The entire period of the measurement is influenced by the power
required to keep the hot chamber at constant temperature. The
test duration is limited during the time that the stationary con-ditions are achieved. Heating wire switching on depends on the
thermal resistance of the specimen; the achievement of the sta-tionary conditions strongly depends on the kind of the specimen
and on its thermal resistance so EN ISO 8990 does not establish
a fixed duration for the test: generally the test has to last at least
2 h for specimens with low thermal transmittance. Polycarbonate
panels are not heavy specimens, so test duration also shorter than
2 h could be considered. In Fig. 13 the air heat and cold chamber
temperature trends are represented.