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What is a Load Cell and How Does it Work?
Whether you’re looking for a load cell for a unique application or have one that needs replacing, there’s a Rice Lake solution to meet your requirements. Not quite sure what you need? A knowledgeable customer service representative can help you select the best load cell for your application, from tank and hopper weighing to bulk material management and everything in between.
Types of Load Cells
Load cells can be categorized into the following types:
Weigh Module Selection
What should you look for in a load cell or weighing assembly?
When selecting a Rice Lake load cell or weigh module, you should consider the surrounding work environment and loading conditions. Based on these and other design specifications, Rice Lake Weighing Systems’ experienced engineering and design staff has created a comprehensive line of Rice Lake weigh modules. Most companies concentrate on one or two types of mounts. However, we offer over 20 different styles of mounts. Coupled with our outstanding selection of Rice Lake load cells manufactured to the highest quality standards, our mounts come complete with everything you need to get started and the unbeatable service support to help you get it done.
The wide variety of Rice Lake weigh modules we manufacture and distribute may leave you wondering what the differences are between certain models and, most importantly, which modules are best suited for your needs. We’ve broken down our weigh module selection into three application categories. Most operations can be classified into one of these:
Aluminum Load Cells
Aluminum load cells are used primarily in single-point, low-capacity applications. When compared to alloy steel cells of comparable capacities, aluminum load cells have relatively thick web sections. This is necessary to provide the proper amount of deflection in the element at capacity. Machining costs are usually lower on aluminum elements.
Alloy Steel Load Cells
Load cells manufactured from alloy steel elements are some of the most popular cells used today. The cost-to-performance ratio is better for alloy steel elements compared to either aluminum or stainless steel designs. This type of steel can be consistently manufactured to specifications, which means that minor load cell design changes don’t have to be made every time a new lot or steel vendor is selected.
Stainless Steel Load Cells
Stainless steel load cells are made from the best overall performance qualities that allow for cell to perform at peak levels. They can be fitted with hermetically sealed web cavities, making them an ideal choice for corrosive, high-moisture applications. This is the best material to use in any application that requires extra protection because of the metal’s environmental resistance properties.
The following are some easy-to-follow steps to help you with Rice Lake load cell troubleshooting when your load cell isn’t working properly. Before you begin, you will need a high-quality, digital multimeter and at least a 4.5-digit ohm meter. There are four tests: Physical inspection, zero balance, bridge resistance and resistance to ground.
How does your load cell look? If it is covered with rust, corroded or badly oxidized on its exterior, corrosion has likely worked its way into the strain gauge area. If the general and physical condition has a good appearance, then you likely need to look at specifics: sealing areas, the element itself and the cable.
In most load cells, areas of the load cell are sealed to protect the internal components from contamination by water and chemicals. To see if any seals have been degraded, look closely at the strain gauge seals.
Is rust concentrated on part of the welded cover? If there is no cover, do you see any tiny holes in the potting? These are indications there has been contamination to the gauge area. Check the load cell cable entrance for signs of contamination as well.
Other items to look for include: element damage, bends or cracks, cracks in the weld, or abrasions in the metal. It may be necessary to remove the load cell and check it for physical distortion against a straight edge.
No inspection would be complete without thoroughly reviewing the cable. The cable should be free of cuts, crimps and abrasions. If your cable is cut and in a wet environment, water or chemicals can "wick" up the cable into the strain gauge area, causing load cell failure. If your physical inspection fails to uncover any identifiable damage, a more detailed evaluation is required.
This test is effective in determining if the load cell has been subjected to a physical distortion, which can be caused by overload, shock load or metal fatigue. Before beginning the test, the load cell must be in "no load" condition. In other words, either the cell should be removed from the scale or the dead load should be counterbalanced. This test is effective in determining if the load cell has been subjected to a physical distortion, possibly caused by overload, shock load or metal fatigue.
Once the cell is no longer under any load, disconnect the signal leads and measure the voltage across the negative signal and positive signal. The color code for determining negative- and positive-signal leads is provided on the calibration certification with each load cell. The output should be within the manufacturer’s specifications for zero balance, usually ± 1% of full scale output. During the test, the excitation leads should remain connected, with the excitation voltage supplied by the digital weight indicator. Be certain to use the exact same indicator that is used for the cell’s daily operation to get a reading specific to the application.
The usual value for a 1% shift in zero balance is 0.3 millivolt, assuming 10 volts excitation on a three millivolt per volt output load cell. To determine your application’s zero shift, multiply the excitation volts supplied by your indicator by the millivolt per volt rating of your load cell. When performing your field test, remember that load cells can shift up to 10% of full scale output and still function correctly. If your test cell displays a shift under 10%, you may have another problem with your suspect cell, meaning further testing is required. If the test cell displays a shift greater than 10%, it is likely it has been physically distorted and should be replaced.
Before testing bridge resistance, disconnect the load cell from the digital weight indicator. Measure across the positive and negative signal leads using a multimeter to find the input resistance. If the reading exceeds the rated output for the load cell, don’t be alarmed; it is not uncommon to receive readings as high as 375 ohms for a 350-ohm load cell. The difference is caused by compensating resistors built into the input lines to balance out differences caused by temperature or manufacturing imperfections. However, if the multimeter shows an input resistance greater than 110% of the stated output value (385 ohms for a 350-ohm cell or 770 ohms for a 700-ohm cell), the cell may have been damaged and should be inspected further.
If the excitation resistance check is within specifications, test the output resistance across the positive and negative signal leads. This is a more delicate reading, and you should get 350 ohms ±1% (350-ohm cell). Readings outside the 1% tolerance usually indicate a damaged cell.
However, even if the overall output resistance test is within normal specifications, you may still have a damaged load cell. Often when a load cell is damaged by overload or shock load, opposite pairs of resistors will be deformed by the stress—equally, but in opposite directions. The only way to determine this is to test each individual leg of the bridge. The Wheatstone Bridge diagram illustrates a load cell resistance bridge and shows the test procedure and results of a sample cell damaged in such a manner. We’ll call the legs that are in tension under load T1 and T2, and the legs under compression C1 and C2.
With the multimeter, we tested each leg and got the following readings:
- T1(–Sig, +Exc) = 282 Ω
- C1(–Sig, –Exc) = 278 Ω
- T2(+Sig, –Exc) = 282 Ω
- C2(+Sig, +Exc) = 278 Ω
NOTE: When testing leg resistance, a reading of 0 ohm or eight means a broken wire or loose connection within the cell. In a functioning load cell in a “no load” condition, all legs need not have exactly equal resistance, but the following relationships must hold true:
- (C1 + T1) = (T2 + C2)
In this damaged load cell, both tension legs read four ohms higher than their corresponding compression legs. The equal damage mimics a balanced bridge in the output resistance test (3 above), but the individual leg tests (1, 2 above) show that the cell must be replaced.
NOTE: On multiple-cell applications for matched millivolt output, excitation resistance values may be higher than 110 percent.
Resistance to Ground
If the load cell has passed all tests so far but is still not performing to specifications, check for electrical leakage or shorts. Leakage is nearly always caused by water contamination within the load cell or cable, or by a damaged or cut cable. Electrical shorting caused by water is usually first detected by an indicator readout that is always unstable, as if the scale were constantly "in motion." The wrong cell in the wrong place is the leading cause of water contamination. Almost always, these leaking cells are "environmentally protected" models designed for normal non-washdown, not the "hermetically sealed" models that could have stood up to washdown and other tough applications.
Another cause is loose or broken solder connections. Loose or broken solder connections give an unstable readout only when the cell is bumped or moved enough for the loose wire to contact the load cell body. When the loaded scale is at rest, the reading is stable.
To deter electrical leakage problems, test resistance to ground with a low-voltage megohmmeter. Use caution—a high-voltage meter that puts more than 50 volts of direct current into the cell may damage the strain gauges. If the shield is tied to the case, twist all four leads together and test between them and the load cell metal body. If the shield is not tied to the case, twist all four leads together with the shield wire and test between them and the body. If the result is not over 5,000 megohms, current is leaking to the body somewhere.
If the cell fails this test, remove the shield wire and test with only the four live leads to the metal body. If this tests correctly (over 5,000 megohms), you can be reasonably sure current is not leaking through a break in the cable insulation or inside the gauge cavity.
Minor water infiltration problems can sometimes be solved outside the factory. If you are sure that water contamination has occurred and that the cable entrance seal is the entry point, try the following remedy: Move the cell to a warm, dry location for a few days, allowing the strain gauge potting to dry. Before putting the cell back into service, seal with silicone around the cable entry point in the load cell body. This prevents the reentry of water vapor into the cell.
Capacity Vs. Resolution
A load cell’s resolution will determine what sensitivity (or readability) it can attain for a given capacity. That is, the resolution of the weighing system (whether it has one or multiple points) equals the system capacity divided by its sensitivity. Simply put, the higher the system capacity, the lower the resolution and sensitivity. For instance, consider that a 5,000-pound system divided by 1 pound would be a 1:5000 system. That same 5,000-pound scale displaying the weight in 0.5-pound increments would change to 1:10,000 (5,000 pounds divided by 0.5 pound).
It is important to understand this because as a scale’s resolution gets higher, the actual millivolt output per increment gets smaller. The smaller each millivolt reading, the more difficult it is to detect small weight changes. This makes it more difficult for the load cell to deliver accurate weighing results and the digital indicator to display stable readings.
What harsh conditions does a load cell have to withstand?
Load cells are critical components of all weighing systems. In some applications, a load cell may be exposed to a hostile environment with corrosive chemicals, heavy dust, high temperatures or excessive moisture from washing down equipment with large amounts of liquid. The load cell could also be exposed to high vibration, unequal loads or other harsh operating conditions. These circumstances can lead to weighing errors or even damage the load cell if the wrong one has been selected.
To choose the right load cell for a demanding application, you should have a solid understanding of your environment and operating conditions, as well as what load cell features are best for handling them. Load cells are critical components in all weighing systems where they sense the weight of material in weigh hoppers, other vessels or processing equipment. In some applications, a load cell may be exposed to a hostile environment with corrosive chemicals, heavy dust, high temperatures or excessive moisture from washing down equipment with large amounts of liquid. Or the load cell may be exposed to high vibration, unequal loads or other harsh operating conditions. Such conditions can lead to weighing errors or can even damage the load cell if it hasn’t been chosen correctly.
What makes an application 'tough'?
Take a close look at the environment surrounding your weighing system and what operating conditions the system must work in.
- Will the area be extremely dusty?
- Will the weighing system be exposed to temperatures higher than 150⁰F?
- What are the chemical properties of the material being weighed?
- Will the system be washed down with water or another cleaning liquid? If cleaning chemicals are to be used to wash down the equipment, what are their characteristics?
- Will your washdown method expose the load cell to excessive moisture? Will the liquid be sprayed at high pressure? Will the load cell ever be immersed in liquid during washdown?
- Will the load cells be loaded unequally because of material buildup or other conditions?
- Will the system be subject to shock loading (sudden large loads)?
- Will the weighing system’s dead load (the vessel or equipment containing the material) be large in proportion to the live load (the material)?
It may be necessary to trim the load cell outputs as a first step before starting the calibration process. Trimming is performed at the junction box to equalize the weight reading from all cells in a system. This ensures the scale weighs correctly regardless of where the load is applied to the scale.
Trimming is necessary if:
- It is a Legal for Trade weighing application
- The location of the center of gravity of the contents is not fixed, e.g., powder material which may accumulate on one side*
- A high-accuracy weighing system is required*
*Assume the vessel’s center of gravity (see 2 and 3 above) rises along the same vertical line as the vessel is filled. Each load cell is always subjected to the same percentage of the weight.
Trimming is not necessary if:
- Matched output load cells are used (as in Paramount weigh modules)
- Weighing self-leveling materials (liquids)
- The vessel is partially supported on flexures
Trimming involves placing the same weight over each load cell in turn, and adjusting the corresponding trim pot in the junction box until the indicator reads the same for all cells. To further illustrate load cell trimming, please review the following examples of signal trim and excitation trimming procedures.
Load Cell Trimming
Many weighing systems use multiple load cells and therefore require a summing junction box to tie or "sum" load cell signals together. This allows a digital weight indicator to read a single "system" signal. The summing process actually wires multiple load cells so all their signal lines and excitation lines are in parallel, providing instantaneous electronic summing of the signals.
Load cell summing is necessary because:
- Weight distribution in multiple load cell systems is not equal at each load cell. The vessel loading process, presence of agitators and the characteristics of the material affect how weight is distributed on the load cells.
- It is virtually impossible to make each load cell exactly alike. Load cell manufacturing process tolerances allow for some variance in individual cell specifications. This variance, if unchecked, would not allow for the kinds of accuracy required in modern process applications.
There are two summing methods: Excitation trim and signal trim.
This is the oldest method of trimming the output from a strain gauge load cell. Excitation trimming adds series resistance to the excitation circuit of the load cell, thereby reducing the excitation voltage at the cell. The load cell with the lowest millivolt per volt output receives the full excitation voltage. All other load cells in the system with a higher millivolt per volt output receive proportionally smaller excitation voltages. This results in matched full load outputs for all load cells in the system.
This form of trimming first appeared as an alternative to excitation trimming for indicators with gated power supplies. Compatible with virtually all indicators and relatively immune to temperature and vibration problems, signal trimming is gaining popularity for all installations. It involves adding a relatively high parallel resistance between the signal leads of each load cell. The added parallel resistance creates a "leakage path" that shunts some of the available load cell signal away from the indicator. The larger this parallel resistance, the more signal available to the indicator from the load cell. Conversely, the smaller this parallel resistance, the less signal available to the indicator from the load cell.