I purchased a set of Horst probes for my Black Tank. While looking at existing probes (see photo) I am somewhat perplexed. The bottom 2 probes are horizontal. It is is the same in Grey tank. You then can see the next probe up and then the top probe is hidden behind metal holder (FR infinite wisdom). I am going to replace the bottom 3. My question is? Why are the 2 probes at the bottom horizontal? I have always assumed empty, 1/3, 2/3, full. In my case it looks like empty and 1/3 are the same? Any thoughts?
Edit, just had a lite come on. Is one of the bottom probes the "reference probe" for the others?