Scales and Load Cells
Weighing technology has been a critical part of industrial activity ever since its earliest development. Simple comparison of materials by weight (whether done with a crude balance or a simple manual comparison—one sample in one hand, one in the other) formed the basis for recipes, inventory measurement, basic custody transfer transactions and determination of selling price.
Dick Johnson, Control Engineering
Weighing technology has been a critical part of industrial activity ever since its earliest development. Simple comparison of materials by weight (whether done with a crude balance or a simple manual comparison—one sample in one hand, one in the other) formed the basis for recipes, inventory measurement, basic custody transfer transactions and determination of selling price. And although these methods were probably not very accurate, they represented an understanding of the basic principle of weighing technology. With time, standardized methods for weighing evolved, equipment improved, and accuracy and repeatability in weighing operations became commonplace and expected.
In the latest online survey, Control Engineering magazine—in conjunction with Reed Research—queried readers involved with specifying, recommending and/or buying industrial scales or load cells as to their application and technology preferences as well as purchasing trends.
Scales for industry
According to respondents to the latest survey, 66% used scales or load cells for in-plant requirements. Of the remaining, 14% used them for OEM requirements, and 18% used them for both in-plant and OEM requirements. Primary applications for sales and load cells include both continuous and batch processing (49%), batch processing only (16%), continuous processing only (14%), testing applications (11%) and discrete products manufacturing (9%). Other unspecified operations accounted for approximately 1% of the responses given.
Types of weighing operations that were specified by survey respondents were diverse. These included basic weighing (61%), pre-weighing or filling by weight (48%), calculation by weight (47%), batching (45%), check weighing (inspection by weight) (36%), force measurement (34%), counting by weight for shipping, interdepartmental transfer and inventory transactions (29%) and web tension applications (21%) (See accompanying graphic). Of all these types of systems, respondents overwhelming used electronic devices over mechanical ones by a margin of almost 3-to-1. Hybrid systems did account for at least as many applications as straight mechanical device among those responding.
Making the measurement
When electronic weighing methods are employed, there are a number of technologies that can be used. According to the survey, stain in the load-bearing member of the scale system can be measured in one of three ways. The oldest and perhaps the most familiar to our readers is the strain gage(s). This technology was still the favorite among scale users and accounted for 54% of the responses. Electrical transducers were used 31% of the time. Sensors accounted for the remaining 15%.
No matter what weighing method is used, industrial scales are defined by their load-receiving elements. According to survey respondents, the scales used in their processes or at their facility contained a wide array of load-receiving elements. The most commonly used type is the floor or warehouses scale, with the bench variety coming in a close second. Portable, hopper or tank, conveyor and hanging (suspended pan or hook) were all well represented. Specialty receiving elements were the least represented. Most of these devices represented elements usually associated with very heavy loads such as truck axle, railway car and overhead crane scales.
Average capacities of scales systems in use varied widely. The most commonly used average capacity represented was 500 to 20,000 lb (35%). This was followed by 100 to 500 lb scales (19%), 5 to 100 lb. scales (18%), and 10 to 100 ton scales (13%). Very small- and very high-capacity units accounted for the smallest usage. Less than 5 lb were used by 10% of respondent. Scale capacities greater than 100 tons represented only 5% of the total. No matter what capacity scales were used, what was most important in a user’s weighing operation, as reported by all respondents, was an equal mix of accuracy and repeatability by a margin of approximately 7-to-1 over either accuracy or repeatability alone.
What else is important?
When deciding what manufacturer to choose when purchasing weighing equipment, control engineers listed the following items in decreasing order of importance:
Wide offering of products,
Ease of product documentation,
Sales representative/distributor support,
Factory technical support,
Availability of complete weighing and solutions
Control engineers also had very specific requests when it came to standards that their company required for its scale systems. Although there were eight separate standard systems mentioned, the top four garnered the most votes. These included UL, FM (hazardous), CSA and CE in decreasing order of mentions. The above wish list rings true for one survey respondent. According to this respondent, “I want support from the factory and long time life and accuracy of the product.”
A few quick tips
Among the responses garnered as part of this latest survey, control engineers had some specific advice to give others about the purchase of weighing systems. In regard to purchase of load cells, one response stated that buying them (either initially or for replacement) from the same company made good sense. The biggest problem: confusion caused during installation and/or retrofit because of disparate color coding on wiring. Another respondent had sage advice for those putting in new scales. “Have the units calibrated every week for the first year after purchase; they drift” was the suggestion. Another respondent wrote “Look carefully at temperature sensitivity errors and hysteresis as they can both adversely affect the measurement and must be considered during the design phase.
Scales, load cell products
Control Engineering subscribers, using a list provided, identified the following vendors as leading suppliers of scales and load cells: Mettler-Toledo, Rice Lake Weighing, GSE Scale Systems, Hardy Instruments Inc., Omega Engineering, Satorius Corp., Kistler-Morse Corp., WeighTronics Inc., Cooper Instruments & Systems, Daytronic Corp., Adtech, Fairbanks Morse Corp., Absolute Process Instrumentation, Calex Mfg. Co., Endress+Hauser, Entran Devices, and Global Weighing. Other manufacturers include: ABB, Cardinal, Cole-Parmer, and Siemens (Milltronics). For more manufacturers visit www.controleng.com/buyersguide . For systems integrators, visit www.controleng.com/integrators . Also visit company Websites listed with each product description.
High-performance weighing terminal line added
The IND 780 has been added to its developer’s existing line of weighing terminals. These high-performance devices are said to be capable of simultaneously supporting four scales and can provide a metrologically approved sum scale. The flexibly of the IND 780 enables it to be used as a single or multiple range weighing terminal for use with analog load cells, high precision scale bases or Powercell technology. The unit comes standard with Ethernet, two serial ports and a USB master for an external keyboard. Fieldbus connectivity options offer compatibility to popular networking protocols, including Allen–Bradley RIO, ControlNet, DeviceNet, EtherNet/IP and Profibus with additional enhancements, such as a new template transmission. The terminal directly controls processes with discrete I/O points. Options can be added to control up to 40 inputs and 56 outputs offering a stand-alone terminal solution. The IND 780 features a stainless-steel enclosure and is approved for both Safe Area and Div. 2 or Zone 2/22 Hazardous Area environment with a single terminal design.
Automate formula selection
The 820i HMI Digital Weight Indicator is said to revolutionize basic instrumentation programming, allowing for more affordable serviceability and programming support. Featuring an intuitive interface, the 820i allows users to automate formula selection, batch start up, data recall or any frequently used process with the push of a button. Built-in software functionality includes programmability for alibi tracking, 25 definable print formats and multi-range (interval). The 820i also conveniently stores up to 1,000 truck in/out records for recall at any time and simplifies truck scale data management with six truck modes and keyed tares.
Ready for food, pharmaceutical
The Model 250x washdown indicator is intended for applications in the wet foods processing, viscous food manufacturing, pharmaceutical, and meat processing and packing industries. The device is rated IP69K for high-temperature, high-pressure washdown. Its enclosure is manufactured from food-grade high-impact plastic. The device itself also features a checkweighing ability. The display, which features a six-key capacitive, visual and audible indication, is an LED backlit liquid crystal display with six 0.8-in. digits with eight annunciators. It can be desk-, wall- or column-mounted.
GSE Scale Systems
Advantage load points and sensors are said to provide optimum performance for process-weighing applications. Made of stainless steel, the load sensors feature a no-cost conduit adapter, true hermetic sealing to IP68 for superior protection from moisture, and matched parameters for easy sensor installation without corner adjusting. Located inside the load cells, its developer’s C2 memory chip allows electronic calibration in seconds without test weights. The mounting hardware is designed specifically to direct the load properly to the sensor, while resisting angular effects and reducing movement sensitivity and still permitting thermal expansion and contraction on the weighing system. Each load point is fit with a grounding strap and anti-lift-off protection. The sensors are available for compression or tension applications, in capacities ranging from 44 to 200,000 lb.
High accuracy, S-beam load cells
LCR Series S-beam load cells are said to offer high accuracy and a five-point NIST traceable calibration at a low cost. Available in common ranges from 25 to 10,000 lb, these rugged, industrial S-beam load cells are protected to IP55 and are constructed of lightweight aluminum for ranges up to 1,000 lb and durable stainless steel for ranges greater than 1,000 lb. Their IR 0.25% interchangeability makes them ideal for multiple load cell applications. Each unit is supplied with a 20-ft, four-conductor jacketed cable. Because S-beam load cells are typically used in tension, a tension calibration is provided. Compression calibration is available upon request. Installation is simplified through the uses of optional rod ends or load buttons. Applications include tank level, hoppers and bins, and industrial weighing applications.
Scales target industrial applications
The Midrics Series industrial scales are intended for use in industrial weighing applications such as incoming inspection and shipping to production, resource planning, packaging and more. The basic Midrics 1 version features what is said to be field-proven simple operation for “weigh-only” requirements, while the versatile Midrics 2 version offers built-in application programs, including counting, checkweighing, classification and weighing in percent. Separate modules and made-to-order versions are also available for custom needs. Ten capacity ranges are available from 3 kg to 3 tons, with standard resolution of 15,000 d and Class III resolution for legal metrology of 1 x 3000 e, 2 x 3000 e. Finishes available include a painted finish with an IP65 protection rating for industrial use or a stainless-steel (AISI 304) finish that has a IP67/IP68 protection ratings for industrial use in wet environments.
Low-cost, wash-down load cell
Using its developer’s proprietary semiconductor strain gage technology, the LD3 compression load cell continuously measures the weight of material for in-process and bulk materials storage vessels. The LD3 is suitable for a wide range of applications, from dry, solid material measurement to high-pressure washdown, corrosive or submerged environments. It is available in capacities from 1,000 to 25,000 lb. The device incorporates a self-leveling feature that provides a single-point “trim” while allowing for up to a 3-in. tilt or rotation in all directions. It is smooth and hermetically sealed so that bacteria is not trapped by its design. It is said to require 10% less cleaning than beam or pancake-style load cells.
Stand-alone/process control weighing indicator
Weigh-Tronix Inc. has introduced the latest addition to its Evolution Series indicators with the E1110 Multi-Application Configurable Indicator for stand- alone as well as process control weighing applications. The E1110 is an ideal weighing solution for a wide variety of applications in the mining, aggregate, cement, waste/recycling, transportation and distribution, food processing and chemical/petrochemical industries. The E1110 can store multiple recipes directly in the indicator and perform process control, with no PLC required. Alternatively, it can be easily integrated with existing industrial networks and SCADA systems. The E1110 indicator is ideal for a wide variety of industrial weighing and process control applications, including simple weighing, counting, check weighing, peak hold weighing, in-motion weighing, truck scale operations and fleet management, pack runs and recipes, batches and filling. It has a stainless-steel enclosure rated NEMA 6/4X and an easy-to-read eight-digit, 1.0-in. high yellow LED display. It displays dynamic status information while the operation is running, stores up to 10 recipes, each with up to 50 programmable steps, and offers an alarm mode with 20 independent cutoffs. It is programmable for target count and high/low limits, records weight of each individual product in a package and total weight and detects products entering and leaving a weigh section with optional photo cell.
Dynamic load weighing system
Loadtronics II is a dynamic load-weighing system used by industrial loaders. The system automatically registers important weighing information during product loading, including product type, product destination and amount loaded. The data is saved in the system’s main unit and can be printed in receipt form. The unit can provide totaling information for each day and each vehicle, customer, material and loader. It can store 2,800 transactions (including date and time of each individual weighing) and information on 999 trucks, 999 products and 999 customers. It features a single-cable harness that interconnects all sensors to the on-board display and can be calibrated for up to five loader attachments. Accuracy is
Fairbanks Scales Inc.
Precision weighing sensors
AD-4212A precision weighing sensors are said to be ideal for high-speed filling and check-weighing applications. The extremely quick proprietary Super Hybrid Sensor technology with 0.8-second response time makes this balance ideal for high-speed weighing and filling, checkweighing and remote weighing applications. The compact weighing sensor can be placed anywhere in a production line and allows for a custom-designed weighing pan. The bright vacuum fluorescent display unit is IP54-rated for use in virtually any environment and can be located up to 6 feet from the weighing sensor. RS-232 interface ensures GLP/GMP compliance with data output of weight and calibration data, as well as unit mass information for the counting mode or over/under limit values. Readability ranges from 0.1 mg to 0.001 g and capacities from 110 g to 1,100 g. Standard features include 11 weighing units (g, mg, oz, ozt, ct, mm, dwt, gn, Tael, tola, Messghal) dust and splash-proof protection of keyboard and time-and-date clock function. Each scale includes a Class F calibration weight.
Heavy-duty belt scale
The MBS is a reliable and economical belt scale, which provides dynamic weighing information for process indication. A basic modular, medium-duty belt scale suited for applications involving aggregates, sand, gravel or crushed stone, the device provides dependable continuous weighing at minimal cost. Designed with an accuracy ofen belt loading or high belt speeds. Its load cells are durable and feature overload protection, which is factory set at 150% of load cell-rated capacity. The MBS belt scale does not have a cross-bridge, which reduces material build-up and allows universal fit on most conveyor widths. It can be moved easily from one conveyor to another.
Hydraulic tank, hopper scales
Guardian tank- and hopper-mounted scale systems are designed to keep weighing accurately, even in the harshest conditions and are said to be the most reliable tank and hopper scales on the market. Constructed of 300-series stainless steel, the hydraulic load cells are built to last a lifetime and require no electricity within the scale itself. They also provide extreme-duty protection for long life. Self-checking three- and four-stand assemblies are available with system gross capacities ranging from 3,000 to 300,000 lb. Each weighing system is supplied with a totalizer and 20 ft of copper tubing per load cell. It has immunity to lightning damage, water/ wash-down resistance, transient voltage/RFI and rodent damage protection, and low maintenance cost.
Control Engineering Resource Center has a library of “Product Research” reports on various topics, free with site registration, under Research, Monthly Product Research. /
To read the prior Control Engineering “Product Research” article on this topic, see below.
Product Research: Weighing Technology
Among companies in the research, not covered above, are
Cooper Instruments & Systems www.cooperinstruments.com/tension-links-crane-scales.htm
Daytronic Corp. www.daytronic.com/Products/3K/3kLVDT.htm
Absolute Process Instrumentation www.api-usa.com
Calex Mfg. Co. (load cell signal conditioner) www.calex.com/cis550.html
Endress+Hauser (flowmeter application note: eliminate need for weigh scales)
Entran Devices (now Measurement Specialties) www.meas-spec.com
Global Weighing (now part of Sartorius, listed above).
ABB was among write-in selections. www.abb.com
Dick Johnson is a Control Engineering consulting editor. Reach him at firstname.lastname@example.org .
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