# Video demonstration: Thermal mass flow sensing technology

## If you’re measuring gas flow, mass techniques will simplify your task. This tutorial and video demonstration explains the concepts.

07/22/2010

Measuring gas flow is more fraught with problems traditionally than measuring liquid flow, primarily because gas can be compressed. Changing gas pressure and temperature has a direct effect on its volume, which is why a gas volume measurement has to be qualified with pressure and temperature assumptions. In the U.S., we use standard cubic feet (SCF) and the corresponding metric measurement is normal cubic meters (Nm3). Both have underlying conditions specified. If you understand those specifications, you can convert the volumetric value to other conditions more relevant to your needs, or a mass value. In any case, when dealing with a gas flow, you have to be clear what you’re talking about. For example, an air compressor’s output can be rated in SCFM, or CFM at a specific pressure and temperature such as 100 psi and 70 °F.

Measuring gas flow using a volume-based approach such as a rotameter or variable area flowmeter is tricky if you don’t understand how it works. Such units have a direct reading scale, but it only gives you a correct value under very specific operating conditions. If you are operating at a different pressure for example, you can correct the value, but you have to know or measure all the relevant data. If you need a reading from the device in a situation where the conditions don’t change, most rotameter manufacturers can provide you with a unit scaled specifically for those conditions. However, you can’t use that device for anything else without corrections.

The best way to avoid all this is to use a mass-based approach. A pound of gas is still a pound at any pressure or temperature. For many applications, a Coriolis flowmeter is best for gas, but these are typically expensive. There are alternatives, and one of those is thermal mass technology.

Thermal mass flowmeters are based on the understanding that a given mass of fluid will remove a known amount of heat from a given body. In basic terms, the sensor uses a heating element and at least two temperature sensors. The first measures the temperature of the process fluid stream. The second measures the heating element. There are two approaches for using the measurements: one feeds a specific amount of current into the heating element and calculates flow by measuring how much lower the actual temperature is than it should be for the amount of current. The other heats the element to a specific temperature, and calculates flow by measuring how much current it takes to maintain that temperature. Both compare the heater temperature to the incoming fluid. The current levels and temperature differences provide the data to calculate mass flow.

When designing flowmeters, the technology can be used in two ways. Inline flowmeters wrap the heater around a section of pipe and heat the wall from the outside. These are typically used in smaller designs. Insertion sensors put the heater and temperature sensor on a probe and place it in the gas stream. These are more suitable for larger pipe sizes where a probe won’t cause too much of an internal flow obstruction.

The video demonstration uses an insertion approach, attaching a thermocouple to a 25 W soldering iron. When air moves through the duct, it carries away heat from the soldering iron. Since the wattage of the heating element is fixed, it simply gets cooler as more air moves through the pipe. In a more sophisticated application, the heater temperature would be compared to the incoming gas. The demonstration doesn’t calibrate the reading, however, it does use a simultaneous differential pressure reading to confirm a change in flow.

If you’re evaluating an actual installation, there are some practical considerations. The effects of these vary between manufacturers so check the specifics with your potential suppliers.

Since the design uses a heating element, it operates continuously (or at least with long on periods) at high power levels to keep the temperature stable, so battery powering probably isn’t a practical option.

Probes can be made in many sizes and for many pipe diameters, so they are scalable and can minimize duct obstruction.

Using a probe for measurement means that it may only be reading a small section of the gas stream. The normal practice is to place the sensing point in the center of the pipe and the final flow calculation is based on characteristic velocity profiles for that pipe size. This means that the probe length has to be adjustable or fixed for a specific pipe size. It also means that turbulence has to be minimal, which calls for flow stabilizers or long sections of straight pipe up and downstream.

For large ducts or where turbulence is unavoidable, some probes have multiple sensing points across the full diameter, reading the flow profile to correct for poor gas distribution.

Dirty and corrosive gas streams can leave deposits on critical surfaces and interfere with measurements or damage fragile sensing points.

Consult with your instrumentation suppliers to work though these variables. The end result can be an economical and practical way to measure gas flow without having to work around complex corrections.

—Peter Welander, pwelander@cfemedia.com
Control Engineering

Visit the Control Engineering Process Control Channel

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Pipe fabrication and IIoT; 2017 Product of the Year finalists
The future of electrical safety; Four keys to RPM success; Picking the right weld fume option
A new approach to the Skills Gap; Community colleges may hold the key for manufacturing; 2017 Engineering Leaders Under 40
Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Power system design for high-performance buildings; mitigating arc flash hazards
VFDs improving motion control applications; Powering automation and IIoT wirelessly; Connecting the dots
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

### Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.