Load Cells Used for Weighing Applications in Industrial Control

Physical science principles are integrated with technology and engineering to build devices critical to modern high speed, high accuracy system operation in industrial process monitoring and control. 

A load cell measures things ranging in size from very small to extremely large and is an essential component of weighing materials and equipment. Their usefulness applies to many process weighing applications across various industries.

In essence, a load cell is a force measurement device that acts as a transducer, transforming force into a unit of quantifiable electrical output in a predictable manner. While there are many different types of load cells, the strain gauge-based cell is the most used in many industries. These strain gauge cells typically have an accuracy range of 0.03 percent to 0.25 percent. Pneumatically based load cells are appropriate for circumstances demanding intrinsic safety and optimal hygiene. There are also hydraulic load cells that run without the requirement for a power supply for regions without a power grid.  These load cells operate on the same principle: a force acts on the cell, which then computes as a value. The value is processed to get an indicator of weight in engineering units. 

The deformation principle applies to strain gauge cells, where highly minute amounts of deformation, directly related to the stress or strain applied to the cell, are produced as an electrical signal with a value proportional to the load applied to the cell. The operating concept enables the creation of devices capable of providing accurate, precise measurements of a wide range of industrial products. 

Load cells' advantages include their endurance, accuracy, and flexibility to a wide range of applications, all of which contribute to their utility in a wide range of industries and applications. 

Learn more about load cells or process weighing applications from Power Specialties. Contact them at https://powerspecialties.com or (816) 353-6550 for more information.

Weight-based Level Control

Weight-based level instruments sense process level in a vessel by directly measuring the weight of the vessel. If the vessel’s empty weight (tare weight) is known, process weight becomes a simple calculation of total weight minus tare weight. Obviously, weight-based level sensors can measure both liquid and solid materials, and they have the benefit of providing inherently linear mass storage measurement. Load cells (strain gauges bonded to a steel element of precisely known modulus) are typically the primary sensing element of choice for detecting vessel weight. As the vessel’s weight changes, the load cells compress or relax on a microscopic scale, causing the strain gauges inside to change resistance. These small changes in electrical resistance become a direct indication of vessel weight.

The following photograph shows three bins used to store powdered milk, each one supported by pillars equipped with load cells near their bases:

When multiple load cells are used to measure the weight of a storage vessel, the signals from all load cell units must be added together (“summed”) to produce a signal representative of the vessel’s total weight. Simply measuring the weight at one suspension point is insufficient, because one can never be sure the vessel’s weight is distributed equally amongst all the supports.

Weight-based measurements are often employed where the true mass of a quantity must be ascertained, rather than the level. So long as the material’s density is a known constant, the relationship between weight and level for a vessel of constant cross-sectional area will be linear and predictable. Constant density is not always the case, especially for solid materials, and so weight-based inference of vessel level may be problematic.

In applications where batch mass is more important than height (level), weight-based measurement is often the preferred method for portioning batches. You will find weight-based portion measurements used frequently in the food processing industries (e.g. consistently filling bags and boxes with product), and also for custody transfer of certain materials (e.g. coal and metal ore).

One very important caveat for weight-based level instruments is to isolate the vessel from any external mechanical stresses generated by pipes or machinery. The following illustration shows a typical installation for a weight-based measurement system, where all pipes attaching to the vessel do so through flexible couplings, and the weight of the pipes themselves is borne by outside structures through pipe hangers:

Stress relief is very important because any forces acting upon the storage vessel will be interpreted by the load cells as more or less material stored in the vessel. The only way to ensure that the load cell’s measurement is a direct indication of material held inside the vessel is to ensure that no other forces act upon the vessel except the gravitational weight of the material.

A similar concern for weight-based batch measurement is vibration produced by machinery surrounding (or on) the vessel. Vibration is nothing more than oscillatory acceleration, and the acceleration of any mass produces a reaction force (F = ma). Any vessel suspended by weight-sensing elements such as load cells will induce oscillating forces on those load cells if shaken by vibration. This concern in particular makes it quite difficult to install and operate agitators or other rotating machinery on a weighed vessel.

An interesting problem associated with load cell measurement of vessel weight arises if there are ever electric currents traveling through the load cell(s). This is not a normal state of affairs, but it can happen if maintenance workers incorrectly attach arc welding equipment to the support structure of the vessel, or if certain electrical equipment mounted on the vessel such as lights or motors develop ground faults. The electronic amplifier circuits interpreting a load cell’s resistance will detect voltage drops created by such currents, interpreting them as changes in load cell resistance and therefore as changes in material level. Sufficiently large currents may even cause permanent damage to load cells, as is often the case when the currents in question are generated by arc welding equipment.

A variation on this theme is the so-called hydraulic load cell which is a piston-and-cylinder mechanism designed to translate vessel weight directly into hydraulic (liquid) pressure. A normal pressure transmitter then measures the pressure developed by the load cell and reports it as material weight stored in the vessel. Hydraulic load cells completely bypass the electrical problems associated with resistive load cells, but are more difficult to network for the calculation of total weight (using multiple cells to measure the weight of a large vessel).

Power Specialties can assist you with all of your process weighing requirements. Visit their website at https://powerspecialties.com or call (816) 353-6550.



Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Mounting Advice for the BLH Nobel KIS and KIM Load Cells

High-accuracy KIS and KIM load cells have several features that distinguish them from other load cells. They are easy to install and extremely accurate, even when subjected to dynamic process forces and severe environmental conditions. Due to their double cantilever design, they have a floating load point and are not affected by side forces up to 100%, making them excellent choices for applications subject to thermal expansion, vibrations and side force disturbances.

You can find mounting advice on the KIS and KIM load cells on the downloadable PDF on this page.

BLH Nobel KIS Load Cell Mounting Advice from Power Specialties, Inc.

Measuring Exact Resultant Forces In Web Tension

Accuracy and consistency of web tension is critical.

Paper quality is determined by several factors including density and wrinkling. Problems in these areas are common in the paper industry. Uneven density and wrinkling of paper reels occurs if there is inadequate control over the positions and forces that control critical stages of the winding process, i.e. winding and spool transfer from the primary arm to the secondary arm. Because paper is sold by the ton not by the foot, the density is obviously extremely important. Wrinkling causes big problems in newspaper manufacture as paper breaks lead to stoppage of the printing presses.

One typical problem occurs when the nip force is controlled by measuring the pressures in the cylinders. Because the cylinders are mounted some distance away from the spool, where the nip force is actually generated, the friction that arises in the machine reduces the accuracy of the measurement results. As the mechanical components in cylinders become worn, this problem grows and the inaccuracy of measurements increases. If you do not have force control on both sides, and do not have control over the prevailing conditions for each reel, the nip forces will lack repeatability. This increases the risk of varying density and wrinkle formation, both when changing reels, and during winding where paper breaks may also occur. When lowering the spool from the primary arm to the secondary arm, changes may occur in the lowering speed, and misalignment between the reels may also result in varying density, wrinkles, and paper breaks.

 

One solution from BLH Nobel is based on mounting load cells directly at the point of force application, as well as on the position sensors in each cylinder. The load cells are therefore installed in the primary arm and secondary arm, and in the spool clamp. This allows us to measure values in real time, which means that we know the actual forces in the critical transfer from the primary to the secondary arm. Acceleration speed and force can then be adjusted by positioning cylinders that are controlled by means of software.

By adopting load cells, paper reels of the right density, with wrinkle free paper, and less paper breaks are the outcome. In a typical mill with 3 to 4 percent of the jumbo role being rejected due to wrinkling, using load cells reduced the waste to less than 0.5 percent.

For more information, visit Power Specialties at http://www.powerspecialties.com or call (816) 353-6550.

Basics of Load Cells Used in Process Weighing

Load cell with many industrial uses
including the weighing of hoppers,
mixing/blending tanks, and conveyors.
(Courtesy of BLH Nobel)   

In industrial application of process measurement and control, principles of the physical sciences are combined with technology and engineering to create devices essential to modern high speed, high accuracy system operation.

Load cells are the key components applied to weighing materials in modern processing. Load cells are utilized throughout many industries in process weighing operations. In application, a load cell can be adapted for measurement of items from the very small to the very large.

In essence, a load cell is a measurement tool which functions as a transducer, predictably converting force into a unit of measurable electrical output. While many types of load cells are available, the most popular cell in multiple industries is a strain gauge based cell. These strain gauge cells typically function with an accuracy range between 0.03% and 0.25%. Pneumatically based load cells are ideal for situations requiring intrinsic safety and optimal hygiene and, for locations without a power grid, there are even hydraulic load cells, which function without need for a power supply. These different types of load cells follow the same principle of operation: a force acts upon the cell (typically the weight of material or an object) which is then returned as a value. Processing the value yields an indication of weight in engineering units. For strain gauge cells, the principle of deformation applies, where extremely small amounts of deformation, directly related to the stress or strain being applied to the cell, are output as an electrical signal with value proportional to the load applied to the cell. The operating principle allows for development of devices delivering accurate, precise measurements of a wide range of industrial products. Advantages of load cells include their longevity, accuracy, and adaptability to many applications, all of which contribute to their usefulness in so many industries and applications.

For more information on any process weighing application, visit Power Specialties at http://www.powerspecialties.com or call (816) 353-6550.

A Better Load Cell for Weighing Heated, Vibration Prone Process Vessels

Most load cells are designed to handle vertical force and cannot discern errors introduced from side loading and/or torque loading. In real world conditions, though, load cells see much more than vertical loading, and unfortunately, can output erroneous values. While they are excellent for static weighing situations, such as scales, load cells typically can’t handle the rigors of process vessel applications.

A case in point is a chemical manufacturer with an several, existing three cubic meter batching tanks. It was decided the tanks needed modifications to provide more accurate weighing of the individual ingredients. The existing load cells were experiencing errors due to thermal expansion of the vessel, and the resultant side loads from expansion. Additionally there was a problem with vibration in the plant. A better solution was needed, and whatever the solution would be, the customer made it clear the new weighing system must provide system accuracy in the range of ±0.1%.

Load cell performance problems causes.

BLH KIS sensor

An approach to deal mechanically with the thermal cycling while using the same type of load cell was discussed. It involved several mechanical modifications that required significant and costly structural changes.

Another suggestion was to evaluate a unique load cell design that was particularly tolerant against thermal expansion, vibration, and high lateral forces - the BLH Nobel KIS series.  The KIS load cell offered some obvious advantages over rebuilding the tanks supports and frame, namely time and expense. Beyond the short installation time and easy modification, the KIS also offers excellent reliability and accuracy.

BLH Instrument

The customer decided to “take the easy way out” and just replace the old, error-prone load cells with KIS load cells. Installation and start-up was very easy, taking very little time. After installation, the customer was pleasantly surprised by the high accuracy of the new KIS load cells, despite the thermal expansion of the vessel and the inherent vibration.

The video below gives an excellent view inside the KIS design and why its ideal for process vessels.