Friday January 28 2022
Silos are used to store a range of different products. For effective silo management, it’s essential to be able to measure the amount of material in a silo.
This can be done manually or by radar, laser, ultrasonic, or weight. The exact method chosen depends on a range of different factors.
At its most basic, manual silo measurement includes climbing a ladder to look over the top of the silo to see how much product remains.
As you can imagine, this poses a number of health and safety issues. It is also the least accurate way to measure a silo.
Some silos have windows which enable operators to look inside them, but this can give a false reading if the product is banked up the walls of the silo.
A more sophisticated form of manual measurement is the weight and cable or yo-yo bin level sensor. This consists of a pulley system attached to the inside roof of the silo with a weight on a cable. The weight is lowered down to the surface of the material inside the silo and the distance is used to calculate the amount of product left inside.
Weighing the contents of a silo is one of the most popular and most accurate ways of determining the volume of product that remains inside. It is also cost effective and more accurate than manual methods.
Also called gravimetric level measurement, it involves fitting load cells underneath a silo or attaching strain gauges to the metal legs of the silo.
The load cells measure the weight of the material in the silo, the strain gauges measure tiny movements in the legs brought about by the strain placed on them by the silo and its contents. From this, the weight and therefore the volume of remaining product can be calculated.
For more information on weight-based level measurement, see our post What Does a Strain Gauge Do?
Radar measurement works with the use of electromagnetic waves. Radar devices fitted within the silo send out signals in short pulses and these reflect off the product inside. The electromagnetic waves bounce back to the radar device in the same way a sound echo is created and the device times how long this takes.
This gives a measure of the level of the product in the silo, and therefore the volume remaining.
One drawback of radar measurement, however, is that it can only measure one spot in the silo and must be angled correctly to give an accurate reading.
We have produced a dedicated post about radar silo level measurement here.
Lasers work in a very similar way to radar, but instead of using electromagnetic waves, they use narrow beams of light. A laser device is fitted inside a silo and emits light waves which bounce off the material surface and are reflected back to the laser device.
As with radar, the level of product within the silo is measured by the time it takes for the light waves to get back to the laser receiving sensor.
Also like radar, lasers are only able to measure one point in the silo, so must be set correctly.
To learn more about laser silo level measurement, visit our dedicated article on the topic.
Similar to both radar and laser, ultrasonic measurement involves sending out soundwaves to measure the volume of product in a silo.
Ultrasonic devices use piezo crystals to generate the soundwave which is reflected off the surface of the material in the silo and back to the sensor. Again, the time difference between sending the soundwave and receiving the echo gives an accurate measure of the level of material in the silo.
Whereas ultrasonic silo measurement does accurate results, older devices without a self-cleaning system can be affected by dust build up or condensation.
We have produced a guide to ultrasonic measurement here: Ultrasonic Silo Level Measurement – a Guide
Many raw ingredients like sugar, flour, and grains are stored in silos before going off to be processed and made into food products.
Accurately measuring the level of these can be challenging because, unlike liquids, they don’t settle out to create an even surface. To get the most accurate picture, both continuous and point level measurement should to be applied.
Flour is often stored in large, 20m high silos, from where it can be taken and put into smaller storage vessels depending on its ultimate use.
Measuring flour levels can be challenging due to the fact it is very light and can bank up around the perimeter of the silo. Dust created in the silo can also have an adverse effect on measuring equipment.
It is for this reason flour silos often use point level measurement and weight as the principal ways of determining the volume of material left in the silo. Point level measurements tell the operator when the silo requires refilling and weighing the material provides continuous measure that are unaffected by any interior banking of the product.
However, this is changing. With advancements in technology including self-cleaning ultrasonic sensors, ultrasonic, radar and lasers are becoming more popular for measuring flour silo levels.
Sugar, like flour, has so many uses in the food industry that it is stored in large, 20m, silos. Although not as light as flour, measuring sugar levels is subject to the same issues, particularly banking.
It is for this reason, both flour and sugar are often fluidized – gas is pumped through them to make them behave more like a fluid, reducing banking and enabling lasers, radars, and ultrasonic level measurements to take place.
However, this can cause issues in itself. It can damage the sensor cables and liquidisation creates clouds of material which can reduce the efficacy of the sensors.
Grain, such as malted barley used in the brewing industry, creates a lot of dust which can interfere with ultrasonic sensors, making them a less efficient measurement process for grain silos.
However, radar systems are less affected by dust particles, so provide an accurate and convenient method of grain level measurement.
Many grain silos employ load cells or strain gauges too, either as the primary source of level measurement or as a back up for internal devices.
Due to their agricultural use, grain silos are also often subject to manual measuring such as the weight and cable sensor, or simply by having someone climb to the top of the silo and look inside.
Alongside agriculture and the food sector, silos are most commonly employed in the construction industry to store a range of products needed for building such as cement and lime.
Unsurprisingly, as powders, measuring these products provides similar challenges to those associated with measuring flour, sugar, and cereals – banking and dust particles interfering with measuring device sensors.
Due to the dust created in a cement silo, ultrasonic level measurement is unlikely to be accurate enough and ought to be avoided.
However, it isn’t just dust that creates issues when measuring silo cement. Cement silos are often very large, sometimes more than 100ft high, and this can create accuracy issues too.
Traditionally, as a result, many cement silos were manually monitored with engineers working at height to assess the amount of material left. But with advancements in technology, this is now changing. Radar level measurement systems are now able to accurately measure the tallest of cement silos regardless of dust, moisture, and accounting for uneven surfaces.
As with cement, lime is notorious for creating a build up of dust which makes accurate level measurement challenging. However, it must be continually monitored to prevent overflow as lime is a hazardous product that can damage the skin on contact.
To avoid this, lime silo operators typically use a combination of radar measurement systems for the level measurement within the silo, and point level detection which alerts the operator once the amount of lime in the silo falls below a certain point.
This way, they can keep a close eye on the precise amount of lime in a silo, and also be confident that come refill time, there will be no leakages due to overfilling.
As well as fine powder substances such as flour and lime, silos are an effective storage solution for bulk solids such as coal and pellets.
Although these don’t produce the level of dust that other materials do, they do present their own challenges when it comes to level measure, however.
There are many factors to overcome when measuring coal silo levels. Firstly, the falling stream of coal can lead to false readings with ultrasonic or radar devices. So-called ‘rat holes’ caused by silos being filled and emptied at the same time leads to inaccuracies in readings too, and the huge variation in particular size, from large pieces of coal to dust, and you can see the problems.
Dust too, is an issue for coal, adding to the difficulties already listed.
For this reason, load cells and strain gauges are popular ways of measuring the amount of coal within a silo. However, with advances in technology, laser measurement is now being taken up by the coal industry as it is proving to be successful at overcoming many of the challenges.
As they are uniform in size and don’t give off any dust, plastic pellets are among the least troublesome materials to store in a silo.
As a result, there are a number of methods employed to measure the levels of plastic and wooden pellets. This ranges from simple methods such as the weight and cable sensor through point level measurements to the most sophisticated laser, radar and ultrasonic systems.
Unsurprisingly, storing different types of materials poses different measurement challenges for silo operators.
The most effective measurement method will depend on a range of factors that go beyond just the product type, but take in silo size, location, the number of silos, and more.
To find out which silo level measurement solution is right for your business, get in touch on (01652) 686000 or email info@minsterport.com.
