Another standard for the WVTR test is ASTM F1249.
The working principle can be shown in the figure above (refer to the link for more details). The major difference between F1249 and E96 is the detection of water. In F1249 water molecules are detected by a IR optical method while gravimetric in E 96. Therefore the major advantages for F 1249 is
- More precise and repeatable. temperature/humidity measure error as well as weighing error (IR Vs balance)
- Less time consuming
- Can measure WVTR less than 0.1g/m^2 day; E96 typically is larger than 0.1g/m^2 day.
In gravimetric method, the accuracy relies on the analytic balance, assuming other errors can be kept very low. For ultra-barrier, the balance must be accurate enough to measure the water weight difference. For example, to test the WVTR with levels of moisture correlating to 0.005 g/m2 ∙ day, the analytical balance would need a resolution repeatable at 2.5 x 10^5 g using a 50cm2 test area to be considered comparable to the F1249 method. Therefore, for ultra barriers, gravimetric method would be limited.