their usefulness. When integrated circuit semiconductor devices began their long and continuing period of exceptional growth, the market for capillary tube thermal mass flow controllers grew with it. In the 1970’s and 1980’s, general industry recognized the advantages of using capillary tube mass flow meters and controllers in a broad range of applications, and several new companies were formed to serve this growing market. The advantages of accuracy, compactness, reliability, and cost-effectiveness continue to make capillary tube thermal instruments the choice for monitoring and controlling smaller mass flow rates of clean gases in general industry and in the fabrication of semiconductor devices.

The primary virtue, and the source of their prominence, is the fact that capillary tube thermal instruments directly measure mass flow rate, as opposed to, for example, volumetric flow rate. This is important because most industries need to measure and control the flow of the molecules, i.e., the mass, of the gas entering their process.

Types of Direct Mass Flow Meters & Applications

Figure 1: Classifications of Mass Flow Meters

There are two kinds of flow meters that directly measure the mass flow rate of fluids—Coriolis mass flow meters and thermal mass flow meters. Coriolis mass flow meters directly measure the mass flow rate of most fluids, both liquids and gases, and do not require knowledge of the identity, or composition, of the fluid. Thermal mass flow meters directly measure the mass flow rate of gases, and do require knowledge of its composition. Coriolis mass flow meters have high accuracy, high pressure drop, work best with liquids, and are relatively expensive. Thermal mass flow meters have medium to high accuracy, low pressure drops, work best with gases, and are relatively inexpensive.

For the purposes of this blog series we will focus on thermal mass flow meters. Specially, capillary tube flow meters and controllers. Capillary tube thermal mass flow meters and controllers have two broad fields of application: general purpose industrial and laboratory applications and semiconductor manufacturing and other high purity vacuum processes. More MFCs are manufactured than MFMs because most users want to control the mass flow rate of the gas in their process rather than just monitor it. Capillary tube thermal MFCs offer a cost-effective solution for controlling the flow of gases because they are compact, require only one penetration of the process line, and have a built-in optimized control system.

Principle of Operation: How Capillary Tube Mass Flow Meters & Controller Work

Figure 2: Typical general purpose mass flow controller

In the case of the capillary tube type of thermal mass flow meter (MFM) described in this blog, the flow enters the flow body and splits into two internal flow paths. One path flows through a heated capillary sensor tube that has a small diameter and relatively long length. The second parallel path inside the flow body passes through a split-flow bypass consisting of a laminar flow element that shunts the bulk of the flow around the sensor tube. The ratio of the flows through the bypass and the sensor tube is a constant. The capillary sensor tube measures its internal mass flow rate by means of the heat capacity of the gas that carries heat from an upstream resistance-temperature-detector winding to a downstream winding, both  on outside of the sensor tube. The difference in the electrical resistances of the two windings provides the measurement of the mass flow rate through the sensor tube, and thereby the total mass flow rate in the flow conduit.

A capillary tube thermal mass flow controller (MFC) adds an integrally mounted flow control valve to the flow body of the MFM and both monitors the mass flow rate and controls it to be equal to a set-point value selected by the user either remotely or on the MFC itself. To learn more about Capillary Thermal Mass Flow Controller & Meter technology, watch this video that follows a molecule of gas through a typical capillary thermal flow controller.


Next up in the “Capillary Tube Thermal Mass Flow Meters & Controllers- User’s Guide” series, Dr. Olin describes the mechanical components of mass flow meters and controllers and the operation of these components.

Get more information on Capillary Tube Mass Flow Meters and Controllers:

Download Dr. Olin’s Complete White Paper “Capillary Tube Thermal Mass Flow Meters & Controllers- A User’s Guide.”

Read more about the History & Evolution of Mass Flow Controllers

Read Core Technology: Capillary & Immersible


Core Technology Series: Major Components of Mass Flow Meters and Controllers

Have you ever wanted to look inside a mass flow meter or gas controller to understand the mechanical components? This blog, as part of our ongoing core technology series based on excerpts from Sierra’s Founder and Chairman, Dr. John G. Olin’s, white paper entitled, “Capillary Tube Thermal Mass Flow Meters & Controllers – A User’s Guide,”  dives “under the hood” and takes a closer look at the flow body, flow rates & sizing, and how flow conditioning works. 

Under the hood – The Major Components of a MFM & MFC

A mass flow meter (MFM) has five major components: flow body, flow conditioning section, flow sensor tube, bypass, and electronics.  A mass flow controller (MFC) has the same components as an MFM, but also has an integral control valve mounted on the same flow body as the MFM (See Figure 1).

SmartTrak 100 inside view

Figure 1.

The Flow Body

General purpose mass gas flow meters and mass flow controllers (MFC) typically have only three flow body sizes covering the entire flow range of the instruments: low flow, medium flow, and high flow. The gas flow ranges of the three flow body sizes are shown in in the table below. Low flow bodies often are machined out of a single piece of stainless steel bar stock. To get an idea of their relatively compact size, MFCs have the following approximate widths and lengths (not including inlet and outlet fittings): low flow—1.0 W x 3.0 L inches (25 W x 76 L mm); medium flow—1.5 W x 4.5 L inches (38 W x 114 L mm); and high flow—3.0 W x 9.0 L inches (76 W x 229 L  mm). MFMs usually have the same widths but have about two-thirds the length in the medium and high flow sizes.

Flow Body Size Maximum Mass Flow Rate Range (slpm)
Low Flow 0 to 50
Medium Flow 0 to 300
High Flow 0 to 1500

The flow body has inlet and outlet flow conduit fittings and houses the flow conditioning section, the flow sensor tube, the bypass/laminar flow element, and, in the case of MFCs, the control valve. The electronics are mounted in their enclosure on the top of the flow body. The wetted parts of a typical flow body and its internal components are made of corrosion resistant materials. Typical wetted materials for the flow bodies of general purpose MFMs and MFCs are: 316 L stainless steel; ferromagnetic stainless steel in the valve; and “O” rings and valve seats of fluoroelastomers and other advanced elastomeric materials. Some lower cost instruments intended for light duty and lower accuracy applications have flow bodies made of plastic or aluminum.

Instruments with elastomeric seals throughout the flow body have relatively low rates of leakage in and out of the flow body. MFMs and MFCs used in vacuum processes use metal seals at all locations in the flow body to further reduce leak rates. Manufacturers should subject every instrument to a helium leak check using a mass spectrometer leak detector, or equivalent instrument. Additionally, all instruments should comply with applicable pressurized equipment standards and codes, and manufacturers should pressure test all instruments to insure compliance.

Process lines typically are tubes with outside diameters of 1/8, 1/4, 3/8, 1/2, 3/4, and 1 inch (6, 10, 12, and 20 mm). The 1/4 inch (6 mm) tubing size is most common. Some MFMs operated at very high flow rates are available in wafer and flange pipe sizes. Manufacturers offer a broad selection of process tube fittings, including: compression fittings, elastomeric “O”-ring face seal fittings, and metal gasket face seal fittings. Since the inlet and outlet fittings contribute to the pressure drop in the instruments, the size of the fittings should be as large as practicable within constraints imposed by the size of the process line.

Semiconductor MFCs

Semiconductor MFCs often have a particulate filter, pressure regulator, and a positive shut-off valve installed upstream of the instrument and may have a positive shut-off valve and pressure regulator installed downstream. General purpose instruments also may include ancillary flow components in their installation.

Semiconductor MFCs used in the fabrication of high-end semiconductor devices have several special requirements to ensure that: (1) no particulates or other contaminants enter the fabrication process; (2) no toxic process gases escape the MFC; and (3) no ambient air enters the process. Typical specifications are: wetted surfaces must have high purity and be highly polished (surface roughness in the range of about 4 to 10 microinches Ra (0.1 to 0.25 micrometers Ra); leak rates must be extremely low; and internal flow paths, as shown in Fig. 3 (no figure 3 shown here), must have no sharp corners, cavities, or dead spaces where particles can form. Semiconductor MFCs are available in both in-line and down- port configurations. Down-port versions reduce the axial dimensions of the MFC and its ancillary flow components, thereby facilitating the compactness required by manufacturers of semiconductor equipment.

The Flow Conditioning Section of an MFM & MFC

The flow entering the MFM or MFC may have non-uniformities in its flow profile due to upstream disturbances caused by elbows, contractions, expansions, and the inlet fitting. This is particularly true for mass flow rates greater than about 50 slpm in the medium and high flow sizes. The flow conditioning section shown in the image above eliminates these upstream flow non-uniformities and conditions the flow so the sensor tube and bypass are able to create the necessary laminar flow in their passages. Downstream flow non-uniformities have no effect on instrument performance.

In operation, the jet issuing from the inlet fitting in enters the flow conditioning section and impacts a central plate. It then flows radially outward and strikes the cylindrical inner wall of a settling chamber. This tortuous flow pattern effectively erases any non-uniform past history of the flow.  A settling chamber then slows down the flow and allows viscous forces to reduce non-uniformities. The flow profile then becomes uniformed and flattened as the stream lines encounter a flow resistance as they pass through the inlet filter plate or screen that also captures any remaining particulate contaminants. After the inlet filter, the flow profile is further flattened as it passes through a flow nozzle. At this point, the uniformed flow splits into the two paths described earlier: one to the sensor tube and the other to the laminar flow element/bypass.

Low flow instruments with mass flow rates less than about 50 slpm, such as semiconductor MFCs, do not require a flow conditioning section. Because of this and the use of flow conditioners for higher flow rates, capillary tube thermal MFMs and MFCs of all sizes do not require straight lengths of upstream and downstream piping (i.e., tubing) that are required by most other kinds of flow meters.

To see how the flow conditioning section works, watch our video “How Capillary Thermal Mass Flow Controller Technology Works.”

Next up in the “Capillary Tube Thermal Mass Flow Meters & Controllers- User’s Guide” series, Dr. Olin describes the mechanical components of the sensor, bypass, and electronics.

Get more information on Capillary Tube Mass Flow Meters and Controllers:

Download Dr. Olin’s Complete White Paper “Capillary Tube Thermal Mass Flow Meters & Controllers- A User’s Guide.”

Read more about the History & Evolution of Mass Flow Controllers

Read Core Technology: Capillary & Immersible

Dissecting a Mass Flow Meter Spec Sheet

7 Need to Know Terms for Successful Specification of a Mass Flow Meter or Controller

Whether you have been on the job for decades or just starting to purchase or specify a mass flow meter or controller to measure fluid flow in your application, there is key terminology to understand. Here are seven terms you should know to assure you are getting what you need to measure or control flow in your process effectively.

1. Flow Ranges (Mass Flow Rate)

The amount of fluid a mass flow meter or controller is best suited to measure flow for.

Example: the table below shows mass flow ranges of three typical flow body sizes of general purpose mass flow meters (MFM) and mass flow controllers (MFCs) for air at 0°C and 1 atmosphere pressure:

Flow Body Size Maximum Mass Flow Rate Range (slpm)
Low Flow 0 to 50
Medium Flow 0 to 300
High Flow 0 to 1500

NOTE: Most manufacturers make no distinction between mass flow meters and controllers and published specifications may vary from manufacturer to manufacturer.

2. Accuracy
The accuracy statement on any spec sheet indicates how precise the flow meter/controller will measure flow within its flow ranges.

Officially, the American Society of Mechanical Engineers (ASME) defines the “accuracy” of flow meters as: the degree of freedom from error, or the degree of conformity of the indicated value of the instrument to the true value of the measured quantity [1].

An accuracy specification includes errors due to: (a) uncertainty in the flow calibration standard; (b) any non-repeatability of the MFM or MFC under test; (c) disagreement between the curve-fitting function and the actual flow response curve; and (d) inability of the flow calibration facility to deliver a sufficiently constant flow rate.

Typically, the accuracy of most general purpose MFMs and MFCs in measuring gas mass flow rate is 1% of full scale, including linearity, and at flow calibration conditions.

These are some examples of how manufacturers express the accuracy of their MFCs and MFMs in measuring mass flow rate:

  • Percent of full scale: X % of full scale
  • Combination: X % of reading + X % of full scale
  • Divided range: X % of reading (≥ Y % of full scale) and X % of full scale (< Y % of full scale)
  • Calibrated Span (%CS)
  • Upper Range Limit (%URL)

3. Repeatability
The flow meter or controller’s ability to produce the same results/readings multiple times with no change in conditions (same flow rate, same operator/engineer, same laboratory, and short intervals of time, etc.).

Typically, the repeatability of general purpose MFMs and MFCs is +/-0.2% of full scale.  Some manufacturers specify a repeatability of +/-0.2% of reading.

4. Reproducibility
The flowmeter’s ability to produce similar results/readings when the conditions of measurement differ. Example: different operators/engineers, facilities, time intervals, laboratories, etc.

5. Rangeability / Turn Down
Turndown or Rangeability is the range of flow (maximum to minimum flow rate) that a flow meter is able to measure in its specified accuracy.

6. Response Time
The time it takes the flow meter to settle on the new flow rate after a sudden change in flow.

7. Leak Integrity
The amount of gas leakage you can expect from seals on your mass flow meter or mass flow co

General purpose instruments with elastomeric seals have maximum leak-rate specifications ranging from about 1 x 10-9 to 5 x 10-9 atmosphere cubic centimeters per second of helium. Semiconductor MFCs and general purpose metal sealed MFMs and MFCs intended for vacuum processes have lower maximum leak rates of about 1 x 10-11 to 1 x  10-10 atmosphere cubic centimeters per second of helium.

This blog, as part of our ongoing core technology series based on excerpts from Sierra’s Founder and Chairman, Dr. John G. Olin’s, white paper entitled, “Capillary Tube Thermal Mass Flow Meters & Controllers – A User’s Guide,”  dives “under the hood” and takes a closer look at the flow body, flow rates & sizing, and how flow conditioning works. Referenced material from ASME MFC-1M-2003(R2008), “Glossary of Terms Used in The Measurement of Fluid Flow in Pipes,” American Society of Mechanical Engineers, 3 Park Avenue, New York, NY.

Get more information on Capillary Tube Mass Flow Meters and Controllers:

Download Dr. Olin’s Complete White Paper “Capillary Tube Thermal Mass Flow Meters & Controllers- A User’s Guide

View Sierra’s Mass Flow Meter and Controllers.

How to Specify the Best Mass Flow Meter or Controller for Your Application

A question we get a lot from our customers is how to determine which Sierra mass flow meter or mass flow controller is best for their application.  Followed up by: What’s the difference between an economical mass flow controller and a premium mass flow controller. Our new video explains the differences between Sierra’s economical and premium mass flow meters and gas controllers (our SmartTrak 50 and 100 product lines) and offers you a simple checklist you can use to purchase the best mass flow meter or controller for your application.


Differences Between an Economical Premium Mass Flow Meter or Controller

SmartTrak 50 is an economical mass flow meter or controller which offers a single gas flow calibration with no local touchpad or control. Maximum flow range of this device is 200 slpm. It’s ideal for single gas and OEM applications. SmartTrak 100 is the premium mass flow meter for you, if you need to measure/control multiple gases and have the ability to change your application parameters in the field. SmartTrak comes pre-programmed with 10 standard custom gases or mixes, with or without a touchpad display, and has a flow range of 4 sccm-1000 slpm.  The SmartTrak 100 is ideal for applications that need flexibility like universities or laboratories. Add an optional Compod™ to the SmartTrak 100 for gas mixing blending/batch control/ratio control applications.

50/100 table

How to Order A Mass Flow Meter or Controller

Using our SmartTrak 50 (economical version) and 100 (premium) as an example, we’ve created a short checklist that will help you specify the right flow meter or controller for gas applications (with most flow meter manufacturers).

  1. Decide if you want to meter or control the gas flow.
  2. Determine your application conditions.
    • What is your application gas?
    • How many gases does your application have? Decide if your application requires the economical (single gas) or premium meter (single/multiple gases with wider flow range).
    • What is your maximum flow rate?
  3. Determine the required body size of your device which is based on your application flow rate. For SmartTraks:
    • Flow rates up to 50 slpm Air equivalent, choose low flow (L) 50 Series
    • Flow rates up to 200 slpm, choose the medium flow (M) 50 Series controller
    • Higher than 200 slpm (up to 1000 slpm) choose high flow (H) flow device. In this case, SmartTrak 100.
  4. Choose flow body material, either aluminum or stainless steel.
  5. Decide on a display option.
    Digital display or no display.
  6. Choose your inlet/outlet fittings depending on the line size and flow rate.
  7. Decide on an output signal/and set point (controller).
  8. Choose the accessory or power supply needed. For the SmartTrak 50 & 100, you can use our purchasing guide to determine the right one for your meter or watch the cabling guide video.

Have an application you need help with? Contact us today.