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Thursday November 11 2021

What Does a Strain Gauge Do?

What is a strain gauge?

When pressure is applied to a stationary object, two forces are at play – stress and strain. Stress is the resisting force of the object which can be considered ‘push back’, and strain is the extent to which the object is displaced or deformed by the force.

Strain can be measured accurately by a strain gauge, a small electrical sensor that alters its resistance in relation to the strain placed on an object. Strain gauges are very sensitive and can detect tiny expansions or changes of shape of the object they’re attached to and convert this into an accurate measure of the strain applied.

Strain gauges have been used to measure strain, pressure, force, weight and tension and are extensively used in silo measurement and management. By measuring the strain exerted on the silo legs, the amount of material in the silo can be accurately calculated.

What can strain gauges be used for?

Strain gauges have a large amount of uses, many of which are vital to safeguarding the public when travelling or using civil infrastructure such as bridges, roads, and lifts.

Here is just a small list of the things strain gauges are regularly employed to measure:

  • Stress on railway lines  
  • Stresses on aircraft wings
  • Aircraft component testing 
  • Rotational strain on turbines, fans, propellers, and motors
  • Testing ships hulls
  • Testing structural components for bridges and buildings 
  • Automotive testing
  • Automotive, marine, and aerospace design
  • Building and architecture, including bridge and tunnel construction
  • Industrial engineering and weighing

However, as important and interesting as these uses are, we are of course most interested in the use of strain gauges in silo measurement and management.

Fitted to the support structure of a silo, strain gauges can be used to continuously monitor the levels of liquids, powders and solids in the silo, ensuring silo owners and operators don’t run out of product, which can lead to costly breaks in production.

How strain gauges work

Strain gauges work by converting the strain force created when a silo is filled up into an electrical signal by use of a transducer.

When mechanical strain is applied to the transducer, the electrical resistance of the wires it is made up of alters, and this can then be used to determine the weight of material in the silo.

The gauge detects strain in the form of movement of the structure it is attached to. The structure moves in relation to the strain that is exerting on it, either by being compressed or otherwise bending out of shape. Often these movements are tiny, meaning the electrical signal created by the strain gauge transducer must be amplified before it can be used to calculate how much material is in the silo.

Given that the measurement is based on tiny movements in structure, it is essential sensors are located in a place that minimises any impact of a change in temperature, as this can cause certain materials, particularly metal, to expand and contract independently from the strain placed on the legs by the silo bin.

Types of strain gauge

There are five main types of strain gauges. These include:

  • Hydraulic
  • Electrical resistance
  • Mechanical
  • Optical
  • Piezoelectric

Whereas they all have their pros and cons, not all of them are appropriate for measuring silos. The two most commonly used types of strain gauge for silo measurement are electrical resistance and hydraulic strain gauges.

Electrical resistance

These are the most common type of silo strain gauge and as the name suggests, they measure electrical resistance. Electrical resistance strain gauges consist of a thin foil with multiple maze-liking wiring attached. The wiring then goes on to connect to two electrical cables which connect to the monitoring software.

The foil strip is secured to the frame of the silo and when the silo is full, the support system deforms under the strain, bending the electrical foil. This either pulls the maze-like wiring apart or pushes it together, altering the resistance through the wiring. Pulling it apart increases electrical resistance as the wires get thinner, making it harder for the electrons to carry the current down the wire. Pushing the wires together and increasing their width has the opposite effect.

The level of resistance can then be used to determine the amount of material in the silo.

 

Hydraulic

One of the difficulties with using strain gauges to measure the amount of material in a silo is that the level of deformation in the support structure can be tiny. Converting this into a readable signal, therefore, is a challenge.

However, hydraulic strain gauges offer a solution to this. Hydraulic strain gauges use a piston system to amplify the data collected, making it easier to read.

This is done by having a large piston in the strain gauge sensor, and when strain is exerted on the silo, the movement in the support structure acts on the large piston. Using hydraulic pressure, this then acts on a smaller piston, creating movement on a scale that accurately measures the amount of material in the silo.

Mechanical

Mechanical strain gauges are the most crude type of strain gauge and are used to measure strain in concrete structures. They generally measure length in millimetres and are used to assess things such as the development of a crack in a building so relevant interventions can be plotted.

As a result, mechanical strain gauges tend not to be used in measuring the contents of silos as they are not sensitive enough.

Optical strain gauge

Optical strain gauges use fibre optic technology to measure strain. They do this by detecting changes in light transmission when the objects they are attached to experiences a load.

Optical strain gauges are extremely small, sometimes the width of a single human hair, which makes them very useful for use in tight places.

They are used extensively in structures such as bridges, buildings, tunnels and pipelines as they have the ability to measure strain in thousands of locations over several metres.

However, due to the dusty conditions inside silos, particularly those used for storing powders, optical strain gauges can struggle to give accurate readings so are therefore rarely used in silo management.

Piezoelectric strain gauges

Piezoelectric strain gauges are made up of slices of monocrystalline crystals that generate an electric charge when strain is placed upon them. Normally, two slices are employed, each sitting on either side of an electrode.

When pressure is placed upon the sensor, the crystals generate a charge, and this is absorbed by the electrode and is transmitted to a charge amplifier. This can then be used to workout the amount of material in the silo.

Piezoelectric strain gauges tend to be slightly less accurate than other types of strain gauge, but they are very resistant to temperature so can be used in situations of extreme heat or cold and still reliably function.

How are strain gauges attached to silos?

New silos tend to be built with measurement devices incorporated into the design. However, strain gauges can be retrofitted to any silo to give owners and operators an accurate picture of how much material is in it.

The first thing to decide is where the strain gauge is to be fitted. Generally, that is one of two places – the bottom of the silo or on the support structure, i.e. the legs.

Which of these you choose is usually be dictated by issues such as ease of access as opposed to one are giving better results than the other. The gauge must be positioned in a place that facilitates access but also where it is least likely to get damaged by the day-to-day operations of the business.

The advantages of bolt-on strain gauges are that they are quick to install and require no structural intervention on site. They can also be installed while the plant is operational, so don’t cause a halt in production, and they have no moving parts so are maintenance free.

Generally, bolt-on strain gauges are secured directly on to the silo support structure with high tensile bolts.

Remote monitoring

If you want to go one step beyond just fitting strain gauges, remote silo monitoring enables silo operators see how much material is in one or multiple silos online to prevent overfill and better track when they need to be refilled.

Our system, MySilo, is compatible with all available strain gauges.

MySilo brings significant efficiencies to a business by enabling silo operators and business managers to be in complete control of their silo levels, making for better forecasting, less deliveries, and less vehicle movements on site.

Silo operators, other relevant staff and even suppliers can sign into the online portal to check levels and ensure there is enough material to keep all projects ongoing and it can also be set to send customisable notifications by text or email and online when re-ordering needs to take place.

As well as preventing dangerous overfills and the resulting over-pressurisation, MySilo enables companies to safely record data and measure silo contents without having to check it manually. It also allows for silos with windows to be tracked accurately with the sensors in case of blockages near the glass, showing a false representation of the contents amount.

The inventory data recorded by the sensors enables operators and managers to make more informed and accurate decisions, as they are able to plan the projects easily through real time monitoring.

To find out more about MySilo, get in touch.