The document provides an in-depth overview of load cells, which are force sensors crucial in various industrial applications, detailing their types, structures, and operations. Key types discussed include hydraulic, pneumatic, and strain gauge load cells, emphasizing their unique components and how they measure force through pressure changes. Additionally, the document highlights the materials used in load cells, focusing on their properties that ensure reliability and accuracy in measurement.

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Load Cell Material
Introduction to Load Cells
A load cell is a force sensor device. Some load cells are electrical transduc-
ers while some are not. A sensor is a device that detects changes in a
physical phenomenon such as force, vibration, motion, heat, pressure,
moisture etc.
Load cells are load (force or weight) measuring devices and are vital com-
ponents of several modern-day instrumentation and industrial processes.
This technology has improved over the past few decades to replace old
weight measurement devices like the Equal-arm balance, steelyard bal-
ance, gradual dial scale, Roberval scales and spring scales.
The advantages load cells have over all these types of weighing devices
listed above has made this possible.
An electrical transducer, however, is a type of sensor device that detects
these changes in the various physical phenomenon and is able to produce
an electrical output signal.
This form of an output signal is analog hence it has to be digitally pro-
cessed to either display the equivalent value of the physical quantity or to
be used in controlling other processes through an actuator.
There are two major types of transducers, which are;
 Active Transducers: The output energy of this device is supplied
entirely by the input signal. In other words, they are self-regenerating
and work on the energy conservation principle. Examples include
thermocouples and piezoelectric accelerometers.

2.  Passive Transducers: These types rely on an external energy
source to supply their output power for their operation. The output
signal of these devices are variations in the material’s resistance, in-
ductance or capacitance. An example is a strain gauge, a force sen-
sor, as we shall see later on in this article.
Types of Load Cells
There are different types of load cells, some of which includes;
 Hydraulic load cells
 Pneumatic load cells
 Strain gauge load cells
 Piezoelectric load cells
 Vibrating wire load cells
 Magneto-elastic load cells
 Dynamic balance load cells
Each of these load cell types has different parts that are made from varie-
ties of materials. However, for each type, its various designs have common
parts of which most are built from the same material.
Detailed Structure and Operation of
Commercial Load Cells
This section shows the detailed structure of some load cell types, their
component parts, functions, the device operations and the material in load
cells.
1. The Hydraulic load cells
These types of load cell measure load through pressure hence they are
pressure based load cells.

3. Figure 1. A simple hydraulic load cell
Component Parts
The structure of a hydraulic load cell consists of the following parts housed
in a cylindrical frame made of stainless steel
 A Diaphragm: This diaphragm membrane is made from an elastic
material. It can also be a corrugated diaphragm which permits a
greater sensitivity and a larger scale of deflection hence they are pre-
ferred to flat ones. The corrugated plate materials include stainless
steel and phosphor.
 The Loading Surface or Platform: This is usually a piston designed
as a loading platform. The elastic diaphragm is attached to the base
of the piston to make contact with the hydraulic fluid. It is usually
made from a thick steel material.
 The Hydraulic Fluid: The load cell is filled with either oil or water.
 A Pressure Output: This might be a bourdon tube pressure gauge
as shown in the figure below or a pressure transducer. The bourdon
tube is made from materials such as phosphor bronze, beryllium
bronze or beryllium copper.

4. Operation of a Hydraulic Load Cell
The hydraulic load cell is a force-balance device. It uses a piston-cylinder
arrangement that uses the hydraulic fluid in the confined cylinder body to
transmit a force to the pressure output when the fluid pressure is raised as
a load or weight is applied on the loading platform.
When the load is placed on the platform, the piston pushes downwards,
hence causing the diaphragm to exert a force over its area on the fluid,
thereby increasing the fluid’s pressure.
Where P is the fluid pressure read on the output, F is the force exerted by
the load or weight and A is the cross-sectional area of the diaphragm.
Hydraulic load cells have to be calibrated before use. Calibration will in-
volve adjusting the reading to a zero pressure value due to the diaphragm’s
pre-load pressure.
The device has a capacity of up to 5MN and it is self-regenerating, requir-
ing no external power other than the energy its input creates. It can, there-
fore, be regarded as a sort of active sensor device (not a transducer as no
electrical output signal is generated). However, a pressure transducer can
be connected to the pressure output, hence producing electrical signals.
Furthermore, its advantages over other types of load cells are that due to
its constituent materials, it can be safely used in explosive atmospheres.
Also, the pressure output can be located several meters from where the ac-
tual device is set up, by improving the design of the fluid tube. The direction
of the force it supports can either be tension (tension load cell) or. Com-
pression (compression load cell).
2. Pneumatic Load Cells
These devices also measure the size of a load through pressure. They are
pressure-based load cells.

5. Figure 2. A simple pneumatic load cell
Component Parts
The structure of this device is similar to that of the hydraulic load cell, how-
ever with some minor differences. The main parts of a pneumatic load cell
are;
 An Elastic Diaphragm: This is made of a flexible material or can
also be a corrugated diaphragm just like that of a hydraulic load cell.
 A Loading Platform: This is where the force is applied or weight to
be quantified is placed. It is usually a steel piston with the diaphragm
attached to its base.
 Air Supply Regulator: This is located at the bottom of the device or
at its side below the diaphragm. It is basically an opening that is reg-
ulated by a valve.
 An Outlet Nozzle: This is also an opening but not directly regulated
by any valve.
 The Flapper or Bleed Valve: This is attached to the body of the dia-
phragm. Its upward/downward movement opens/closes the outlet
nozzle.
 A Pressure Gauge or Manometer: This indicates the pressure as a
measure of the weight or applied force. The pressure gauge is made
from materials such as glass, plastics and basic metals like alumi-
num. Also, alloy materials like brass and steel are used with polycar-
bonate and polypropylene.
Operation of a Pneumatic Load Cell

6. The pneumatic load cell is a force-balance device whose operation relies
on the pressure exerted by the force or weight exerted on a volume of air in
a confined space.
The force is applied to the loading platform to which a diaphragm and a
flapper is attached. This applied load deflects the diaphragm and as a re-
sult, causes the flapper to move downwards and shut off the outlet nozzle.
While this is happening, a back pressure is also acting on the diaphragm
thereby producing a counteractive upward force.
The resultant of the upward and downward force acting over the area in-
side the cylinder causes a deflection in the pressure gauge, hence, giving
the measure of the applied load. To return the diaphragm to its pre-loaded
position, the air supply regulator is turned.
It should be noted that this device also has to be calibrated before use and
it can hold up to 250KN.
3. Strain Gauge Load Cell
It is the most common type of load cell used widely in various industries for
a large variety of load cell applications.

7. Figure 3. A simple single point strain gauge load cell
Component Parts
 The Elastic Element: This makes up a large visible portion of the
load cell’s body. It is part of the device to which the strain gauge is
bonded and it is also called the structural member.
 The Strain Gauge: This is the underlying mechanism if the strain
gauge load cell. It is called the secondary passive transducer as it ac-
tually transforms the strain force on the elastic element to electrical
change in resistance.

8. Figure 4. A strain gauge
The Housing Unit: This unit serves to protect the electrical components
and parts of the load cell from unfavorable environmental factors, espe-
cially in industrial areas. It also allows for external fixings like mounting kits
to be attached to the load cell for proper transfer of load to the elastic ele-
ment.
Operation of Strain Gauge Load Cell
When the weight is applied on the load application point (mounting top) of
the load cell, the strain is then transferred by the elastic elements along its
axis to the strain gauges attached to the beam or structural member (elas-
tic elements).
This causes a deformation in the dimensions (length and cross-sectional
area) of the strain gauges hence creating an electrical signal at the output
terminal of the load cell device when power is supplied to its supply input
terminals. In other words, the strain gauge deforms when weight is placed
on the load cell.
It should be known that the strain gauges are arranged and electrically con-
nected in a Wheatstone bridge circuit configuration. It is the output of this
Wheatstone bridge that is the output of the load cell.
The Wheatstone bridge has four arms and its configuration depends on the
number of strain gauges bonded onto a beam or structural member. A load

9. cell that has a single strain gauge will assume the quarter-bridge Wheat-
stone configuration. A two-strain gauge load cells assume a half-bridge
configuration and a four-strain gauge load cell forms a full bridge configura-
tion.
Fig-
ure 5. Basic setup of a full-bridge configuration
The arrangement of the strain gauge is such that each strain gauge is
placed at alternating arms of the Wheatstone bridge. This is to maximize
the response of the load. Also, temperature compensation is catered for by
using dummy gauges or by designing the strain gauges themselves in a
way that they cancel out the effects of temperature changes in them.
The figure below demonstrates this concept of temperature compensation
by using a dummy gauge

10. Figure 6. Temperature compensation using a dummy gauge
The General Properties of Load Cell
Materials
Each of these materials has properties that make them suitable for use in
designing the load cells. These properties are verified by using various test-
ing machines.
Few of such properties include high strength, low weight, high malleability,
corrosion resistance, good thermal conductivity, good electrical conductiv-
ity, high cryogenic toughness, attractive appearance, high work hardening
rate, easy machining, low vapor pressure, low viscosity, low coefficient of
thermal expansion and low volatility.
SOURCES
 The Essential Guide to Load Cells
 Guide to the Measurement of Force
 Instrumentation Reference Book
 Measurement of Pressure
 Pressure Gauge
 Types of Transducers
 Lec 07(sensors 2)

11. Phone Number: 1-800-550-0280
Contact Email: contact@tacunasystems.com
Website: https://tacunasystems.com/