INTRODUCTION

The measurement of mass flow, a fundamental requirement for any fluid, has been an elusive goal due to the problems of developing a suitable flowmeter. Despite this, the availability of mass flowmeters has increased greatly over the last 15 years. This is partly due to the increasing value of products (Hall 1990), but it is also due to an increasing realization that volumetric flow measurement is often inappropriate. In addition, the advent of the Coriolis flowmeter has stimulated engineers to find other mass flowmeters. General reviews of mass flowmeters were given by Sproston et al. (1987), Betts (1990), and Medlock and Furness (1990).

Mass flow measurement is commonly categorized as direct (true) or indirect (inferential). However, it may be useful to allow a few more than two categories.

1. a. True (direct) mass flow measurement by a single instrument is rare. It appears that to achieve it we need to use one of the fundamental acceleration laws. We can do this by creating:

• the force (or torque) resulting in a linear (or angular) acceleration (Chapter 16 describes an example) or

• the force that produces Coriolis acceleration (Chapter 17).

1. b. Fluid-dependent thermal mass flow measurement uses the temperature rise resulting from heat addition but is affected by other parameters such as the specific heat of the fluid (Chapter 15).

2. c. Multiple differential pressure flowmeters used in a dedicated system (Section 14.2) depend on the nonlinearity of the flowmeter equation.