Q: Some ASTM test methods request that one brings the specimens into equilibrium with the atmosphere prior to the test (some of them determine the conditions: 21±2ºC and 60±10% relative humidity). Considering that we just produce polyethylene geomembranes, and they are not significantly affected by the relative humidity, do you have any suggestions about how long the specimens should be kept under those conditions?
A: You are correct, polyethylene geomembranes are not affected by relative humidity. Nor are most geosynthetics for that matter. This is demonstrated in the reference Baker, T., and Koerner, G. (2000). “Recommended Atmosphere for Geotextile Testing.” Geotechnical Fabric Report, 18(1), 16–17. That said, temperature matters. One will need to bring the geomembrane to 21±2ºC prior to the test. This usually takes half an hour in the lab and not the 40 hours prior to the test that the method suggests. You need to address this in your quality manual and show support for the actions you take in your manufacturing quality control (MQC) lab.
I can well imagine that your statistical process control (SPC) data would be marginalized if you had a 40-hour time lag with production.