Silica gel sachets or bags are simply flexible 'containers ' of silica gel. The sachet material has to be robust and capable of withstanding normal handling whilst importantly being able to breathe.
Usually, 5 grams of silica gel are required per cubic foot of enclosed volume (or 170 grams per cubic meter). For large volumes, using a few well spaced smaller sachets will reduce humidity levels faster than if using a single large sachet. Often, environments cannot be considered 'hermetically sealed', there will be a number of factors which will increase the amount of silica gel that is needed.
Large silica gel sachets (1 kg. to 5 kg.) can prevent moisture damage to items in transit in packing crates and shipping containers, large machinery and enclosed vessels such as tanks or pipes