Introduction

There are two types of thermal flow meters in industry. Industrial thermal mass flow meters, also known as thermal dispersion or immersible mass flow meters comprise a family of instruments for the measurement of the total mass flow rate of a fluid, primarily gases, flowing through closed conduits. A second type is the capillary-tube type of thermal mass flow meter. Many mass flow controller

Mass Flow Controller

A mass flow controller is a device used to measure and control the flow of fluids and gases. A mass flow controller is designed and calibrated to control a specific type of fluid or gas at a particular range of flow rates...

s (MFC) which combine a mass flow meter, electronics and a valve are based on this design.

Both types measure fluid mass flow rate by means of the heat convected from a heated surface to the flowing fluid. In the case of the thermal dispersion, or immersible, type of flow meter, the heat is transferred to the boundary layer of the fluid flowing over the heated surface. In the case of the capillary-tube type, the heat is transferred to the bulk of the fluid flowing through a small heated capillary tube. The principles of operation of the two types are both thermal in nature, but are so substantially different that two separate standards are required. Additionally, their applications are much different. Thermal dispersion flow meters are commonly used for general industrial gas flow applications in pipes and ducts, whereas capillary types are primarily used for smaller flows of clean gases or liquids in tubes. This type is most widely used for thermal mass flow meters in industry. Nevertheless, the capillary type is not the subject of this discussion.

Thermal Dispersion Mass Flow Meters

The operation of thermal dispersion mass flow meters is attributed to L.V. King who, in 1914, published his famous King’s Law revealing how a heated wire immersed in a fluid

Fluid

In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 flow measures the mass velocity at a point in the flow. King called his instrument a “hot-wire anemometer”. However, it was not until the 1960’s and 1970’s that industrial-grade thermal dispersion mass flow meters finally emerged.

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.

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 flow meters using this method
have two temperature sensors — a heated
sensor and another sensor that measures
the temperature of the gas. Mass flow rate
is computed based on the amount of electrical
power required to maintain a constant
difference in temperature between the
two temperature sensors.

A second, and less popular concept, is called a constant current
method. Thermal mass flow meters using
this method also have a heated sensor and
another one that senses the temperature of
the flow stream. The power 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
measure mass flow based on the
measured effects of cooling in the
flow stream.

Industrial Applications

The primary reason thermal mass flow meters are popular in industrial applications is the way they are designed and built. They feature no moving parts

Moving parts

The moving parts of a machine are those parts of it that move. Machines comprise both moving and fixed parts. The moving parts have controlled and constrained motions....

, nearly unobstructed straight through flow path, require no temperature or pressure corrections and retain accuracy over a wide range of flow rates. Straight pipe runs can be reduced by using dual-plate flow conditioning

Flow conditioning

Flow conditioning ensures that the “real world” environment closely resembles the “laboratory” environment for proper performance of inferential flowmeters like orifice, turbine, coriolis, ultrasonic etc.- Types of Flow :...

elements and installation is very simple with minimal pipe intrusions.