Instrumentation Pressure Measurement

What is a load cell?

A load cell is a kind of transducer that would convert force into electrical output and this would be easily readable. The load cell can be considered as a force transducer because it can convert the weight or force into an electrical signal. The load cell can be used to measure weight it can be used to measure small things such as a needle to heavy things such as drilling machines. The load cell operates in a way that if a force is exerted in a specified manner then the load cell would create an output that will be proportional to the applied force. Mostly one or more load cell is used to measure the force on a scale.

How to select a load cell?

  • It must be selected according to the size
  • Range of measurement
  • Vibration or signal noise
  • The frequency of the oscillation should be considered
  • Changes in temperature should be considered
  • It must be selected according to the environmental condition like if there is a presence of moisture above the limit could affect this

How does a load cell work?

A load cell consists of strain gauges, the strain gauges are elastic materials and are made up of stainless steel. So if the strain is placed in the load cell it would be stretched due to force or strain and thus the strain gauge movement could occur. The strain gauge would go through physical variation because of the movement in the strain gauge and at the same time, its resistivity will be varied too.

The sensor (strain gauge) in this device would work if there is any compression, so if there is any variation in the length of the wire then there would be a change in frequency. The load cell would only work if there is any change in the strain gauge resistance and this change can be measured with the help of a wheat-stone bridge. So a load cell would do the measurement of the force according to the strain that it receives. There could be a problem where the load is not uniformly distributed because of the eccentric loading, a multi-wire system is used to solve this problem by averaging the load in different wires. Each of the wires would perform as a separate sensor for the measurement of the load at that portion and the average value will be taken to get the actual load.

What are the types of load cells?

Load cells are classified according to their operation

  • Operating by using liquid pressure or air pressure
  • Uses elasticity for its operation
  • The operation could be based on magnetostriction or piezoelectric effect

Types of load cell

Strain gauge load cell

This kind of load cell can be used to measure the level of any storage vessel. So in this load cell if the pressure is applied then its length would change and because of this the conductor resistance would change and we have discussed it earlier. This device will be connected to the wheat-stone bridge and because of this a small change in the resistance will be converted into an electrical signal.

Advantages of strain-gauge load cell

  • It is used in the automotive industry to determine the performance of the materials which is used in, doors, trunks, etc
  • In the process industry, it can be used for material testing
  • It is used in tensile test machines
  • They are not costly

Disadvantages of strain gauge load cell

  • Electrical energy is required for the entire operation
  • It can’t be used for the pressure measurement of highly reactive or corrosive material because it could badly affect the gauge.
  • In order to generate the output, it would need an amplification circuit, due to the fact that the signals which are created by the gauge are very low voltage  

Tension load cell

This type of load cell can also be used to measure the level

This device is composed of a vibrating wire transducer and it would be fixed in a metallic cylinder and it can be used to check the tensile loads in batch systems and also in process weighing systems. So if we apply load in this load cell then the tension of the internal vibrating wire would increase, and due to this the frequency of the wire would change and this change in frequency will be proportional to the applied weight and it can be measured.

Advantages of tension load cell

  • Good stability and sensitivity
  • The measurement is done by checking the varying frequency and because of this it is not affected by the change in the cable resistance
  • The output doesn’t need any amplification because the produced signals don’t have low voltage

Disadvantages of tension load cell

  • It can’t be used to measure high capacities, or for weight measurement
  • The high temperature would affect the wire

Pneumatic load cells

This type of load cell operates by using the force balance principle. The sensing element of this load cell will be introduced to the place where the liquid weight or the pressure is to be measured. In this load cell the force transferring medium would be air, so when the fluid applies force on the sensing part of the load cell then it would transfer this force to the inside air and this force will be transferred to the potentiometer and it is in contact with the Wheatstone bridge. So due to the force that acts on the potentiometer, its resistance would vary and this variation will be proportional to the force.

Advantages of pneumatic load cell

  • It is used in smaller loads where safety is really important
  • Good accuracy
  • This type of load cell is not affected by variation in the temperature
  • Explosion-proof
  • It doesn’t have any fluid in it so it won’t contaminate the process, in case if the diaphragm got ruptured

Disadvantages of pneumatic load cell

  • Not suitable for high-quality measurement
  • Accuracy can be affected at high temperatures

Hydraulic load cell

This type of load cell can be used to measure the magnitude of the applied force and the working principle is the same as the pneumatic load cell, it works by force balanced principle. The major difference between a hydraulic load cell and the pneumatic load cell is the transferring medium. The force transferring medium is air in the case of a pneumatic load cell while in the case of the hydraulic load cell liquid could be transferring medium.

In this type of load cell, there will be a piezoelectric crystal and this would convert the force into an electrical signal and this conversion will be according to the weight or pressure. Initially, the applied force would be affected by the diaphragm and the diaphragm would sense the force and transfer it to the fluid and the fluid will transfer this force to the piezoelectric crystal.

Advantages of hydraulic load cell

  • It can be used to measure the weight of the material in storage tanks
  • Output is linear
  • Good accuracy
  • This device doesn’t have any electrical components so that it can be used in hazardous areas

Disadvantages of the hydraulic load cell

  • The diaphragm in this load cell would limit the force
  • Expensive and complex

Shear load cell

This load cell is suitable to measure the weight or the level, in this load cell, a shear of elastic material is used. It consists of a web and a frame and the web is attached to the frame. This material will be placed in a storage tank and the liquid in the tank would pressurize this elastic material and this stress level will be according to the fluid column load. So by measuring the stress the level or weight of the fluid column can be measured.

Advantages of shear load cell

  • This technology is used in reverse transducers
  • These load cells can be easily sealed and it can be protected from environmental effects
  • It can be used for any type of medium

Disadvantages of shear load cell

  • The sensitivity of this load cell is really low so they are not suitable for small scale measurements
  • These load cells are costly
  • Overloads can badly affect this device

What are the applications of load cells?

  • It is used in the geotechnical field
  • Quality control
  • Test machinery
  • It is used to do the level control in tanks and silos
  • Dams and underground powerhouse
  • It is used to determine the roof convergence in the underground mines
  • It can be used for load testing in piles
  • It can be used to measure the compressive load between the structural members
  • It can be used to detect the load in rock bolts, cables, struts, etc

What are the advantages of load cells?

  • Good reliability
  • They are temperature compensated
  • There are no moving parts
  • It can be used in severe industrial environment
  • The load cell can be combined with any strain gauge bridge measuring readout device

What are the disadvantages of load cells?

  • Overloading could damage it
  • Chemical or moisture ingress could damage it too
  • Electrical surges could be affected

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