Engineering Handbook for Free / Load Cell Cable / Load Cell accuracy ¦ METTLER TOLEDO
Know How

Engineering Handbook for Tank Scales and Customized Scales

Know How

Comprehensive System Design with Best in Class Load Cell Technology

Our Engineering Handbook for weighing offers more than 150 fact-filled pages with a wide range of information on applications with to help you create an ideal weighing and inventory control system.

 


This comprehensive library of information on tank-weighing and customized scale design will help those in engineering, operations or integration.

Weigh Module Systems is the most comprehensive handbook on how to design tank-based weighing systems available in the industrial market today. If you work in engineering, operations, or integration and desire to learn more about the applications and use of tank-based storage and weighing systems, then this free, downloadable English-language resource is for you.

This comprehensive guide also contains a chemical resistance resource chart that compares more than 30 types of steel, composites and finishes with more than 400 chemicals, solvents and materials to help ensure tank-scale system safety and longevity. More than 10 additional useful appendices combine to make your tank weigh-system design process faster and easier than ever.

This includes critical information such as:

  • Assessing your tank, platform or conveyor system performance needs and setting specific performance criteria.
  • Engineering calculations that account for the effects of wind, seismic, shock, thermal effects and load cell cables.
  • Creating tank piping-connection designs in the context with support structure and foundation.
  • Qualification guidelines for structural, foundation, electrical and mechanical considerations to ensure best accuracy.
  • A complete set of engineering specifications for compression load cell, tension load cell, static load cell, and dynamic load cell applications.
  • How calibration, verification and service help to ensure long-term tank scale performance and system uptime.
  • Types of load cell technology available for weighing systems, and why certain load cell technology types are better than others.
  • How does the load cell cable length influence the reading of a tank scale.
  • Common specifications such as load cell accuracy, and load cell material types
  • Tools such as a load cell accuracy calculation and a load cell capacity calculation

This comprehensive library of information on tank-weighing and customized scale design will help those in engineering, operations or integration.

Weigh Module Systems is the most comprehensive handbook on how to design tank-based weighing systems available in the industrial market today. If you work in engineering, operations, or integration and desire to learn more about the applications and use of tank-based storage and weighing systems, then this free, downloadable English-language resource is for you.

This comprehensive guide also contains a chemical resistance resource chart that compares more than 30 types of steel, composites and finishes with more than 400 chemicals, solvents and materials to help ensure tank-scale system safety and longevity. More than 10 additional useful appendices combine to make your tank weigh-system design process faster and easier than ever.

Weigh Module and Load Cell Technology Helps to Improves Overall Efficiency

 

One of the load cell technologies featured in the handbook is METTLER TOLEDO’s  PowerMount™ weigh modules and POWERCELL® load cell technology. This load cell technology provides predictive maintenance features to improve Overall Equipment Efficiency (OEE). Their condition monitoring features continuously monitor performance, discovering variances before they impact productivity. This allows manufacturers to stay in control of processes and avoid mistakes before they cost money.

Without the kind of continuous monitoring offered by POWERCELL® technology, errors such as load cell overload, poor communication caused by damaged load cell cables between modules, out-of-symmetry errors, and out-of-range temperatures can go unnoticed for long periods. When they do, out-of-specification batches and poor quality products are the result. Significant costs and damage to company reputation can occur.

PowerMount™ - equipped with POWERCELL® load cell technology with on-board microprocessors not only alerts operators to performance degradation — they also adjust the weighing signal to compensate for environmental changes. This allows PowerMount™ weigh modules to provide accurate weighing regardless of the effects of temperature, linearity, hysteresis, and creep. In the unlikely event of load cell failure, the design of PowerMount™ also makes individual load cells easy to replace. POWERCELL® is available in many different capacities. It is also important to note that when you are reviewing various load cell options, considering load cell price should be only one point of consideration. Load cell technology, load cell accuracy, load cell material – and more should all be a part of your purchasing and system design decisions.

What is a load cell cable?

On multi-load cell systems each load cell is connected by a load cell cable to a junction box, which adds the individual load cell signals together to provide one signal that can be transmitted to the indicator. Analog is the most common operating mode, but some scale suppliers offer proprietary and non-proprietary digital operating systems. When using a digital system, consult the supplier’s technical manual for proper junction box wiring.

Normally, each load cell is supplied with a standard length of cable. Do not lengthen or shorten load cell cables in the field. Changing the length of a load cell cable will affect the output signal from the load cell. If a cable is too long, simply coil the excess cable and place it in or near the junction box. You can order junction boxes in sizes that are large enough to hold coiled cables. Never attach excess cable to a live portion of the weighing system. Nonstandard lengths of cable can be ordered for applications that require them.

How do I connect a load cell? How do I connect multiple load cells in parallel?

A load cell cable is used to connect the load cell to the terminal (a system with just one load cell) or to a junction box (a system with more than one load cell / multiple load cells in parallel). On analog load cells this cable is usually an integral part of the load cell. Our Engineering Handbook offers insight on how to connect a load cell and how to connect 4 load cells.

What is the difference between 4 wire and 6 wire load cell?

6-wire load cells have additional voltage sensing lines as part of the load cell's circuit; 4-wire load cells do not. The additional 2 wires in a 6-wire load cell allow terminals designed for the system to maintain the ideal voltage.  This improves the system performance compared to systems with 4-wire load cells, especially in applications where the terminal is further from the scale.

How do I install a load cell? How do I mount a load cell?

Our Engineering Handbook provides general information about how to install a load cell or weigh module and tips on proper load cell mounting and load cell mounting design. Each application has unique requirements and should be planned by a qualified structural engineer. When installing load cells or weigh modules, refer to the Installation and Service Manual for the specific model to ensure proper load cell mounting and load cell mounting design. This is the best resource for learning how to install a load cell.

How do I manage compression load cell installation?

Compression load cells and compression weigh modules are suitable for most weighing applications. These modules can be attached directly to the floor, piers, or structural beams. The tank or other object is mounted on top of the weigh modules. For proper compression load cell installation, consult the Installation and Service manual for the specific model. Our free Engineering Handbook also offers tips on compression load cell installation.

How do I learn how to wire a load cell? Where do I find a load cell wiring guide or load cell wiring diagram?

There are many different considerations for how to wire a load cell. On multi-load cell systems each load cell is connected by cable to a junction box, which adds the individual load cell signals together to provide one signal that can be transmitted to the indicator. Analog is the most common operating mode, but some scale suppliers offer proprietary and non-proprietary digital operating systems. When using a digital system, consult the supplier’s technical manual for proper junction box wiring and a load cell wiring guide or load cell wiring diagram.

Our Engineering Handbook offers general information on how to wire a load cell, as well as load cell wiring diagrams for analog or smart weighing systems.

Where can I find a load cell application guide?

A load cell application guide helps you match your requirements to the appropriate load cell design. When selecting weigh modules for an application, it is important to consider how the load will be applied to the load cells. Most weigh module applications on tanks, hoppers, and vessels are subject to static loading. Under normal operation with static loading, little or no horizontal shear force is transmitted to the load cells. Applications such as conveyors, pipe racks, mechanical scale conversions, and high-powered mixers or blenders are subject to dynamic loading. With dynamic loading, the way in which products are placed on a scale or processed transmits horizontal shear forces to the load cells. Our free Engineering Handbook offers a load cell application guide, which explains how to select the appropriate load cell or weigh module for your weighing need. 

How to convert a load cell reading into total weight?

Most weighing systems use an analog junction box, which requires an analog-compatible indicator. An analog junction box can sum up to four load cells, acting as a load cell reader. For weigh module systems with more than four load cells, you will need to connect several junction boxes together. Our Engineering Handbook provides more details on how to connect a load cell for accurate weights.

 

How is load cell accuracy calculated?

The load cell accuracy calculation is complicated. There are a number of electronic and mechanical tests that are performed in accordance with a number of metrological a/o commercial standards (as well as basic industry standards), but the basic theory is this: a thin wire network (strain gauge) is bonded to a piece of metal and change in electrical properties when the metal is bent is monitored.  Metal has to bend in a repeatable way, the gauge (the wire network) has to report it as signal in a repeatable way, and the terminal must interpret the signal as weight. When we place a reference weight, we tell the terminal that the change in signal is proportional to the change in loading.

 

What is load cell capacity?

Load cell capacity is the maximum amount of weight a load cell can bare while providing accurate weight readings and without being damaged. Knowing the load cell capacity will impact your load cell accuracy for any weighing application.

How do you determine load cell capacity? How do you calculate load cell capacity?

A load cell capacity calculation contains many components. The load cell is a sensor that must cover the largest and smallest measurement required of it.  More specifically, it must not break when maximum load is applied, and it must be able to make the smallest measurement requested of it.  In the case of weigh modules, which are load cells plus additional engineered hardware designed to optimize load introduction and may also have additional structural capabilities, keeping a structure from moving or tipping over may also be desired.  In any case, knowing the weights and forces required to maintain structural safety/integrity and complete the required measurements is key to successful recommendation.  Most engineers receive training to calculate these forces, and we provide limit values of the forces our modules can withstand, so the choice for selection becomes clear.

Load cells measure both the dead load and the net load of a load cell scale, so this needs to be considered when calculating scale load cell capacity. How the goods are positioned on the scale also matters. Using load cells that are larger than they need to be for a given application is not recommended as accuracy suffers. For scales where the goods are mostly centered — which is true for most tank, silo and vessel scales — the formula is:

C >= sf*(TDL+SC)/N (eg. tank scales)

For scales with four load cells where the goods are likely to be positioned off-center — which is true for floor, small truck and conveyor scales — the formula is:

C >= sf*(TDL/4 + SC/2)  (eg. floor, small vehicle scales, conveyors)

C = LC or Weigh module capacity

TDL = Total dead load

SC = Scale capacity

sf = Safety factor (typically 1.25)

N = Number of load cells or weigh modules

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