The industry's most authoritative handbook on flow measurement provides a road map to the field of flow measurement. This best-seller discusses strategies for problem solving and puts the whole array of types of flowmeters at the reader's disposal. The text includes laminar flow elements, critical flowmeters, statistics for measurement, laboratory primary standards, and uncertainty in flow measurement. Emphasis is placed on the importance of accuracy in measurements and ways of ensuring accuracy and avoiding equipment damage through correct forecast of operating conditions, flowmeter selection, installation, calibration, and maintenance. Fundamental considerations such as mixed-phase flow, piping effects, and flow conditioning are examined at length. The problem of attaining a meaningful flow signal through linearization, compensation, and totalization is discussed. Join the thousands of engineers, technicians, managers, and salespeople that have found this reference text an invaluable resource.
TABLE OF CONTENTS Chapter 17 - Thermal Mass Flowmeters and Controllers
There have been numerous applications for the thermal mass flowmeter since
they were first introduced commercially. Early development was spurred by the
space program, which required a small, low-powered mass flowmeter to measure
air flow in space suits. The rise of air pollution concerns demanded accurate, electronic
mass flowmeters and controllers for a new generation of pollution-monitoring
equipment and gas blending apparatus. Unfortunately, this market shrank
when funding for environmental concerns was cut.
Thermal mass flowmeters came of age when the semiconductor manufacturing
industry required gas flowmeters that would accurately measure and control small
flows of all types of clean gases being used in their diffusion process. Additional
applications include leak testing of automotive parts and medical analytical equipment.
There are also numerous special niches such as measuring uranium hexafluoride
in the nuclear industry or gas velocity in flare stacks.
Principles of Operation
The thermal mass flowmeter, as its name implies, depends on the variation of
one or more of the heat characteristics of the fluid as a function of flow. While in
theory such flowmeters will measure liquids, in practice most commercial versions
are limited to measuring gases. There are two types of thermal mass flowmeters:
those that measure the "rate of heat loss to the flow stream" and those that
measure the "temperature rise of the flow stream."
Rate of Heat Loss Flowmeter
This type of flowmeter measures the "rate of heat loss" to the flow stream from
a heated element such as a resistance wire, thermistor, thermocouple, or thin film
sensor (see Figure 17-1). These flowmeters are characterized by the classic King
equation for a hot wire:
 | (17-1) |
| where: | |||
| qt | = | rate of heat loss per unit time | |
| ?T | = | mean temperature elevation of wire | |
| d | = | diameter of wire | |
| k | = | thermal conductivity of the fluid stream | |
| Cv | = | specific heat of the fluid stream at constant volume | |
| ? | = | density of fluid stream | |
 | = | average velocity of the fluid stream |
