Material testing can sometimes be challenging. Once I was material testing a scale in a plant that converted municipal sludge to fertilizer pellets. The entire plant was in a large warehouse type building and the smell was similar to what you would probably imagine. After the first day of working on the scale, on my way back to my hotel, I stopped at a grocery store to pick up a few things. As I’m walking through the store, I realize I am getting some very odd looks. The smell from the plant was in my clothes, but because I was around it all day I could not smell it. Once I realized this, I left the store and immediately went to the hotel to clean up. Although some materials may be more pleasant (and less embarrassing) to test than others, material testing is an important aspect in validating a conveyor belt scale’s accuracy.
A material test is the only way to verify that a conveyor belt scale is accurate, and it is recommended that all conveyor belt scales be material tested. Material testing consists of weighing a sample of material on a known accurate, static scale either before or after it has crossed the conveyor belt scale. Finding a way to capture a material sample or introduce one in the process can take a bit of creativity. Sometimes rotating a screw conveyor or chute can give you enough room to put a small hopper under the discharge. Another common way of material testing is to empty the bin before the scale and partially filling it with pre-weighed material. There have been very few cases where I have not been able to come up with some way to perform a material test, but sometimes the difficulty in weighing a material sample is not practical for the increase in accuracy that will be achieved. If the scale being installed in a system is still in the design stages, consider adding a diverter gate that will allow you to divert the material into a small hopper that can be weighed.
Prior to starting a material test the belt scale should be calibrated and the static scale used to weigh the material sample should be checked to verify its operation. Verifying the operation of the reference scale is a step that is often overlooked when performing a material test, but I have spent many hours looking for a problem with the conveyor scale when the error was in the reference scale.
To correctly material test a conveyor belt scale you should run three weighed material samples. The first two tests are used to determine a repeatable error, if any. Each material sample should be run across the conveyor belt scale continuously at a rate over 50% of the scale’s rated capacity. You may find after the first two tests that the scale has a small repeatable error. If that is the case, the conveyor belt scale can be adjusted and a third test should be done to verify the correction. If there is an error in the scale that is not repeatable, the source of the non-repeatability must be found and corrected.
How large should the material sample be?
Generally speaking, you want the material sample to be as large as possible. Most of the time, the container used to transport the material between the two scales is the limiting factor. Because of this, perhaps the more appropriate question is, “What is the smallest material sample that can be used for a valid material test?” Desired accuracy, resolution of the test scale and the conveyor scale, and condition of the belt are factors that should be considered when determining the minimum material sample size.
The first consideration is the resolution of the conveyor scale and test scale. The desired accuracy must represent at least 2 totalizer counts. For example, let’s say you have a scale that totalizes in tenths of a ton (0.1 tons per count) and you try to test it with a 1 ton material sample. If the scale is just about to increment to the next count (1.1 tons), the totalizer will read 1 ton when you actually have 1.0999 tons. In this case, the scale will appear to be very accurate, but in fact your error is almost 10%. So, for a 0.5% scale I recommend at least 400 totalizer counts and for a 0.25% scale I recommend at least 800 totalizer counts. The resolution of the test scale should be at least double that of the conveyor scale.
The size of the sample is a function of the rate you will be running for the test and the length of time the test lasts. Ideally, the material test will be performed at the rate you will be running under normal conditions. If that is not possible, you want to make sure you are well above the turn down of the scale. I typically recommend a minimum of 50% of the scale’s rated capacity.
One last consideration is minimizing the affects of variations in belt weigh and the error caused by the ramp up and down of material. The National Institute of Weights and Measures recommends a material test for a certified scale be at least 10 minutes, although they do list a few exceptions. This can sometimes be difficult because of the amount of material required to do a test that large. I find in most cases a minimum of 5 minutes or 3 belt revolutions of continuously loaded belt is enough to provide an accurate material sample.
Alternative methods of verifying the scale’s operation
Although material testing is the only way to truly verify a conveyor belt scale’s accuracy, there are other methods people have tried. One of the most common is using “belt cuts”. This is where the belt loading, in lbs/ft, are noted just as the conveyor is stopped. A length of material is removed from the belt and weighed. Varying belt tension, belt weights, and material loading, as well as, the insufficient size of the material sample can induce large errors in the accuracy of this type of test. This method of testing will often produce errors of 3% or more. Because of this, it is not considered a valid method of material testing a conveyor belt scale. Another common method of testing conveyor scales in blending applications to chemically test the blend. Using chemical analysis may yield results close enough to provide proper blending when a repeatable error is found. However, if any of the other scales in the blending process are not weighing accurately, these errors will be passed on to the scale being tested. Because of this, we do not recommend this method to test your conveyor belt scale.