The mass flow meter is a device that measures the mass of gas flowing through it by measuring pressure differences. There are two types: hot wire and thermal. The accuracy of these meters depends on how well they measure these pressure differences.
Types of Mass Flow Meters
A. Hot Wire
Hotwire mass flow meters are commonly used in the chemical industry for process gas measurement. They can be used in high-temperature environments, such as ovens and furnaces. The meter is a tube with wires set inside at regular intervals. The wires heat up when hot gas flows over them.
The hot wire meter operates on a fuel-free open flame principle. The heat generated by an electric current causes a thin wire (e.g., platinum or nichrome) inside the pipe to become red hot and glow. The gas flowing through the pipe heats the wire, which allows for the mass flow measurement.
wires produce a voltage proportional to the gas velocity that they sense. A time-to-digital converter (TDC) has an electric pulse proportional in length to the duration of the sensed voltage, i.e., its width is proportional to the velocity of the gas flow.
B. Hot Film Mass Flow Meter (also called "Hot Film Air Density")
The hot-film mass flow meter is a modified version of the hot wire type: it uses an electric current to heat a wire (e.g., platinum or nichrome) instead of natural gas. A mass flow meter measuring the temperature difference between a heated film and ambient air is a hot film air density meter.
The utility of this model is that it is essentially independent of pressure changes across the flow line, making it suitable for use in high-pressure air applications.
E. Laval Flowmeter
Laval flowmeters are used to measure high-viscosity liquid flow. It works on the principle of measuring the buoyancy force between a hot and cold moving fluid. This mass flow meter is commonly seen in crude oil refineries, power generating stations, petrochemical plants, and natural gas processing facilities.
The Laval flowmeter sensor consists of a corrugated tube with internal fins coated with a heat-resistant coating. The fluid is continuously heated at the inlet and cooled at the outlet through a cooling coil or air blast. Heat removal from the hot to the cold liquid in parallel plates results in net buoyancy, which varies linearly with flow rate.
Accuracy Type of Mass Flow
Accuracy of Mass Flow Meter (Hot Wire and Thermal)
The most common type of sensor is the hot wire mass flow meter. The accuracy decreases as the operating pressure increases, so it must be used at pressures below 100 psi.
It can also vary with gas temperature and humidity variations and varies with changes in gas composition such as oxygen, CO2 level, and water vapor level.
For accurate measurements, the flowmeter is calibrated with the gas of known composition.
Accuracy of Mass Flow Meter (Vortex and Pitot Tube)
The vortex mass flow meter is exact since its measurement accuracy is independent of pressure and can be used for various process conditions.
However, it can only measure steady gas flows and has no orientation dependence making it unsuitable for bi-directional (multiple input) applications.
The Pitot tube type flow meter consists of a length of tubing having two openings that produce three types of pressure drop measurements:
The pitot tube flowmeters are very reliable and can handle a high flow rate while providing good repeatability. The accuracy of the Pitot tube type flowmeter depends on the measurement error inherent in each measuring element in the system. Thus, it has limitations to measuring gas mass velocity when process conditions are not steady and the flow rate range is small.