What is a thermal mass flowmeter?
There are various styles of flowmeters for natural gas measurement, which can be categorized by their operating technologies, such as vortex shedding, Coriolis technology, differential pressure, positive displacement, turbine, ultrasonic or thermal. While there are pros and cons of using each of these types of flow meters, this entry explains, “What is a thermal mass flowmeter?”
A thermal mass flow meter measures gas flow based upon the principle of heat transfer. Heat is introduced into the gas flow stream, and then the rate of how the heat dissipates is measured. For this to work, the meter needs a probe or “flow body” containing two sensors (resistance temperature detectors) which will be in contact with natural gas within the pipe.
One of the sensors is heated and serves as a self-heated flow sensor while the other is a reference sensor and measures the gas temperature. As gas flows by the heated sensor (flow sensor), the flowing gas molecules carry heat away from the sensor, so the flow sensor cools, and energy is lost. The temperature difference between the flow sensor and the reference sensor has changed, and within a second, the circuit heats the flow sensor to replace the energy that has been lost. The mass flow is calculated based upon the energy (electricity) required to restore and maintain a constant overheat temperature.
Diagram courtesy of Sage Metering – gas mass flow meters
Among the advantages of thermal mass flow meters is there are no moving parts or orifices, which eliminates replacing bearings and prevents undetected shifts in accuracy. They also don’t require temperature or pressure corrections and provide good overall accuracy and repeatability over a wide range of flow rates. This style of meter calculates mass flow rather than volume and is one of the few categories of meters that can measure flow in large pipes.
The thermal mass flow meter is cost-effective and accurate, making it an excellent choice for most natural gas mass flow applications.