Load Cell, Weigh Module, Load Sensor

How does a load cell work?
The basics of load cell technology is converting a load - which is a force applied to any mass in the earth gravity field – into an electrical signal. Inside most load cells have so called strain gauges that convert a mechanical strain into a resistant change. Learn more on strain gauges below. The mass applies a load enforced by the gravity field, that load stresses the body of the load cell and lead to deformation – strain. The strain is converted into resistant change and finally into a voltage output. That output is proportional to the applied mass.

What is a load cell?
A scale load cell, or load cell sensor, is used in variety of industries in which precise measurements are needed. Load cells accurately measure the weight of tanks, vessels, hoppers or conveyors. Load cell technology is built to withstand the demands of a variety of rugged industrial applications. The capacity of load cells can greatly vary. Some load cells are more appropriate for laboratory applications, while others are more appropriate for high-capacity batches or logistics applications.

Load cells shall meet all globally-required approvals and standards. They include single-ended beam load cells that allow you to change your structure into a scale. Beam load cells are used in multiples in floor scales, belt weighers, weighing systems, pallet scales, checkweigher scales and conveyor scales.. Canister/ring load-cell technology features robustness for harsh environments as well. These are typically used in tank scales, vessel weighing, silo scales and truck weighing.

What is a Weigh Module?
Weigh Modules are made from load cells and steel hardware. They are optimized for tank and conveyer scale systems. Other than load cells weigh modules also provide safety functions for coping with wind and seismic loads. Since the load cell sensor integration is done by the manufacture one can assume the best accuracy and lift time for the scale load cell.    

What are load cells used for?
Load cells are used to measure the weight of any possible material (e.g. with a 50 kg load cell, load cell 50 ton or any other capacity) . They reach from microgram in micro load cells to several hundred (100) tons in high capacity load cell scales. Low weights are required for pharma applications where fractions of a gram can make a key difference. Mid-size load cells are typically used for processing and trading materials. Large capacities are used for storage applications.

How accurate are load cells?
Very often accuracy and resolution are mixed. Resolution is how many digits are displayed, but more digits does not mean better accuracy. Accuracy describes how true the weighing results are (load cell accuracy calculation and load cell capacity calculation). For good reason, legal-for-trade scales limit resolution to eliminate getting into the interpretation of results. Therefore, the best benchmarks for accuracy are legal-for-trade approvals. Strain-gauge load cells typically provide accuracy from 1/3,000d up to 1/10,000d (also referred to in the load cell specifications). Load cells using force restoration reach 1/200,000 or even better. Under ideal conditions, which include constant temperature and humidity, the accuracy can be 5-10 times better.
But it is fairly complicated to calculate the final system accuracy of a load cell system. Many impacts like temperature, time, resolution etc. need to be considered. It is recommended to seek advice from load cell manufacturers. The Weighing Component Selector is providing these calculations in a professional manner.

What is load cell calibration?
Calibration is a process that assesses the load cell accuracy to ensure weighing results remain within predefined tolerances. Load cell sensors need to be calibrated  where they will be used. The primary reason for onsite calibration is that gravity changes by location. A load cell manufactured in location A and used in location B would create an error without onsite calibration. Another reason onsite calibration is important is the ongoing wear on the device itself to which all technical instruments are subject. Depending on the importance of accuracy in the process in which the scale is used, it is good practice to calibrate a scale every 1-2 years. 

An efficient method calibrating larger tanks scales is RapidCal.

What is the output of load cell?
Most load cells provide an analog output of 2-3 mV/V. Since these are passive devices, they need an external power source for excitation. The output signal varies related to the relative excitation voltage and current load. Thus, it is crucial to have a very stable excitation source that eliminates effects cause by the power source. A 2mV/V load cell provides as little as 20mV output at full capacity when powered with 10V output. Modern terminals (load cell reader) provide both a stable power source and accurate amplification of the output signal to convert results into digital information. It is important for the signal to perform proper load cell wiring.

How many types of load cell are there?
Load cells are first distinguished by their form. Single-point load cells are used for small and mid-size applications. Beam load cells are used for floor scales and mid-size tank scales. Canister load cells are used for heavy-capacity applications. Another separation criteria for load-cell type is the output provided. Analog load cells need an external terminal for amplification and A/D conversion while digital load cells have in-built "intelligence" with the advantage of better accuracy, much more robust data transfer and the potential to monitor the condition of the load cell to help with maintenance. All load cells have a different load cell price, depending on the performance needed.

Getting started with load cells?
Load cells measure both the dead load and the net load of a scale, so this needs to be considered when calculating 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

Seek professional consulting for more complicated systems.

More details you can find in the Weigh Module Systems Handbook addressing engineering topics.

Tips and tricks using load cells
A common problem for using load cells is to choose the wrong protection. Load cells are very sensitive to humidity and regular wash-down. Although claiming high IP rating the protection is not good for damp heat. When it comes to high humidity and sudden temperature changes (cold environment, hot wash-down) it is advised to always choose stainless steel hermetically sealed load cells. So different parameters need to be taken into account when you buy load cells.

Calibration costs for load cell systems are often underestimated. If your quality system requires regular calibrations of such weighing systems, seek advice for the most efficient method by one of the largest load cell manufacturers globally. And ISO9001 complying method needs an independent calibration equipment traceable to international standards. Under no circumstances, it can be assumed that calibration is never changing and a load cell system does not need re-calibration at all.  An efficient method calibrating larger tanks scales is RapidCal.

Load Cell wiring or cabling is often under estimated on accuracy impact. In any case shielded cables are recommended since the load cell signal in very low voltage and easily disturbed. Chose the shortest cables possible as copper changes resistance over temperature and the lower cable resistance, the better.

Load cells measure force, sadly not masses which would be ideal. That leads to the fact that any other vertical accidentally occurring force in a load cell system has direct impact on the accuracy. Typically piping forces in a vessel, contact to ground by eg. debris collection, cable touching for micro load cell and small capacity systems, vertical vibration etc. are to mention. Touch base with expert to avoid those problems in your system.

Speed of the load cell system is often critical. A filling process is more accurate the higher the update rate is. Same is true for a batching system where the recipe is more reparative the faster the load cell system measures. Speed comes a little costs, but can make a huge difference.

Very often the integration of load cells is cumbersome and requires engineering knowledge. Weigh Modules are higher integrated and provide safety and ideal load conditions for the load cell. See details how to select Weigh Modules.  

How do strain gages work?
This short tutorial explains the function of strain gages. Strain gages use the physical effect that prolonged wires increase electrical resistance while compressed wires reduce resistance. Applied on the body of a load cell the deformation induced by the load the strain gages need to follow that deformation and change resistance. That resistance change is proportional to the deformation. The resistance change is very small – typically less than 1 Ohm. That is difficult to measure over a longer distance. To convert resistant change into voltage change, the Wheatstone bridge is uses. Typically, four strain gages are used for a reasonable voltage change. To transfer voltage change over longer distance is much easier and state of the art.

What is the difference between load cell and strain gauge?
Strain gauges are components of load cells. Load cells are a counterforce that converts load into strain. That strain is taken by the strain gauge which changes its resistance into an electrical quantity that can be measured and reported as a weighing result. 

What is excitation voltage for a load cell?
Load cells and load cell cable are passive devices that need an external power source for excitation. Related to that excitation, voltage can change based on the current load, causing errors. Therefore, it is crucial to have a very stable excitation source to eliminate errors caused by the power source. Modern terminals provide a stable power source, accurate amplification of the small output signal, and conversion of results into digital information.

What is hydraulic load cell?
Hydraulic load cells use the principle of pressure change when loading an enclosed fluid. The pressure change is measured by a pressure transducer. Hydraulic load cells are not very accurate and not very common these days. The advantage of robustness that they offered in times when electronics were not as accurate are long gone.

Are load cells linear?
Typically load cells are very linear to 0.03% or even much better. Scales today are primarily only calibrated at zero and a maximum span point indicating that the range in-between is linear.

Would 1 or 2 load cells be sufficient for a 3 or 4 leg tank scale?
Using fewer load cells than tank legs is possible for liquid level control systems. The key is to ensure that the center of gravity in the system does not shift horizontally with load. Accuracy for such a system may not be ideal. Do not use such a system for bulk material.

Will a PT1000 or PT100 electronic work with analoge load cells?
Analog load cells have a mV/V interface which is different from passive temperature sensors such as PT100, PT100, thus special electronics is required.

What measurement tolerances can I expect from a weighing system?
Weighing system measurement tolerances are influenced by many things, including mechanical, electrical, and environmental impacts. Our weighing experts can provide more information and help you design a weighing system that meets your requirements.