I am in process of designing a homemade capacitance sensor and I'd like to have an approximation of the resulting capacitance of the following geometry. The plates are placed on the outer surface of a food grade plastic cylinder. The distance z between their edges is many times smaller than the diameter d of the cylinder (z<<d). Each plate has an area of A and it's dimentions are comparable to d, but, according to what I have in mind, smaller. the dielectric is air. is there any approximate value as a function of A and d that I could use to determine the sizes at which this geometry would produce a measurable capacitance? i am away from my books and the calculation method escapes my mind right now (plus, this is more complex than anything we did on the respective course), I'd appreciate any help. It is not crucial to have an analytical formula, but something to give me an idea of what to expect depending on the dimentions used.