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Wednesday March 2 2022

Ultrasonic Silo Level Measurement – a Guide

Mention ultrasonic and the first thing you’re likely to think about is bats. Love ‘em or hate ‘em, those clever winged beasts, the world’s only truly flying mammal, use sound to navigate in a technique called echolocation. Of the more than 900 species of bat worldwide, more than half rely on echolocation to detect and avoid obstacles, forage for food, and find their way to roosts.

What on earth has this to do with silos, silo measurement or anything else silo related, you may well ask!

Well, more than you might think. You see, the echolocation that bats rely on uses ultrasonic sounds – soundwaves in the 20 to 200 kilohertz (kHz) range, mainly inaudible by the human ear – and they send these out almost constantly to create a soundscape of the world around them.

Using this, they can assess how close or far objects are away from them by timing how long the sounds they create take to return. They then use this information to navigate through their environment.

Exactly the same principle can be applied to silo level measurement. Sending ultrasonic soundwaves from the top of the silo down to the target surface and timing how long the signal takes to return can be used to calculate how far away the target surface is, and therefore how much material is left in the silo. This is called time-of-flight measurement.

However, it’s not like we can train bats to measure silo levels now, is it? So, how do we go about using ultrasonic to do this, and what materials are ultrasonic measurement best suited to?

In this article, we take a look – or should that be listen – at all things ultrasonic.

Ultrasonic sensors

Unsurprisingly, ultrasonic silo level measurement is based on ultrasonic sensors that mimic what bats do. They use piezo crystals to generate a mechanical pulse, usually between 23kHz and 40kHz, and direct this towards the target surface. They then time how long the signal takes to return.

Because the speed of sound in air at room temperature is a known constant of 343 metres per second, calculating the distance from the sensor to the target surface is easy. It can be done with the following formula:

Time taken for the signal to return (seconds) X the speed of sound in room temperature (343 metres per second)



= the distances in metres.

From this, the volume of material remaining in the silo can be accurately calculated, enabling silo operators to plan refills and inventory management far better than by manual measurement.

Benefits of ultrasonic sensors

Ultrasonic sensors have a number of advantages over other forms of level measurement systems. The main ones are:

  • They are unaffected by the colour of the objects being measured.

This makes ultrasonic sensors particularly useful for measuring transparent materials and liquids such as water. Because the soundwaves rebound off the surface and are not scattered or refracted as light might be, they do not give false readings when used to measure colourless materials.

  • They can be used to detect materials near and far.

The minimum and maximum ranges for ultrasonic sensors are wide. They are capable of measuring materials a few centimetres away to those up to 20m away. This makes them a very flexible solution for large, industrial silo users.

  • As a mature technology that’s been around for decades, they’re reliable and consistent.

Ultrasonic technology and the sensors based on it are well understood and used in many applications. This means they have been refined to the point where they deliver reliable, consistent results.

  • They offer measurements that are precise enough for the majority of applications.

Although not the absolute most precise sensor available, ultrasonic sensors tend to be accurate to within one per cent which is acceptable for most industrial and manufacturing users. However, they can be tuned to greater levels of precision if required.

  • They tend to be inexpensive.

As ultrasonic silo level sensors contain no rare materials, they tend to be relatively inexpensive. This is particularly useful if used in industrial processes where damage might occur.

  • They are resistant to electronic and acoustic noise

Background electrical or acoustic noise does not impact ultrasonic sensors, meaning they can be used in a large range of environments without the risk of false readings.

  • They can withstand a wide range of temperatures.

Ultrasonic sensors are relatively unaffected by temperature and have been known to work in temperatures as low as -25OC and those as high as +70OC. This makes them very versatile for use in outdoor settings such as in quarries and mines.

That said, ultrasonic sensors work best where the temperature is constant and not changing, as changes to air temperature affect the speed of sound through air, meaning false readings might result.

Drawbacks of ultrasonic sensors

Whereas there are a number of benefits of ultrasonic sensors, they do have some drawbacks. These include:

  • Environmental factors can alter the speed of sound in the air.

Temperature and humidity can alter how fast sound moves through the air, but most ultrasonic sensors a calibrated to the air at room temperature, meaning changes in the ambient environment can produce inaccurate measurements.

  • False readings can be produced by banked or a build-up of material.

As ultrasonic measurement relies on soundwaves rebounding off the target surface, if the material in the silo builds up in a particular area, and the sensor is pointed towards that, it might take a false reading and show there is more material in the silo that there actually is. 

  • They don’t work well in dusty environments.

Because ultrasonic sensors work by emitting sound which then bounces off a target surface, it can be affected by thick dust. Instead of reaching the target surface, the soundwaves bounce off the dust particles and give a false reading as to how much material is in the silo.

  • They are affected by dirt and condensation.

Dirt or condensation on the sensor can also mean they return inaccurate readings, so sensors must be cleaned on a regular basis. However, self-cleaning sensors are now available, meaning this is no longer the problem it once was.

  • They don’t work in a vacuum.

Although this is not something that should impact a silo operator, as ultrasonic measurement relies on soundwaves, it doesn’t work in a vacuum.

Uses of ultrasonic level measurements

Ultrasonic sensors are effective for measuring a range of materials including rock, gravel, grain and other food ingredients, liquids, and some powders.

However, when being used to measure powders, particle size and the propensity to produce dust must be taken into consideration.

Another factor that needs considering is the conditions within the silo itself such as the presence of obstacles and moistures levels.

However, if these are at acceptable levels not to interfere with the ultrasonic sensor or signal, ultrasonic silo level measurement is an accurate, reliable, and relatively inexpensive technology.

Remote silo monitoring

Another advantage of ultrasonic sensors is they are fully compatible with remote silo monitoring software.

Our remote monitoring platform, MySilo, brings significant efficiencies to businesses by putting silo operators in complete control of their silo levels, making for better forecasting, fewer deliveries, and fewer vehicle movements on site.

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

For advice on whether ultrasonic sensors are right for your business, or any other type of silo level measurement system, or for more information on MySilo, get in touch.

Want to learn more about the different types of sensors used to measure silo levels? See our complete guide How to Measure Silo Levels by Product Type which provides a great overview.