The concept of hydraulic capacitance is applied to pneumatic bellows assemblies. This concept is then used in the analysis of a number of different basic types of pneumatic controllers in common use. Its application allows the effect of the bellows system used for delaying the derivative action, and the effect of bellows movement generally, to be conveniently included in the analyses. Numerical values determined for a particular controller show that the effect of bellows movement can change the transfer function constants by as much as 20 percent.

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