General remarks on nomograms and alignment charts

Spreadsheet and electronic calculator pretend high accuracy – but most time input data is inaccurate. Hence what's the purpose of those 3 to 7 figures after the decimal point?

Although there are many devices powered by a small solar panel – the battery is low in many cases, if you do not use the calculator daily. A stained photo copy of a nomogram can still be used.

I hear some people complain that alignment diagrams require a straight edge or ruler to be read. Take any sheet of paper, even with ripped off borders and fold it once: there you have a straight edge!

Technical magazines and books can present nomograms in an easy way. However they must not be scaled down to much, because many lines will clinch together then. A copy of such a graph may well be unusable.

To calculate results according to a given formula requires data to serve all variables to the left of the equal sign. Only few users of such formulas are capable to re-arrange the formula to allow to find a variable from a given result and the other variables. If part of the formula reflects empiric knowledge you have no chance to re-arrange it.

A nomogram does this automatically: just use those scales you know.

This alignment chart (a form of nomograms) is used to calculate the equivalent pipe length for various valves and fittings to calculate the head loss in piping.

Faulty diagram:
This diagram as presented here can be found in many brochures, even on the web. It is – however – wrong and misleading. A correct nomogram could be found in Levens, A. S. (1948): Nomography. New York: Wiley, page 151.

Artists have copied and copied this (proven by subtle details in various copies I have found) this diagram without knowing what they do. They needed more room for the middle column of drawings and moved the pivot line to the left. The correct location of the pivot line is in pink.

The underlying formula is very simple. The empirical element are values for K marked on the pivot line which depend on the type of fitting or valve. You get the equivalent length L by this formula: L = K x D. When dividing L by D you get a particular K. with 10*L / 10*D you should get the same value K. This is indicated by the red lines.

Since K can be found only by experiments, the source of the used data must be given in a diagram. This is missing here also.

I have re-developed this diagram for the Durchfluss-handbuch (ISBN 3-9520220-3-9), english version Flow Handbook (ISBN 3-9520220-4-7), published by the company Endress + Hauser Flowtec in Reinach/CH (see this page)

The gases are numbered in the charts: