Level systems: How accurate are they?

by Emily Wilson
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It’s the end of the month and you take your bulk material inventory. After you report to accounting, you get a call. Where did you get the extra material? Or worse, where did your material go?

The receiving tickets and production utilization don’t match with the materials on hand. You send three different operators at different times up on the deck to verify the level measuring system, and now have more confusion. The readings conflict with one another, which conflict with the level measuring system. Additionally, none of the readings match the production draw.

So who is right? Why don’t the weights match?

When trying to account for material in a vessel, many people mistakenly try to measure by the level of the material. If trying to verify the weight, there are numerous problems that can occur within the conversions that must take place from level to weight.

First, there is a known amount of material delivered by a rail car, determined by a certified rail scale. The material is brought into the storage silos and eventually emptied into a certified weigh hopper that batches out in 300-pound increments for the production process. There is some material loss from the connection leakages and dust collection system, but nothing to the extent of what accounting is trying to reconcile.

The automated level measurement system then becomes a target. To see why it might be the culprit, let us review how you arrived at a weight value using the level measurement system.

There are two methods to determine the weight of material in a bulk vessel:

•Find the location of the top surface of the material with a level measurement device and convert it into a calculated weight, or

•Directly weigh the material using a sensor attached to, or placed under, the vessel supports.

All weight indicators take a physical measurement from a sensor (capacitance, strain, time) and convert it into a weight value. Ultrasonic technology, pulsed radar and Time Domain Reflectometry (TDR) measure how much time it takes the signal to reach the material and return to the sensor. Capacitance systems measure the difference in the dielectric constants of air and the material. Mechanical bobbers count rotations of the cable feeding mechanism.

Any level measuring system locates a point on a material surface. The location of that point across the surface greatly affects the value computed for weight. This measurement must be converted to weight using several steps.

First, time is converted to distance. Then distance is converted into volume. Finally, volume is multiplied by the material density to approximate weight. Each of these conversion steps introduces an opportunity for errors.

All level measuring systems for bulk solids assume that the material is flat and horizontal. In the real world, all bulk solids have an angle of repose. This angle is produced when the material fills into or empties from a vessel.

The method of filling a vessel can produce a wide variety of shapes to the material surface. Off-center fill points, a moving tripper car, and angled or multiple fill pipes all produce multiple cones and peaks. A very large vessel with multiple draw points produces multiple funnels in the material surface. Some of these shapes can be extremely irregular with multiple high and low points.

In some cases, the level on one side of a vessel can be 30% to 50% different in height than the other side (see illustration). Vessels designed for mass flow usually produce an almost level surface when emptying but possess a cone shape on filling. This non-level material surface can produce a significant error in the conversion to weight reading.

The conversion from distance to volume is one of the biggest sources of error. The claim that a level system can make a direct conversion from a level to weight reading is misleading. A level system must first make a conversion to volume using the vessel height and cross section dimensions.

All linear volume conversions assume the vessel shape is a cylinder, yet most bulk vessels do not have flat bottoms. Non-linear conversions of distance to weight must account for the variation in the vessel and particularly the shape at the bottom. Cones, square, pant-leg, and internal mass flow devices can produce significant level-to-volume conversion errors when the level is low in the vessel.

Using the wrong bulk density when converting from volume to weight is another major source of error.

Considering all the opportunities for errors, the best method of determining the weight of material in a vessel is to weigh it. Often a level technology is mistakenly used to determine weight. If you must account for weight or process by weight, you should measure by weight.

Many vessels with level systems have the support structure to allow for a weighing system to be installed with such equipment. When using a weighing system there is only a single conversion needed and much higher accuracies achieved with these instruments. The volume/material density conditions inside the vessel are irrelevant. After all, weight is weight and always will be.

This article was provided by Kistler-Morse Corp., Bothell, Washington, U.S.