Load Cell &DP Cell

tobascothwackUrban and Civil

Nov 15, 2013 (3 years and 6 months ago)

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2007
-
CHEM
-
17


Load Cell &DP Cell

Load Cell


A load cell is a transducer that converts a load
acting on it into an analog electrical signal. This
conversion is achieved by the physical deformation
of strain gages which are bonded into the load cell
beam and wired into a Wheatstone bridge
configuration
.


A load cell is a type of transducer and this
transducer is used for converting forces into
electrical signals. For example, changes in pressure
or weight may be converted into signals which can
then be used within an electronic
circuit.



Load Cell Operating Principles


Load cell designs can be distinguished according to
the type of output signal generated (pneumatic,
hydraulic, electric) or according to the way they
detect weight (bending, shear, compression, tension,
etc.)

Types of Load cell

Load cell consist of two basic types


Mechanical Load Cells


Strain Gauge Load Cells


Mechanical Load Cell

This load cell consist of two major
classes


Hydraulic


Pneumatic



Hydraulic Load Cell


It is force
-
balance devices, measuring weight as
a change in pressure of the internal filling fluid.
In a rolling diaphragm type hydraulic load cell, a
load or force acting on a loading head is
transferred to a piston that in turn compresses a
fluid confined within an elastomeric diaphragm
chamber. As force increases, the pressure of the
hydraulic fluid increases
.


This pressure can be locally displayed or
transmitted for remote indication or control.



Hydraulic Load Cell

Pneumatic Load Cell


Pneumatic load cell also operate on the force
-
balance
principle. These devices use multiple dampener
chambers to give higher accuracy than can a hydraulic
device.


The advantages of this type of load cell is that they are
explosion proof and not sensitive to temperature
variations. Additionally, they contain no fluids like
hydraulic type that might contaminate the process if the
diaphragm
destroyed.



Disadvantages of this type is that they are relatively
slow speed of response and the need for clean, dry,
regulated air or
nitrogen.

Pneumatic Load cell

Strain Gauge Load Cells


This converts the load acting on them into electrical
signals. The gauges themselves are bonded onto a
beam or structural member that deforms when the
weight or force is
applied.


When
force or weight is applied, the strain changes
the electrical resistance of the gauges in proportion
to the load
.


Other load cells are fading into obscurity, as strain
gage load cells continue to increase their accuracy
and lower their unit
cost.

Strain Gauge Load cells

Strain gauge load cell further classified
into

different
types
.


Bending beam load cell


Shear beam load cell


Canister load cell


Ring and pancake load cell


Button and washer load cell


Bending Beam Load Cell


Bending beam load cells
need particular care
when mounting to ensure
good load introduction
and to prevent damage
from side or non axial
forces. Fully weld
-
sealed
stainless steel low
capacity beams down to
5kg are now available
offering excellent
solutions to low capacity
weighing in hostile
environments.


Shear Beam
L
oad Cell


Single Ended Shear

Beam:


Provide the ideal solution
for medium capacity
weighing.


Their ability to withstand
100% side load without
problems makes them
particularly suitable for
weighing systems with
mixers and agitators.


However these units
become costly



Shear beam Load Cell


Double
-
ended shear
beam:


For medium to high
capacity applications,
the double ended
shear beam offers a
number of advantages
over other designs.


Canister load cell


Traditional canister
load cells have stood
the test of time and
provide a compact and
cost effective solution
for many high capacity
weighing systems.


Canister load cells can
be fully weld sealed
and stainless steel
construction is now an
industry standard.

Selection of Load cells


Basic type and mode of operation.



The Number of load cells.


Capacity.


Accuracy level related to system requirements.


Method of mounting.


Approval requirements (metrological, safety,
environmental)


Sealing level and construction material.


Cost.


Applications of Load Cell


Tank/ silo


Tank dispensing


Wind tower


Dual tank level controller


Bag filling machine


Differential Pressure Cell



It is a device that measures the differential
pressure between two inputs.


Example:

we have a pressure container where we connect the
H
-
side (high) of the DP
-
cell, L
-
side (low) leave in the
atmosphere (open air). The DP
-
cell will tell us the
pressure in the pressure container relative to
atmosphere pressure. Depending on what class the
DP
-
cell is, it will give you feedback with a current
signal. Normal in Europe is 4
-
20 m A, where 4 is
lowest and 20 is highest.

Types of DP Cells

DP cell has the following types.


Diaphragm capsules DP cell


Pneumatic DP cell


Electronic DP cell


Diaphragm Capsules DP Cell


It consist of 2 circular
metallic diaphragms


Diaphragm capsule act as
sensor


Diaphragms have
concentric corrugations to
optimize their flexibility and
rigidity


Space between backing
plate and diaphragms is
filled with silicone oil

Pneumatic DP cell



In this type of cell
capsule is held between two
flanges
castings

which form
chambers on either side as
high and low pressure sides of
dp cells


Electronic DP Cell


These cells are much
the same as pneumatic
cells


Convert the deflection
of diaphragms into a
change in resistance,
inductance or
capacitance

Applications


Level measurement



Density measurement



interface measurement