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September 2020 / 23 material into the scale; the control system that han- dles the recipe automation; the required cycle time or process speed; and the environment where the scale is going to be installed. Weighing sensor. In most modern applications, the weighing sensor, known as a load cell, is commonly an electronic strain gauge, like the kind shown in Figure 2. Standard strain gauge technology measures the deflec- tion of a load-bearing element when force is applied to that element. The load-bearing element that's deflected must have a high degree of repeatability in the degree of deflection for a given load and a high degree of repeatability in the load-bearing element's ability to return to its original position. The typical strain gauge is bonded to the load-bearing element and as the load-bearing element bends, the gauge's resistance changes. When a voltage is applied to the strain gauge, the change in voltage is directly proportional to the amount of load that's placed on the load-bearing ele- ment. To make sure a load cell is accurate, the load cell must respond to a minimum and maximum amount of weight in the same fashion throughout the cell's rated capacity. For example, when weighing 1 certified pound on a load cell that has a capacity of 100, if you put the 1 certified pound on the scale 100 times, the scale should go up by 1 pound each time. And if you take the 1 cer- tified pound off the scale 100 times, the scale should go down by 1 pound each time. Weighing instrument. According to NIST Handbook 44, a typical load cell should be capable of being accu- rate with 5,000 divisions. accurate a scale should be and how accurate a scale can be. In NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirement for Weighing and Mea- suring Devices, the National Type Evaluation Program (NTEP) discusses how electronic scales can be set up, qualified, and calibrated. The handbook covers the capabilities of both the weighing sensor and the instrumentation that's used to display and calibrate the signal from the weighing sensor. Following these guidelines is necessary when selling LFT scales, and these guidelines are certainly useful in the process of scale qualification too, but available technology for in-process weighing may in some cases be better than what the handbook's specifications require. Determining ingredient requirements With ingredient batching automation, there will typically be a recipe or formula that you want to man- ufacture. The first thing any manufacturer should consider is what its own ingredient needs are in order for a recipe to be made accurately. Specifically, for each weighed ingredient think about what an acceptable amount would be above or below that ingredient's tar- get weight (also called the amount of error) that would still meet your recipe for batch requirements. For example, if the process calls for a 1 percent maximum error and you're trying to weigh 1 pound of material, then the weighing system would need to be able to weigh to a specificity of ±0.01 pounds (1 pound divided by 100 equals 1 percent). If your process also requires weighing another ingredient in your formula on the same scale used for heavier weighments, then you'd want to make sure that the minimum weight wouldn't go below 1 pound if ±0.01 pounds is the scale's accu- racy. For example, you wouldn't want to weigh 0.01 pounds of material on this scale because your possible error could be 0.01 pounds, which could possibly be a 100 percent error in measuring that ingredient. Once you've established what the acceptable amount of error is for each ingredient that's being weighed in your process, then you can look at the available automated ingredient weighing and batching systems to find one that suits your needs. Selecting the right system There are several factors that come into play when needing to achieve an accurate weight in automated ingredient weighing and batching systems. These fac- tors should be taken into consideration when looking to purchase an automated weighing and batching sys- tem and include the weighing sensor; the weighing instrument; the number of scales, their size, and how they're mounted; the feeder size that's used to meter FIGURE 2 Electronic strain gauges known as load cells are used to weigh and batch ingredients in automatic systems.