Introduction Series from Paul Chase
This is the first in a series of articles discussing beltweighers (belt-conveyor scales in the United States). This introductions identifies some of the items intended for discussion as the series progresses. Future sections could also include discussion of questions that may be posed by web-site users.
In the late 1960s the author gave a talk to some industrial users and stated (to the horror of belt weigher manufacturers attending) “If you want to weigh accurately on a conveyor scale, don’t.” The statement is only partially facetious because belt weighers have error sources that are not normally present in static weighing. The primary problem is that the belt weigher can only be calibrated by using another scale, not a mass standard, so the accuracy can never be as good as the reference scale. A variety of simulated tests have been devised over time, but none will provide traceability except the collection and weighing of the material that passes over the belt weigher. The author has participated in improving belt weigher accuracy over the years, both from a design and regulatory standpoint, but the basic traceability problem remains.
The need for accurate weights for commerce dates from antiquity. The Bible references weights and measures in several places.
See for example:
Proverbs 11:1 (NIV) The LORD abhors dishonest scales, but accurate weights are his delight.
Today the need for accurate weighing is combined with the need to accomplish the weighing in a shorter time to aid the flow of commerce. The belt weigher (or belt-conveyor scale) is a way to weigh bulk materials without interrupting the flow of material in the process. Bulk materials are bought, sold, and shipped in large quantities. Coal is typically shipped in rail cars containing 100-110 tons each. Loading a train needs to be completed in as short a time as is feasible and a length of 100 cars is typical. Other applications include ship loading. A belt-conveyor scale can conveniently handle the weighing task and several are in use at rates from 4000 to 5500 tons per hour.
Although the term “weight” is used rather casually to include both force and mass, legal metrology really deals with mass. The “weight” of a standard mass varies slightly from place to place because the acceleration of gravity varies and therefore the “weight” or force also varies slightly. Although that variable will not be considered in this book, most of the systems described herein are really force measuring systems. For systems performing weighing in motion, the possible error induced from changes in the acceleration of gravity are small compared with the other error producing influences.
To narrow the scope even further, the weighing machines discussed use a load cell as the basic weight (mass) sensor. In most cases these are strain gauge load cells and have accuracies as described in OIML (Organisation Internationale de Metrologie Legale) document R60 and NIST (National Institute of Standards and Technology) Handbook 44.
Historically at least one weigh-in motion device, Merrick Model E Weightometer® was actually a mass balance device. It is described later in the series. This venerable device was a good belt conveyor scale and with proper maintenance they remained in use for many years.
This series is intended to discuss belt conveyor scale systems as defined in NIST Handbook 44 and in OIML R50. There are other devices that may use a conveyor in the weighing process such as weigh-price labelers and check weighers (catch weighers in OIML R51). These devices have many similarities to belt conveyor scales, but for the sake of brevity and simplicity they are not included here. In-motion railway track scales have similarities to a belt conveyor scale but are not included here.
Many belt conveyor scales are used in applications which do not need to meet the requirements of HB44 or OIML R50. These scales are often used in process situations where a measurement is useful even at reduced accuracy. The principles discussed in the following chapters apply to these lower accuracy scales as well as to certified scales. Some of the error-producing effects have a greater influence on the lower accuracy scales than on certified scales..
Several future sections are planned. The first part of the series, Sections 1 through 4, contains a description of belt scale weighing on a largely theoretical basis. To emphasize the relationship of the belt conveyor scale to its installed and operating conditions, these early sections disregard the effects of the electronics and any errors resulting from the electronics. Considerations of the electronics are included in later sections. Section 1 discusses the weigh platform and load receiving element. Section 2 adds motion to the weighing process. Section 3 discusses the design of the load receiving element as it applies specifically to weighing in motion; this is often called the scale carriage. Sections 1-4 do not include any reference to the effects on weighing of the conveyor belt itself.
Sections 5 and 6 discuss some of the factors that can produce errors. These two sections do not include the effects of the electronics, but only the effects of the belt and the environment.
Section 7 discusses some types of electronics currently in use to show the inter-relationship with the weighing process. The characteristics of the electronics, including the load cells, is an important background for the discussion of approvals and regulatory agency requirements as discussed in Section 8.
Section 8 discusses the approval process in general. In most countries the approval will be in accord with OIML R50, in the United States the approval is in accord with NIST HB44. In addition to these sections discussion from website readers is encouraged and will be included at appropriate times in the discussion.