What is a Thermal Mass Flow Meter?
- What is a Thermal Mass Flow Meter? It is a meter that directly measures the gas mass flow based on the principle of conductive and convective heat transfer from a heated sensor.
- All Thermal Mass Flow Meters have probes (Insertion Style) or Flow Bodies (In-Line Style) that support a pair of sensors, which are in contact with the gas.
- The sensors are precision platinum RTDs, which are resistance temperature detectors. They consist of highly stable reference-grade platinum windings. In fact, we use the same material that is used as Platinum Resistance Standards at the NIST.
- The RTDs are clad in a protective 316 SS sheath for industrial environments.
- One of the RTDs [See Diagram below] is self-heated by the circuitry and serves as the flow sensor. The other RTD acts as a reference sensor and measures the gas temperature. Essentially it is used for temperature compensation.
- 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), the molecules of flowing gas carry heat away from this sensor, and the sensor cools down as it loses energy. The circuit equilibrium is disturbed, and momentarily the temperature difference between the heated sensor and the reference sensor has changed. The circuit will automatically (within 1 second) replace this lost energy by heating up the flow sensor so the overheat temperature is restored.
- The current required to maintain this overheat represents the mass flow signal. There is no need for external temperature or pressure devices.
How Thermal Mass Flow Meters Work
Thermal mass flow meters are used almost entirely for gas flow applications. As the name implies, thermal mass flow meters use heat to measure flow. Thermal mass flow meters introduce heat into the flow stream and measure how much heat dissipates using one or more temperature sensors. This method works best with gas mass flow measurement. It is difficult to get a strong signal using thermal mass flow meters in liquids, due to considerations relating to heat absorption.
Constant Temperature Thermal Mass Flow Meters by Sage Metering
While all thermal flow meters use heat to make their flow measurements, there are two different methods for measuring how much heat is dissipated. One method is called the constant temperature differential Thermal Mass Flow Meter. Thermal Mass Flow Meters manufactured by Sage Metering use this method that utilizes two temperature sensors — a heated sensor and another sensor that measures the temperature of the gas. The mass flow rate is computed based on the amount of electrical power, or energy expressed in milliwatts, required to maintain a constant difference in temperature between the HEATED sensor and the passive temperature sensor.
Constant Current Thermal Mass Flow Meters
A second, and less popular concept, because of it’s slow response characteristics, is called a constant current thermal mass flow meter. Thermal mass flow meters using this method also have a heated sensor and another sensor that senses the temperature of the flow stream. The power or energy delivered, to the heated sensor is kept constant. Mass flow is measured as a function of the difference between the temperature of the heated sensor and the temperature of the flow stream. Both methods are based on the principle that higher velocity flows result in a greater cooling effect.
Both types of Thermal Mass Flow Meters measure mass flow based on the measured effects of cooling in the flow stream as heat is taken away from the heated sensor.
Sage Metering is an industry leader in Thermal Mass Flow Meters
Sage Metering has been an industry leader in high-performance Thermal Mass Flow Meters for Gas Flow and has pioneered numerous innovations, including introducing a digital method of driving the sensor (versus the traditional drift-prone Wheatstone Bridge). We are the only Thermal MFM Manufacturer that provides an “In-Situ Calibration Check” that assures that the Flow Meter still retains the original NIST Traceable Calibration.
Sage Meters feature a simple In-Situ calibration check
The ability to provide a convenient In-Situ calibration check is unique to Sage, and it is a direct result of having developed a digital method of driving the sensors (versus the traditional drift-prone Wheatstone bridge). Since the Sage Flow Meter has extraordinary reproducibility, even at a “No Flow” (0 SCFM) condition, it is an easy matter for the customer to validate that this unique data point hasn’t changed since the original NIST Traceable calibration.
Sage Thermal Mass Flow Meters provide the following benefits
This proprietary technology provides additional benefits as well: Enhanced signal stability; improved temperature compensation; greater sensitivity to flow changes; improved resolution; and even the ability to digitally adjust the dynamic operating range of the meter for the specific process conditions the customer has specified.