I have finally managed to fine tune the design of my black boxes to get more air in the nutrient and less humidity in the pods, resulting in better seedlings.
The main difference you can see here is that the stem remains very healthy while in the previous crops, it would shrink and the salad would not grow properly. I think I now have the system to maintain the pod out of water and let the seeds push their roots in the box down in the nutrient.
Salads were growing fine but we still need to find a better mix for the nutrient and probably better storage. I think the heat affects the quality of the nutrient.
Another issue is that I also had lots of precipitation in the solution which ended up clogging my pump or the fine pipes feeding the nutrient. This is still an issue that I need to resolve.
I wish I could visit a proper industrial system and talk to the guys who design them instead of reinventing the wheel…