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Applications Note 1-6:Deadweight test apparatus possess the important inherent ability of applyingexactly the same loads to the sensor being calibrated regardless ofwhether loads are increasing or decreasing. Insight into the value of thisinherent advantage of deadweight calibration systems follows in the discussionof hydraulic test systems.HydraulicIn higher-load-range calibrations, a reference load cell is installed into theload path in series with the device to be calibrated where the assembly ishydraulically loaded and the output of the test load cell is compared to theoutput of the reference load cell. Calibrations of this type are known ascomparison calibrations as the sensor under test is being compared to thereference load cell. Moorehouse Instruments of Pennsylvania manufacturesload frames of this type. As one might suspect, the precision of this type ofcalibration is limited by the precision of the reference load cell otherwiseknown as the Òtransfer standardÓ. A calibration showing zero nonlinearityand hysteresis simply means that no substantial difference exists betweenthe load cell being calibrated and the reference load cell. Such results donot imply zero error. Typically, the reference load cell is chosen to showbetween three and ten times lower nonlinearity and hysteresis errors thanthe sensor being calibrated. Accordingly, to calibrate a load cell to ±.1% offull scale output (FSO) nonlinearity and hysteresis, the reference load cellshould show between ±.03% and ±.01% FSO nonlinearity and hysteresis.To achieve traceable high performance results with this type of apparatus,one must consider that the loads applied will decay with respect to time dueto seal leakage around the hydraulic cylinder piston seal. High pressurehydraulic lines provided with this type of system are often elastomeric innature and it is common to change to stainless steel tubing thereby reducingthe applied force decay rate due to hydraulic line expansion. Often,high speed digital data acquisition systems are used to collect many samplesof both the transfer standard and the sensor under test at each inputload level where this data is averaged. This has the effect of averagingnoise to zero and reducing the uncertainty due to load decay.The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-31