In earlier posts, we established a need to measure greenhouse gas emissions. Many flow measurement technologies have inherent disadvantages, including the expense, having moving parts, requiring pressure and temperature measurement, or poor low flow sensitivity. When evaluating thermal dispersion or thermal flow meters, it’s clear that they excel in key areas which would benefit greenhouse gas monitoring:
- The thermal mass flow meter (TMFM) measures mass flow and is not impacted by temperature and pressure changes. Therefore there is no need for separate temperature or pressure transmitters.
- TMFMs offer high accuracy and repeatability.
- TMFMs offer excellent rangeability with over 100:1 and resolution as much as 1000 to 1.
- TMFMs have no moving parts.
- TMFMs are easy to install by insertion into a pipe.
- The SAGE meter offers an easy onsite in-the-pipe calibration verification.
- The TMFM offers a negligible pressure drop; therefore, it doesn’t impede the flow or waste energy.
- TMFMs are ideal for monitoring very low velocities, which is common to GHG monitoring applications.
- Most TMFMs offer analog and digital communication options to interface with emission management systems to track GHG emissions.
Principals of Thermal Mass Flow Meter Operation
Thermal mass flow meters (TMFMs) measure gas flow based upon the principle of convective heat transfer. Either insertion style probes or in-line flow bodies support two sensors that are in contact with the gas. The sensors are resistance temperature detectors (RTDs), and the SAGE METERING (SAGE) sensors consist of highly stable reference-grade precision-matched platinum windings clad in a protective 316 SS sheath for industrial environments.
One sensor is heated by the circuitry and serves as the flow sensor, while a second RTD acts as a reference sensor and measures the gas temperature. The SAGE proprietary sensor drive circuitry maintains a constant overheat between the flow sensor and the reference sensor. As gas flows by the heated sensor (flow sensor), molecules of the flowing gas transport heat away from this sensor, the sensor cools, and energy is lost. The circuit equilibrium is disturbed, and the temperature difference (ΔT) between the heated sensor and the reference sensor has changed. The circuit will replace the lost energy by heating the flow sensor within one second, so the overheat temperature restores.
The electrical power required to maintain this overheat represents the mass flow signal. There is no need for external temperature or pressure devices.
One of the advantages of TMFMs is that they have no moving parts, which reduce maintenance and permit their use in difficult application areas, including saturated gas. They also do not require temperature or pressure corrections and provide good overall accuracy and repeatability over a wide range of flow rates. This meter style calculates mass flow rather than volume and is one of the few categories of meters that can measure flow in large pipes.
Thermal mass flow meters excel in many conditions commonly associated with measuring greenhouse gases. The Sage Metering difference, which is the topic of part IV of this series, reveals why Sage’s meter may very well be the best choice for measuring GHG in many environmental applications.
To read the Sage Metering white paper in its entirety, visit “Greenhouse Gas Emissions Monitoring Using Thermal Mass Flow Meters.”