Hi There, I have two gas bottles that hold 45kgs of gas each which equals 0.088 m3. When 1 is empty it switches over to the other one. I have the following sensors:
rockgas_bottle_1_weight in kgs
rockgas_bottle_1_level in %
rockgas_bottle_1_weight is updated by a NodeMCU with HX711 and load cells, then rockgas_bottle_1_level is a template. Any ideas on how I would go about calculating the burn rate (over X days?) and days left? I literally have no idea how to start! 
A derivative helper should provide you with what you’re after (although you will need to combine it with another template sensor to get the days left part).
I’ve actually got a very similar setup in my trailer, but I haven’t had a chance to test it out since installing the sensors and so haven’t tweaked things or decided if it’s actually very useful for me yet. 
Super cool, I’ve been wanting to measure my oil tank level, had thought about a camera to read the float gauge had never considered a strain gauge. Any learnings on strain gauges that I should know?