Chapter 6 - Level Measurement and Control

Introduction

This chapter discusses the basic principles of level measurement and control
in industrial process applications. Sensing and controlling product
levels in containers involves a wide range of materials, including liquids,
powders, slurries, and granular bulk. In all level measurement a sensing
device, element, or system interacts with material inside a container.
A wide variety of physical principles are used to measure level. Common
types include sight, pressure, electric, sonic, and radiation—each of which
we discuss here. We will also explore level switches and a level control
application.

Sight-type Instruments

There are three common sight-type level sensors: glass gauges, displacers,
and tape floats. Glass gauges are the most widely used instruments for
measuring the level in a process tank.

Glass Gauges

Two types of level glass gauges are used to measure liquid level: tubular
and flat. The tubular type works in the same way as a manometer, that is,
as the liquid level in a vessel rises or falls the liquid in the glass tube will
also rise or fall. The gauges are made of glass, plastic, or a combination of
the two materials. The material from which the transparent tubes are
made must be able to withstand the pressure in the vessel, and they are
generally limited to 450 psi at 400°F. Figure 6-1 shows two common applications
of tubular sight glasses: an open or vented process vessel and

a pressurized vessel. For the pressurized tank, the upper end of the tube is
connected to the tank. This creates an equilibrium pressure in both ends of
the tube, and the liquid in the tube rises to the same level as the liquid in
the vessel. A calibrated scale is normally mounted next to the sight gauge
to indicate the level in the tank.