Level Measurement

Muhajir Ab. RahimSchool of Mechatronic Engineering

Universiti Malaysia Perlis

Introduction

• Most industrial process use liquids such as water, chemical and fuels.

• These materials are stored in container ready for on-demand use.

• It is very important to know the level and remaining volumes of these materials.

Level Formulas

• We can correlate the value of displacement or height of liquid based on the following formulas.

∆P = ρg∆hSo, ∆h = ∆P/ρg

Cd = Caμd + Ca

So, d = (Cd-Ca)r

r

μd

∆h

dr

Ca = capacitance with no liquid

μ = dielectric constant of the liquid between the plate

r = height of the plates

d = depth or level of the liquid between the plates

Level Sensing Devices

• There are two categories of level sensing devices.

1) Direct sensing

2) Indirect sensing

Direct Level Sensing

Examples of direct level sensing are;

1) Sight glass

2) Float

3) Probes – conductive, capacitive, ultrasound

Conductive probe

Capacitive probe Ultrasound

transmitter receiver

Indirect Level Sensing

• In this method, the property of the liquid such as pressure is sensed to determine the liquid level.

• Example of indirect level sensing techniques are;

1) Bubbler technique- measuring pressure

2) Radiation technique- measuring radiation

3) Resistive tapes- measuring resistance

4) Load cells – measuring weight

5) Paddle wheel- measuring torque

Bubbler technique Radiation technique

Resistive tape technique Paddle wheel technique

transmitter

rece

ive

r

How to Select?

• You need to understand the characteristic of your system.

• For examples, the pressure, liquid temperature, turbulence, corrosiveness, accuracy..

• You also need to consider the limitation of your instrumentation.

Quiz

• What is the depth of a liquid in a container, if the specific weight of the liquid is 82 lb/ft3, the container weight is 45 lb, with 21 inch in diameter? Assume a load cell measures a total weight of 385 lb.