Load cells are transducers that are used to convert work, force or pressure into an electrical signal. Load cells are also called as load transducer or load sensors. The force is measured on the basis of deflection or increase in fluid pressure. There are electronic and pneumatic/hydraulic type load cells:
Electronic load cells:
The electronic load cell uses the physical principle that a force applied to an elastic element produces a measurable deflection. Load cells exist in both compression and tension forms. In the compression type, the measured mass is placed on top of a platform resting on the load cell, which therefore compresses the cell.
In the alternative tension-type, the mass is hung from the load cell, thereby putting the cell into tension.
The elastic elements used are specially shaped and designed as shown below:
Several types of displacement transducers are used to measure the deflection of the elastic elements. Of these, the voltage meter is used more frequently, since it provides the best measurement accuracy, with an inaccuracy figure less than ≤ 0.05% of the full-scale reading that can be obtained.
The design of the gauge should be such that it should ensure a uniform strain distribution over the gauge area with the gauges. Load cells including strain gauges are used to measure masses over a very wide range between 0 and 3000 tonnes.
Advantages:
- low cost
- wide measurement range
- tolerance of dusty and corrosive environments
- remote measurement capability
- tolerance of shock loading and ease of installation
Pneumatic/hydraulic load cells:
Pneumatic and hydraulic load cells translate mass measurement into a pressure measurement task.
Application of force to the loading member increases the fluid pressure which is measured by the pressure transducer.
For the pneumatic load cell, in an application of a mass to the cell, it causes the deflection of a diaphragm which acts as a variable restriction in a nozzle-fin mechanism. The output pressure measured in the cell is approximately proportional to the magnitude of the gravitational force on the applied mass.
For the hydraulic load cell, the gravitational force due to the unknown mass is applied, through a diaphragm, to the oil contained within a closed chamber. The corresponding increase in oil pressure is measured with a suitable pressure transducer. These instruments are designed to measure masses much larger than pneumatic cells, with a load capacity of 500 tons.
Advantages:
- Rugged and compact construction
- No moving parts
- Highly accurate
- Wide range of measurement
