what is the difference in load cells

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TECHNICAL NOTE What are the differences? Will these differences affect your system performance? Those who make purchasing decisions should understand the differences in order to make an informed decision as to what will work in their applications and yield the desired results. Taking risks should only be done from a point of knowledge. To the typical consumer all load cells look basically the same. The purpose of this Tech Note is to educate and enable our Customers to discern the various levels of load cell quality and make informed purchase decisions. The topics discussed include: • Review of costs to design and manufacture load cells • Design and manufacturing steps often skipped to reduce costs • Consequences of skipping various manufacturing and design steps • How to identify a load cell where steps have been skipped Load Cell Provider Profit and Costs The profit realized by any manufacturer is the difference between the revenue from sales and their costs. The costs for a load cell provider includes the cost to build each load cell and overhead costs such as Engineering, Sub vendor management, and quality control. A provider who shortcuts engineering staff and passes the savings on to the consumer will see a short term price advantage in the market with a medium term technical disadvantage. The provider who cuts manufacturing steps sacrifices some level of product performance. Before we can evaluate the affects of manufacturing steps reduced or skipped we must first have a common understanding of the complete and proper load cell manufacturing process. Manufacturing Load Cells The steps involved in manufacturing load cells include: 1. Purchase metal and test material received 2. Machine metal into the sensing element shape 3. Heat treat the sensing element 4. Apply strain gages and wire them in a Wheatstone Bridge 5. Measure bridge unbalance and apply correction resistance 6. Verify bridge balance 7. Measure temperature effect and apply correction 8. Verify that temperature effects are within limits 9. Verify that creep, linearity and Hysteresis are within specifications 10. Measure rated output and apply correction 11. Verify that rated output is within specification limits


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12. Seal the load cell from the environment 13. Perform final bridge readings and zero balance, carefully pack and ship Each of these steps will be discussed in appropriate detail below: 1. Purchase metal and test material received - We think of metal as simply metal but there are many different alloys available on the market. Each alloy has different results when used in sensing element design. There is a choice of quality levels of metal available on the market today. The quality level required depends on its use. Load cells, sensors, require a high level of quality. Different mills deliver varying alloy compositions within relatively broad standard limits. The subsequent rolling, straightening, annealing, and pickling processes can vary, too. Within a single mill, the manufacturing can vary and affect the consistency from lot to lot. Additionally, the variance in process control at each mill results in the reality that metal from one mill will yield considerably different results than metal from another, and metal from one mill is not guaranteed to remain at a consistent level of quality and performance. To remove, or greatly reduce, these variables, a quality load cell manufacturer must invest in people and time to investigate different alloys, potential metal vendors, test material and continuously monitor successful vendor's quality. This represents an overhead cost to the manufacturer and a higher material costs for higher quality metal. 2. Machine metal into the sensing element shape - The mechanical design of the sensing element structure governs the precision of the measurement. A good design can be largely immune to typical stray forces where another might be affected. The ultimate overload strength of the load cell is determined by the mechanical design and yet can be in conflict with the sensitivity of the sensor measurement. Mechanical designs with uniform stress distribution return considerably better results than those with gradients of stress through the gage area. Those providers committed to optimum designs must invest in talented and experienced engineers to design and test top performing load cells. 3. Heat treat the sensing element to obtain prescribed strength, toughness and grain structure - You may be familiar with heat treating metal to make it stronger but in the manufacture of load cells, the sensing element is carefully heat treated to achieve an optimum combination of yield strength, toughness, creep, change in creep with temperature and Hysteresis. A properly heat treated sensing element will undergo up to four separate processes in controlled atmospheres and with the use of expensive equipment. The heat treating process is proprietary to each load cell manufacturer. Quality load cell manufacturers must invest time in perfecting the heat treating process for their designs and then monitor that process for consistent practice. Those who make large investments in their own heat treating ovens have rapid feedback of problems and ultimate control over the proper heat treating process being followed on each load cell sensing element.


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4. Apply strain gages and wire them in a Wheatstone Bridge - Designing and manufacturing strain gages are disciplines in themselves resulting in different gages having a variety of performance levels. Gage design and application can affect linearity, creep, Hysteresis, temperature effects on zero and span. Load cell providers who design and produce their own gages have ultimate control over the fit of the gage design with the load cell sensing element mechanical design. For example, some manufacturers design gages with varying amounts of negative Hysteresis used to offset positive Hysteresis in the sensing element material. Providers who buy off-the-shelf gages do not benefit from the ability to design a gage specifically for each load cell designed. Additionally, the application of the gage must be performed in a clean environment by people trained and equipped with the proper tools to do a precision job. Those performing this tedious and precise job must be carefully selected and trained. Precision fixtures must be designed to properly locate the strain gages, since the ultimate performance of the load cell is no better than the precision of the strain gage location. Quality load cell manufacturers must invest in engineers who select or design, and test gages for optimum results and in equipment and training of employees who apply the gages. 5. Measure bridge unbalance and apply correction resistance - Resistors must be added to balance the Wheatstone Bridge so that the "no load" output of the load cell is within prescribed limits An unbalanced bridge means that you would measure different levels of output from different load cells with the same load applied. 6. Verify bridge balance - A step often skipped, the bridge balance must be checked to verify proper adjustment in step 5. 7. Measure temperature effect and apply correction - Each load cell must be temperature compensated for effects of temperature on zero and span. An uncompensated load cell will vary its output as temperature changes. The temperature effect on span is consistent with each lot of metal and production as long as each step leading up to this test is controlled. Temperature effect on zero varies from load cell to load cell and therefore must be measured individually and corrected by adding temperature compensation elements or resistors. 8. Verify that temperature effects are within limits - Most of the time the compensation added in step seven is sufficient, causing many providers to skip this verification step but leaving the user wide open to temperature effected load cells. Verification is important since there are finite differences between the temperature characteristics of the individual strain gages and the temperature compensating


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components which must be "compensated out" in precision load cells. For precision load cells, the temperature compensation on zero should be verified on a 100 percent basis. The temperature compensation on span should be verified on a sample basis. 9. Verify that creep, linearity and Hysteresis are within specifications - Creep, linearity and Hysteresis have already been determined by design and process control in the prior steps. Many providers sample test the production run to spot check the performance. Quality providers conduct 100% verification to ensure that each and every load cell has been produced within advertised specifications. This 100% testing is important since there are finite differences between the temperature characteristics of the individual strain gages and temperature compensating elements which must be "compensated out" for precision load cells. 10. Measure rated output and apply correction - The rated output sensitivity of the load cell must be measured and adjusted to be within a tight tolerance for each unit produced. This is often referred to as "matching the mV/V output" of the load cell to others of the same model produced. The process of matching load cell outputs involves measuring the output and applying correction resistors. The objective is to have the same level of output change for a given load applied regardless of which load cell in the system supports the weight. Most manufacturers attempt to match mV/V of each load cell to within a tolerance. The tightness of the tolerance varies between manufacturers and only two manufacturers (to our knowledge) match mV/V/ohm output characteristics of their load cells and only one manufacturer matches both mV/V and mV/V/ohm characteristics. 11. Verify that rated output is within specification - It is important that the load cell manufacturer verifies that the calibration resistance added has, in fact, properly adjusted the rated output to within prescribed tolerances and that no problems have been introduced as a result of making electrical adjustments. In steps 5,7 and 10 resistance has been added to adjust for bridge balance, temperature effect on zero and output sensitivity. In these steps the resistance added should be of the high precision type, e.g. 10 parts per million (ppm), as opposed to lower cost and readily available 50 ppm resistors. In steps 6, 8 and 11 these corrections have been verified to be within spec and that no new problems have been introduced as a result of the assembler or technician physically adding resistance. There is a lot of time spent in these measurement, correction and verification stages to meet performance specifications that usually are not checked by the customer. Quality load cell manufacturers must develop sound manufacturing processes and monitor themselves to make sure all steps are followed. Without an outside checking function and with tremendous price pressure from the market, it is tempting for some manufacturers to spend less time in these adjustment


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and verification stages. Only those manufacturers with the strongest desire to uphold quality will continue to hold themselves accountable for proper and thorough testing and performance. Questions that should be asked when evaluating a load cell manufacturer include: • How many specifications are verified on each load cell produced versus quoting typical performance? • What data is recorded on the specification sheet? (measured or typical?) • Who monitors the results of the testing? • Are the test technicians given conflicting directions or requirements? (Accuracy of measurement versus more units produced in a shift) • Are test instruments regularly checked and matched with each other? Performance parameters that are not guaranteed to be consistent from one manufactured load cell to the next and therefore should command 100% production testing include: Ø Creep Ø Linearity Ø Hysteresis Ø Temperature effect on zero Ø Bridge balance Ø Bridge input and output resistance Ø Insulation resistance, bridge to element, at 50 VDC Ø Output Sensitivity 12. Seal the load cell from the environment - One of the most misunderstood specifications for load cells is the level of sealing. Load cells must have good sealing, therefore protection from the environment, to yield longevity of the load cell and consistency of the measurement. Moisture in the load cell or the cable will quickly result in erroneous readings. Moisture entering the gauged cavity of the load cell is absorbed by the organic strain gage backing causing relaxation of residual stress, lower strain transmission and lower insulation resistance to the element. Anyone of these factors will cause zero drift and span changes. Other foreign materials entering the gauged cavity may cause similar effects and even corrosion of the strain gages and associated connections. Load cell life is shortened, as a result. One source of confusion is created by the load cell manufacturers themselves who mislead consumers as to the level of seal provided. A strong example of this is misuse the term "Hermetic Seal". According to Webster's Dictionary "hermetically", as in hermetically sealed, means "A vessel or tube is hermetically sealed when it is closed completely against the passage of air or other fluid by fusing the extremity; -- sometimes less properly applied to any air-tight closure." Many manufacturers claim hermetic "sealing" and point to welded caps over the gauged


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area but fail to discuss the sealing at the cable entry of the load cell and the subsequent passages of the wires to the gauged area. Very few, maybe just one, manufacturer(s) actually provides a load cell closed completely against the passage of air, gas or fluids by fusing the extremity and is prepared to discuss the level of the seal effectiveness by testing. The only way to be certain that a load cell has been properly hermetically sealed at the gauged cavity and at the cable entry is to perform a helium leak test in those areas. That is done be connecting the load cell to a mass spectrometer and evacuating it to a "hard" vacuum. A source of helium gas streaming through a small needle is brought close to the welded joints and if there is a leak the helium enters the load cell and is detected by the mass spectrometer. The leakage rate is measured in units of cubic centimeters of air per second. A water vapor molecule will slip through an opening which exhibits a leakage rate of 10 exp-6 cubic centimeters of air per second. One load cell manufacturer requires a leakage rate of lower than 3 exp-8 for acceptance, one thirtieth of that required to admit a water vapor molecule. While the load cell is evacuated it is good practice to back fill it with dry nitrogen when the leakage test has been completed. This ensures a good, dry, inert environment for the strain gages and associated circuitry. For applications not requiring hermetic sealing, potted load cells can be sufficient. In all cases the manufacturer should be talking from a point of knowledge about the seal and not quoting specifications typically requested or those found in competitor's literature. 13. Perform final bridge readings and zero balance, carefully pack and ship - One final check is in order to make sure that the product performs as specified. Specifically, it is a good practice to make final electrical bridge readings, insulation resistance to element and zero unbalance. Many load cell manufacturers omit this step to save cost.


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Making a Low Cost Load Cell and Realizing the Consequences We now have a common basis for the production of a quality load cell. We also understand that manufacturers can only lower the price by reducing overhead expense such as engineering or quality control or by skipping steps in the production of the load cell. We are now ready to consider what steps might be eliminated from the production process to save money. The table below highlights areas to save money during the production and the resultant consequence:


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In many instances, taking short cuts in the production of a load cell results in poor and/or variable performance of one or more load cell parameters. Unfortunately, these areas of poor performance are not quickly identified on the user's production floor. Typically operators and quality inspectors realize symptoms such as "That scale is not consistent", "Our product yield is not as good from that scale", "The deviation of product from that scale is greater than others", "That scale is noisy" or "The accuracy of that scale is not good enough". Troubleshooting poor performance often involves exhaustive trial and error approaches to adjust piping, alignment of load cells, and rewiring and rerouting load cell wiring to protect it from electrical noise. Many hours are spent trying to make a good scale out of a bad one. When the root cause of the problem is inadequate load cells, the problem is rarely identified and the company replaces the scale or learns to live with substandard results. Both of these paths lead to magnitudes of higher costs than if the consumer had purchased quality load cells to begin with. With knowledge, short cuts can be taken without negative results. If the application calls for low accuracy, for example weighing a silo to know its weight within one or two percent accuracy, then low cost load cells may make sense, provided, of course, that costs savings have been done only affecting the quality of measurement and not affecting the longevity of the load cell.


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Most, if not all, process control weighing applications require quality load cells. The cost of an inadequate scale and the subsequent costs of replacement load cells far outweigh the initial savings. How To Distinguish Quality Load Cells? So how do you know if you are buying a quality load cell or one in which the manufacturer has focused on cost reduction, at the expense of quality? Here are some quick guidelines with the comment that nothing can take the place of good vendor qualification: Price: If it seems too good to be true, it is. Individual specification sheets: Does each load cell produced have its own specification data sheet with measured specs? Can you trust that this data was accurately and actually measured? Does the company have any outside audit process to ensure that these products meet spec? Measured versus generic specs: Take a close look at the specification sheet to determine which specs have been measured versus ones where the generic product family specifications are quoted. Any specification with a plus or minus deviation is a generic family spec and most likely has not been measured on this load cell, or else they would have published the measured performance. Yield: Ask the salesman what happens to product that doesn't meet the manufacturer's published specifications. All load cell manufacturers have product that doesn't meet specification, unless they don't check or they pass it along to save the cost of scrap material and labor. Specifics on tests: Ask specific questions about how the testing is done. Ask how they know that each load cell produced has a hermetic seal. How do they know each load cell has been temperature compensated or how much creep each cell has. Measuring creep ties up expensive testing equipment, something that a low cost supplier doesn't want to do. Load cell expertise: Inquire as to the company's expertise in the area of load cell production, testing and refinement. Ask who designed the load cell and determine if the design was done in house or copied from another design. Weighing expertise: Ask about the final accuracy of the scale with these load cells. If the vendor quickly quotes percentage figures, ask them if that is a percentage of load applied or of full scale. Most load cell specs are a percentage of full scale but few really know this. Most importantly, if the vendor doesn't start talking about other factors that


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are significant to weighing accuracy then they are not knowledgeable about process weighing applications or they are trying to impress you with a number you'd like to hear. The truth is that, given a quality load cell and instrument, the most significant deterrents from accuracy of a weighing system are installation related. If the vendor can't speak intelligently about possible error sources which are installation related then they haven't been students of effective weighing technology. There are many load cell sensor providers but who lack process weighing application knowledge, but very few weighing system providers with command of load cell technology. The combination of load cell, instrument and weighing application knowledge yields the best performance of any process weighing system. Reality or Scare Tactics? Is Hardy Instruments just trying to justify a higher price? Is the author of this paper defending technology for technology sake? The profit that Hardy Instruments makes is the difference between the selling price and the cost. It would be easier for us to lower our costs by skipping manufacturing steps, pass the savings on to our customers, and have a competitive price to earn more sales. We would not have to spend any time educating our sales channel and our customers. We would not be trying to convince the market of the importance of quality in load cells. We could instead spend our time reaching more customers and winning in the volume of business available. We could take such short cuts in production starting immediately so there is no barrier to us lowering costs, at the expense of performance. The highest profit achieved for any manufacture is to charge a high price and skip steps in the manufacturing process. This holds costs low but retains a high price. It is difficult and short sighted to adopt such a strategy. Short term sales growth is the reward for this approach but long term customer respect will not be attained. Hardy Instruments' perspective is that we believe it is best for our customers, for Hardy Instruments and the evolution of weighing technology to spend the money, don't skip steps unwisely and deliver a truly quality product. Our cost is the effort required to educate the market. Our reward is lasting relationships as our customers learn that they can trust Hardy and our products. We will continue to look for ways to lower costs, through sound engineering, but will not sacrifice quality.

what is the difference in load cells

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