Load Cell &DP Cell 2007-CHEM-17
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.
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.
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 Load 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