The surface morphology of the sensor paper can be seen in Figs. 1-3. A good dimensional comparison between the diatoms and the cellulose fiber is shown in the middle of the image of Fig. 1. size distribution of the diatoms was investigated by visual analysis and was averaged from 10 randomly selected areas (100 µm x 100 µm) with coat weight of 6 g/cm2. The average size distribution can be seen in Fig. 4. Roughness value (Ra) was measured by first subtracting a linear regression from scanned profiles to level the profiles. Three different samples had Ra values ranging from 5.7 µm to 6.9 µm, average being 6.2 µm. The cross-section SEM images were used to evaluate the thickness of the coating and the diatom particle distribution in the z-direction. We used targeted coating weights of 6 g/m2 and 20 g/m2 and this corresponds to 100 µm and 235 µm, respectively. Higher magnification with the SEM was used to see the diatom pores were visible mainly in micro-scale and to investigate if the diatom pores were filled with the PVOH coating (Figs. 5-7). The SEM images show that the coating formulation is homogenously spread and that the majority if the micro-pores are open. This is essential to maximize the large surface area of the diatoms and their favorable structure for gas permeation.