We are often asked how frequently an operator should balance the load cell during testing. Many lab managers require the operator to balance a load cell before the start of a new sample; others require balancing the load cell before testing every specimen. We believe that either procedure is acceptable, as long as you follow one major rule:
If you notice a change on the load channel display after gripping, you can remove the load using automated software features such as preload or specimen protect, or by manually adjust the position of the crosshead.
Never balance the load cell when there is a specimen clamped in both grips
Instron load cells can detect a change in load as a result of simply gripping the specimen. If the load is balanced after a specimen is gripped, you risk zeroing out a real load. This real load will be subtracted from or added to reported results, thereby falsely increasing or decreasing actual values depending on whether or not there was a compressive or tensile load on the specimen before the load cell was balanced.
If you notice a change on the load channel display after gripping, you can remove the load using automated software features such as preload or specimen protect, or by manually adjust the position of the crosshead.
3 comments:
Another option to corect for speciemn tension or slack after gripping a specimen is to let the software do the correction. Most software should be able to extrapolate the true "0,0" (zero load at zero displacement) point via correction calculations. These calculations apply a correction to the specimen to compensate for any preload or slack in the
specimen and is always the first calculation and subsequent calculations use
the corrected gauge length. Very pertinent to ASTM D638's Toe Compensation (Annex A1).
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