OxyFog analysis:
Ozone generation

AAPlasma’s open-source OxyFog system utilizes an asymmetric dielectric barrier discharge (“surface plasma”). In atmospheric air, containing predominantly nitrogen, oxygen, and water, we should observe significant ozone generation. On this page, we go into the details of what we have observed while operating one or both plasma sources with and without using one or both nebulizers.

To measure the ozone concentration, we drilled a ~0.5” hole in the center front of the box (Figure 1), placed a 1/4” tube push-fit quick-connect (see Amazon link for example), and positioned the recirculation fan approximately in the center of the box, fixing it to the bottom plate with a double-sided tape (Figure 2). The ozone monitor (model 106-M, 2B Technologies) was connected to the quick-connect by a 12” 1/4” PTFE tubing through the water and debris filter (provided with the ozone monitor). The model 106-M is designed to operate in the 0–1,000 ppm range and was recently calibrated by the manufacturer.

Figure 1. Photograph of the OxyFog system with the ozone monitor connected to the center of the chamber.

Table 1 shows the experimental setup for the measurements. The ozone monitor is operated continuously throughout every experiment, recording ppm measurement every 2 seconds. We analyzed the behavior of the first plasma unit, the second plasma unit, and both together, with and without tuning on one or both nebulizers.

For example, in Experiment #10 (Table 1), Plasma 1 and Nebulizer 1 were turned on after the ozone monitor recording was started, then Nebulizer 1 was turned off after 30 seconds. The recirculation fan was kept on for the entire duration of the experiment. Once ozone concentration was observed to peak, the plasma unit was turned off and the Heated Cleaning cycle was turned on. The experiment and the recording were stopped once the ozone concentration in the box dropped below 5 ppm.

Disclaimer: AAPlasma performs all of our ozone experiments in a well-ventilated environment to protect the operator from exposure to ozone. A personal ozone monitor (available on Amazon) was placed on the bench to ensure a safe ozone level in the room. Anyone performing similar experiments should take the same precautions.

All of our experimental data (in MS Excel format) and data analysis (in GraphPad Prism 9 format) are available for download here (Zip file).

Figure 3 shows the results of our measurements. The general trend that can be observed from these measurements is that there is little difference between the plasma generators with or without moisture. However, the effect of moisture is quite strong, taking the gaseous ozone from around 800 ppm in room air (60% RH) to around 300 ppm when either one or both nebulizers are used. This may be due to ozone and other plasma-generated reactive species dissolving in the water (as will be demonstrated in later experiments).

Based on these results, we decided to repeat these measurements (this time, n=2 for each experiment), comparing one or both plasmas with and without moisture.

Table 1. Experimental setup for ozone measurement with and without moisture.

Figure 3. Results of the first set of ozone measurement experiments with OxyFog system, outlined in Table 1.

Figure 4. Ozone measurement inside of the OxyFog system with plasma 1 on, recirculation fan, and no moisture. A gradual decrease in the ozone concentration is observed after the initial rapid increase, until the plasma is turned off and the heat-clean cycle is run for 20 minutes.

As can be seen in Figure 4, we observe a gradual decrease in the ozone concentration, after an initial rapid increase. This is happening inside a closed and air-tight OxyFog chamber; thus, we can attribute the observed decrease to the system’s trend for an equilibrium ozone generation/destruction balance, of around 500 ppm. This figure may serve as a reminder to be mindful of the time and location in the chamber where the measurements are performed.

Table 2 shows the experimental plan for the follow-up experiments: we performed the ozone measurement with one or both plasmas, with and without water droplets, and we repeated these measurements twice.

Table 2. Experimental setup for the second group of ozone measurements.

Figure 5. A repeat of ozone measurements (n=2) with range of measurements platted.

Figure 5 shows the results of the measurements repeated twice for every experimental point. The observed ozone concentration is higher without moisture than with, and it is lower with both plasmas turned on.

All experimental data (in MS Excel format) and data analysis (in GraphPad Prism 9 format) are available for download (Zip file).

Conclusion

In this report, we performed an essential set of measurements for an atmospheric pressure plasma system: ozone generation inside of the OxyFog system. We demonstrate the dependence of ozone concentration on time and other parameters of our system, making it a valuable educational and research tool.

All of the measurements were performed by AAPlasma LLC staff members. If you have any questions, please contact Dr. Greg Fridman at greg@aaplasma.com.