Silicon water cures as does Silane based sealer so they would give a falsely low reading whereas decorators filler is water based so can lead to a false high. I've known people use plumbers mate or glaziers putty before both of which should be fine. As long as it does not use or give out water it should be ok. also bear in mind that digital hygrometers are affected by temperature. If its warm itll give a higher reading than if its is cold despite the screed having the same moisture content. this is to do with the way the hygrometer works. Carbide Bomb is not affected by temperature.Is that because it gets a reading off the moisture in the Silicon ? Anything else you can other than the tape ?