Load Cell Q&A
Question: Loads calculated from my load cell readings do not agree with loads calculated from my hydraulic jack. What's wrong?
Answer: There are a number of reasons why calculated loads from load cells and jacks do not agree.
One factor (Sellers, 1994) is mismatch between the diameter of the jack and the diameter of the load cell. This causes bending of the bearing plate between the load cell and the jack and affects the response of the load cell. If the diameter of the jack is larger than that of the load cell, load cell readings will be low. Conversely, if the diameter of the jack is smaller than that of the load cell, load cell readings will be high. To minimize this effect: (1) use thick bearing plates: at least 1-inch (25 mm) thick when the diameters of jack and load cell are the same. Use significantly thicker plates when the diameters are different. (2): Use a step-up or step-down loading ring on the jack side of the bearing plate. The idea is to load the bearing plate with a diameter that matches that of the load cell.
Another factor is that axial misalignment of the jack (typical in field conditions) results in hydraulic pressure within the load cell that does not match the laboratory calibration of the load cell. Tests (reported in Dunnicliff, 1988) show that during loading (of piles), the pressure reading of the jack can indicate loads 10 to 25% higher the actual load, and during unloading, loads that are 5% lower than the actual. Ideally, the jack should be calibrated using the same method of loading as used in the field.
Sellers, Barrie. "Load Cell Calibrations" in Geotechnical News, September 1994.
Dunnicliff, John. Geotechnical Instruments for Monitoring Field Performance, John Wiley and Sons, New York, 1988, pages 301 - 303.
Question: How do I calibrate my load cell to read zero when there is no load? I'm using the DataMate MP.
Answer: Use the MP Manager program to set up the sensor definition and determine the engineering units to be used. Take an initial reading with no load. This reading will be used as the “Datum” in the “Datum Reading tab” in MP Manager. Enter the value as expressed in desired engineering units. Check the box to subrtract this datum from following readings. Leave “Offset” and “Sensitivity” as %FS, as noted on the cal sheet.
The “Offset” is defined in units of %FS and is divided by “Sensitivity” after being subtracted from the reading. On the other hand, “Datum” is subtracted from the calculated result without further modification. Consequently, the “Sensitivity” and “Offset” are normally entered directly from calibration sheet while “Datum” is used to zero the no-load reading.
The no-load reading is usuallyslightly different from zero because the calibration
uses a best fit method.