Point level sensors measure material in a vessel at a specific point. The three types of point level devices are vibrating points, rotating paddles and sensors. It is important to understand where to install a point level device on top fill silos. If you install the device too close to the center or edge of the top of the silo, measurements can be inaccurate. Angle of repose, bridging and fill stream are factors that contribute to the best location for point level devices.
When granular material is dispensed in a vessel, it settles in a pile. Angle of repose is the steepest point of a pile of material. At this angle, the material on the slope face is on the verge of sliding. An angle of repose can range from 0° to 90°. A point level device installed at the center of the top of the vessel, measures the highest point of the pile. This same device at the edge of the top of the vessel, measures the lowest point of the pile. You can avoid inaccuracy such as this by installing point level devices between the center and edge of the top of the silo.
For narrow, top fill silos, avoid installing point level devices on the side of the vessel. In this situation, bridging can occur. Bridging is when material gets stuck at a certain point and is unable to flow. Material can bridge between the device and the side of the vessel. This may cause the point level device to state that the vessel is full when, it’s not. If you are working with longer probes, avoid installing them on the side of the vessel. If you must place the device on the side, rather than the top, try turning the end of the probe down.
You may also consider installing point level devices between the center and edge of the top of the vessel due to the materials fill stream. Fill stream is the flow of material from outside to inside the vessel as it is being filled. You can fill silos from the top or side, depending on its design. Always install point level devices on the opposite side as the fill stream. This is important so material does not hit the device as it is poured in, causing a deceiving measurement.