Dear Ed,

Thanks for your interesting posting.

SHIN-ETSU Sifel® is a rather specific sealant which stays flexible below - 50 degrees Celsius and is chemically resistant to Amine based oil additives.

As a consequence of this low temperature applicability, the glass transition temperature is low - lower than -50 degrees Celsius - and therefore the free volume in the sealant will be considerable - at room temperature, and of course at higher temperatures. The Tg is -65 degrees Celsius. The available free volume determines the diffusion rate of water in the sealant.

As such, the diffusion rate of water in Viton® or Viton® ETP-S is lower. And since the matrix is slightly less polar, the water permeation in these elastomers is less than in SHIN-ETSU Sifel® elastomer.

Nevertheless, whether water vapor diffusion is an issue also depends on the shortest path length between outside environment and the cavity. Secondly, the units of the given permeation figure seems to be related to the path length, since a common SI unit would have been [ gram / m day] following from the Diffusion coefficient [m2/s] times Solubility [gram/m3]. If one divides this unit by the path length [m] one obtains [gram / m2 day].
Hence, do you have any idea of the path length used, then we can calculate the actual permeation figure and compare this to other materials / get a better indication of the role of water diffusion in the oil filled pressure sensor.

Kind Regards,
Sijmon
Composite Analytica