Load Cell Measurement Errors Can a load cell adapter plate or block be interchanged without introducing additional errors?
Load cell measurement errors - Hardness of the top block used for calibration can result in errors as high as 0.5 % of applied force.
We have seen large errors from customers switching out top blocks, or worse, not sending a mating top block for calibration. A simple rule is always send the top block adapters with the load cell for calibration.
If switching out adapters, the new adapter must consist of the same material and have identical characteristics, it may be substituted without introducing additional error. Though some materials will harden with repeated use; there could be a difference. The material of the top block is softer and has a lower hardness than the load cell it is used with. If a top block is replaced, the recommendation is to have the force measuring equipment checked or calibrated to ensure errors are accounted for.
Load Cell Measurement Errors Figure 1 A Multi-Column Load Cell
Using a top adapter with a different hardness value may affect the strain level in the load cell column or web; and, therefore, result in different measurement outputs. We have observed errors of up to 0.3 % from varying the material on top compression pads. We highly recommend the end-user send us the top adapter they are using with the load cell and even load cell bases. If either adapter is not ground flat, additional errors could result. We have conducted several tests and have found repeatability errors to be about 3 times higher when the compression pads or load cell base is not flat. Morehouse has a full machine shop and can grind top adapters for a nominal fee (typically $40 to $45 per block).
Real-World Example: A customer brought in a 1,000,000 LBF load cell for calibration. Morehouse performed a calibration. The load cell's output was recorded as 1,500 LBF higher than the previous calibration for a force applied of 1,000,000 LBF.
Is this a stability issue or an adaptor issue?
After calling the customer, we were informed a new top-loading block was supplied with this load cell for the current calibration. When we told them what was happening, they sent the original top-loading block. When tested, the original block resulted in an output of 1,000,180 LBF when loaded to 1,000,000 LBF.
Load Cell Measurement Errors Figure 2 Individual Uncertainty Contributors Graph
What causes this error?
Load Cell Measurement Errors Figure 4 Load Cell Stress Analysis Example
There are two points to make.
#1. Material with different hardness experience different amounts of lateral deflection under the same amount of load. This causes different amounts of stress between the block and the load cell.
# 2. The flatness and smoothness of the block are important in that it will change the contact position on the load cell. The assumption is the load cell has a radius of maybe R17 and is designed to be loaded exactly at the center of the spherical section, but an unbalanced or nonflat block can shift the contact point off-center. As the stress analysis above shows, a small amount of shift will change the stress distribution. The key is to use the same adapters in use as used in calibration. The adapters should be manufactured not to produce off-axis loads.
Load Cell Measurement Errors - Top Block Adapter Hardness and Flatness - Conclusion
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#Load Cell Measurement Errors