A clamp assembly for an external fixation system includes a first clamp member for engaging a first elongated member. The test circuit includes a substrate, a plurality of pixel structures, a plurality of signal lines and a plurality of shorting bar sets. The process comprises the determination of a target value V of the partial pressure piC4 of the isobutene in the gas phase of the reactor corresponding to the property P, by virtue of an empirical relationship established beforehand between the property P of the polyisobutene produced and piC4.