Thursday June 9 2022
“This is the crux of it, how can you best measure the true weight in your silo?”, asks Minsterport business manager Peter Widdowson. “Radars can be good in the right setting, but the reality is they can be a lot less accurate.”
Silos are critical to any production process and it is vital operators know exactly how much product is in it any time. If a silo runs dry, production has to stop and the costs associated with that are significant.
So, what is the best way to measure the contents of your silo and why can radar sometimes be a less reliable method?
How do you measure the contents of a silo?
Aside from manually checking the contents of a silo by climbing a ladder and looking inside – which presents a number of serious health and safety issues – there are three main ways of measuring the contents of a silo.
These are weigh cells, which sit underneath a silo and measure the total weight of the silo and its contents, and strain gauges, which are placed on the legs and measure tiny movements brought about by the strain placed on them which can then be used to calculate the weight and volume.
The third option is radar, which sends out short electromagnetic pulses and these reflect off the volume inside, giving you the height of the material and allowing you to then calculate the volume below. Radar can only measure one spot in the silo however and it must be accurately placed to provide reliable readings.
Peter explains: “When weighing, the best choice is always going to be weigh cells. Strain gauges work using the same principal as weigh cells but, the benefit is that they can be retrofitted to a silo. They get an accurate reading of weight by simply measuring the bend in the metal.
“The third option for measuring the contents of a silo is radar, but they cannot measure weight and that can present significant issues. Radars can only measure distance. They measure how far down it is to the material in the silo and that number is then used to calculate the weight in the silo.
“However, the accuracy of radar is dependent on a number of factors, including the density and viscosity of the material, and it can often not be as reliable as the other methods of measuring the contents of a silo.”
What are the challenges of radar measurement in a silo?
Materials in a silo can be unpredictable, particularly with granular products which can be prone to a range of blockages and structural build-ups inside the silo.
These blockages and other issues are more likely if your silo isn’t carefully managed, cleaned and serviced. We wrote about the types of blockages and how you can prevent them in this earlier blog.
It’s these blockages and build-ups that present significant challenges for radar measurement and can fool them into providing false and inaccurate readings.
Peter says: “Radar is only capable of measuring what is in front of it and, in silos, that can be problematic. For example, if you get a build-up of material stuck to the walls of the silo, you could still have 50% of the contents still in the silo but the radar could miss this if it is not in the line of sight. This is because the radar is shooting a beam straight down to a point in the silo and can’t see the material built-up on the sides. Weigh cells and strain gauges would still be able to read that the silo is half full in this situation.”
The image here gives an example of this. In this situation, the radar beam is shooting down the centre of the silo but a substantial amount of material is still in the silo, built up on the sides. In this instance, the radar would register the silo as empty.
A similar issue can be caused by “ratholing” which is where narrow passages form in the silo as the product is fed out at the bottom. Again, this could result in the radar beam shooting down one of these holes and providing an inaccurate reading.
Peter adds: “Another issue with radars is bridging in silos. This is when material creates a ‘bridge’ between the silo walls holding up a small amount of material while everything empties below. With radar, your silo would be showing as full, but the reality would be that you are about to run out of material, halting production.”
When should you use radar measurement in a silo?
Radar does have a place in silo measurement, but only in very specific settings as the other measurement options offer better accuracy and reliability. For Peter, he says radar is always the last option when he’s providing his expert services to customers.
He concludes: “Radars are good in the right settings, but they aren’t always accurate. I would only fit radar sensors in a silo if the option to fit weigh cells or strain gauges wasn’t available.
“For example, one of our customers has radar sensors on their silo for a granular product but we also fitted strain gauges to compare the accuracy as the customer keeps claiming the silo is empty because their radar sensors are telling them there’s nothing in it.
“However, our strain gauges show there is 200 tonnes in the silo and this is confirmed when we fill it up with 300 tonnes of material a day and can see from our online Mysilo platform that 300 tonnes has come out of the silo via the historical graphs.
“The strain gauges can be relied on to show the truth because they are measuring the weight. The radar keeps telling the customer the silo is empty because the product is building up around the sides, while the radar is getting an unobstructed shot to the outlet at the bottom.”