What is the coverage area of a sensor?

Imagen de cereal dentro de un silo

In this article we’d like to contrast one of the great myths that exist around temperature sensors for stored cereal: their coverage area or radius of action.

In the world of manufacturers of temperature control system, one can frequently find brochures or reviews indicating that sensors cover or have a radius of action of about 2 or 3 meters approximately, with some indicating more and others a bit less.

But which of these statements is correct?

The answer is as simple as it is revealing: none.

Why?

The thermal conductivity of seeds and grains – their ability to transfer or spread heat – is very low, and their behaviour is very different from liquids or other materials such as metal. Therefore, it cannot be scientifically affirmed that there is a percentage of the silo covered by the sensors beyond the cereal or grain that is in direct contact with the sensor itself.

Any statement indicating that a percentage of the silo is covered by its sensors cannot be scientifically confirmed.

So how can the area of coverage in a silo be improved to help detecting a hotspot as soon as possible?

Without a doubt, introducing more sensors inside the silo.

Can one predict where a hotspot will occur? 

Unfortunately, it is not possible to predict where a hotspot will occur inside the silo or warehouse. It could occur near or far from a sensor. As a matter of fact, all sensors available commercially measure the temperature of the grain that is in direct contact with the sensor and cannot measure the grain temperature 3 meters away.

GESCASER sensors are not different from other sensors in this regard and, therefore, when monitoring the grain conditions, it is safe to assume that the temperature in the readings correspond to the temperature of the cereal that is in direct contact with the sensor.

When there is a hotspot somewhere in the silo, the heat is slowly transmitted to other cold areas. The time it takes from the hotspot until it is detected by the sensor depends on may factors (type of cereal, amount of cereal, impurities, degree of compaction, etc.) 

Indeed, the thermal conductivity of the grain is very low. To get a better understanding, the material with the best thermal conductivity is silver, with a coefficient of about 418 W/mK, while cereals such as corn, soybeans or sunflower seeds vary between 0.3 W/mK and 0.1W/mK depending on its humidity. With increased moisture levels, more conductivity (W/mK).

Comparative table of the thermal conductivity of some materials in W/mK
Comparative table of the thermal conductivity of some materials in W/mK

How many probes and sensors do I need?

At GESCASER, manufacturers of temperature control systems for silos with more than 40 years of experience, we provide recommendations with regards to the number of probes and sensors taking into account the type of silo or warehouse and its technical characteristics (diameter, height …).

Based on this information we determine the optimal number of probes and sensors (see graph).

Esquema de las sondas en función del diámetro del silo
Sondas de GESCASER
Our objective is to help finding the right balance between an absolute control of the silo temperature and the economic cost. It is no surprise that with more sensors, one can achieve greater control, but of course at an increased price.

At GESCASER we control the entire manufacturing process: from the receipt of raw materials, to the probe manufacturing, the shipment and the subsequent installation. Thus, probes and sensors can be totally customised to our customers’ needs. 

We hope this article helped resolving some doubts about the sensors and their radius of action. 

Don’t get confused!