Measurement of gas flow is a key point in the field of gas application. In recent decades, thermal mass flow (TMF) meters are widely used in measuring the mass of gas. The TMF meters have many advantages such as wide applicable fields it can be applied to many kinds of pipelines and different types of gasses), wide measurement range, and high measurement accuracy and repeatability.
Thermal mass flow meters generally use combinations of heated elements and temperature sensors to measure the difference between static and flowing heat transfer to a fluid and infer its flow with a knowledge of the fluid’s specific heat and density. The fluid temperature is also measured and compensated for. If the density and specific heat characteristics of the fluid are constant, the meter can provide a direct mass flow readout, and does not need any additional pressure temperature compensation over their specified range.
Today, thermal mass flowmeters are used to measure the flow of gasses in a growing range of applications, such as chemical reactions or thermal transfer applications that are difficult for other flow metering technologies. This is because thermal mass flow meters monitor variations in one or more of the thermal characteristics (temperature, thermal conductivity, and/or specific heat) of gaseous media to define the mass flow rate.
In recent years, the gas industry also demands novel techniques to replace the traditional electric heat flow meter to reduce the hidden danger.
The sensor-based on optical fiber, for example, the fiber Bragg grating (FBG), has many advantages over conventional electrical sensors, such as no electrical measurement, anti-electromagnetic interference, intrinsic safety, and high sensitivity. It can easily realize multi-parameter measurement and is very suitable for measuring the temperature and pressure on the occasions with inflammable and explosive gas. For the fiber-based heat flow measurements, the structure design for heat dissipation in a sensor is the challenging aspect. The sensor is based on the principle of convection of heat transfer using a metal sensing head.
When measuring, the sensor head is placed in the gas to be measured, there will be natural convection heat transfer and forced air heat transfer between the sensor head and gas. There are many factors related to heat transfer, the most critical one is the flow velocity of the gas to be measured. If other factors are fixed, the convective heat transfer is a function of the flow velocity. When the sensing head which is covered by a metal is heated inflowing gas, between the metal and the gas, the heat exchange is carried out in three ways: conduction, radiation, and convection.