Introduction

There are broadly two concepts of thermal flowmeter now available for gas mass flow measurement, one of which is also applicable to liquids. I shall follow the useful terminology in ISO 14511:2001 for the two types and encourage the reader to make use of the ISO document. The first is the capillary thermal mass flowmeter (CTMF), which has broad applications in the control of low flows of clean gases, but which can also be used with a bypass containing a laminar element to allow higher flow rates to be measured. The arrangement of heaters and coils between the various manufacturers differs, but the basic approach is the same, with heat added to the flowing stream and a temperature imbalance being used to obtain the flow rate.

The second is the full-bore thermal mass flowmeter (ITMF), which is available as both insertion probe and in-line type. It has a widely used counterpart in the hot-wire anemometer for measurement of local flow velocity, but, as the need for a gas mass flowmeter has become evident, it has been developed as a robust insertion probe for industrial usage and then as the sensing element in a spool piece flowmeter. It has been produced by an increasing number of manufacturers in recent years as a solution to the need for such a mass flowmeter.

However, the advent of the MEMS (micro-electro-mechanical sensors) or CMOS (complementary metal-oxide semiconductor) designs has opened a new set of flowmeters based on silicon chip technology. Their operation is similar to the CTMF, but usually on a smaller scale.

Capillary Thermal Mass Flowmeter – Gases

In various examples of the CTMF, the gas flows through a very small diameter tube that has heating and temperature-measuring sensors. For larger flows, this is used with a bypass laminar flow element in the main gas stream. In many cases, the mass flowmeter is in one unit with the control electronics and the gas control valve. However, in this chapter our interest is in the meter itself. These meters are designed for clean dry gases.

Description of Operation

In this design of thermal flowmeter, the gas flows through a very small tube (possibly with ID in the range of 0.2 to 0.9 mm), with a sufficient length to diameter ratio to ensure fully developed laminar flow, and on the outside of which there are heating and temperature-sensing windings.