Having tried numerous commercially available pads - among which there are both good and very bad - I'm mitterweile gone over to use of the above powder cartridges (one was as a free supplement in buying my Petra 31.17 here). Here, a small circular filter are as shown in the picture) is inserted into the cartridge, it painted the ground coffee added and then smooth with my spoon. Finally, the "cover" closed into place - ready. Sounds harder than it is, but with a little practice this works without any problems. The preparatory work is worth it - the coffee tastes, especially when (fresh) ground, worlds better than the pads from the market. Added to this is the cost savings. Especially with larger cup amounts incurred but considerable cost. When using cassettes can thus be substantial savings on top of that. You do not recommend I can empty the pads, which are then used as purchased without pads cartridge. The coffee you get manual very difficult purely not to mention the required amount. I am really very pleased with the Petra powder cartridges.
