Horizontal Pitch in a Spiral

"Sensors shall be located along a spiral pattern (the vertical spiral pitch of which shall be a 10th of tank wall height) with an horizontal pitch between 2 sensors of 25 m." I get confused on horizontal pitch. Could somebody help me to give me a clear vision of above situation.
 
Assuming a vertical cylindrical tank and a vertical spiral, its axis is also vertical as is the pitch. The term vertical pitch is somewhat redundant but it is the height of one complete turn of a vertical spiral in this case.

Now comes the hard part. In my understanding of geometrical shapes, a spiral (or helix) has only one pitch. I could be mistaken, but let us proceed as though I am not and surmise what is meant by this "horizontal pitch" term.

Assume the tank is wrapped with a piece of paper such that the left side touches the right, the paper is taut, and the edges are perfectly plum (as is the tank :)). The spiral pathway is drawn on the paper. The edges represent a [vertical] pitch reference line.

If we now take that paper and roll it out flat, we have several angled lines going across the sheet (if we assume the spiral made it more than one round around the tank, that is :)). If we draw a line from where the spiral's path stops at one edge and resumes at the opposite edge. We should have a perfectly flat horizontal line (as if when the paper was still wrapping the tank). The length of this horizontal line is what I believe to be the horizontal pitch.

Anyone buying it?
 
Wouldn't that just be the circumference of the tank?

I would just go back to the person who wrote the specification and ask them to clarify. Just to be sure...
 
>Wouldn't that just be the circumference of the tank?

Yes. And a distance longer or shorter would put the sensors along the spiral path such that they are not one above the other.

>I would just go back to the person who wrote the
>specification and ask them to clarify. Just to be sure...

I agree.
 
Thank you @Smart & @Joe. The example of wrapping a paper over a cylinder and rolling it out flat makes a clear understanding about the horizontal pitch.
 
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