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76 SMT007 MAGAZINE I JULY 2021 Four tests were run with the chamber under the conditions shown on the psychometric chart of Figure 3: (1) 15% relative humidity, 40°C; (2) 15% relative humidity, 50°C; (3) 31% relative humidity, 50°C; and (4) 75% rel- ative humidity, 50°C. As mentioned earlier, the humidity was maintained using saturated salt solutions. e durations of various elec- trical and temperature conditions are listed in Table 1. Testing involved test periods at various tem- peratures and even though the intent was to keep temperature constant during each test period, rise in film resistance with time increased film temperature accordingly. e effect of temperature was compensated, as shown below, by calculating the value of resis- tance that would be if the film were cooled to temperature, T o . e temperature compensated (corrected) resistance, R(t,T o ), can then be used to cal- culate the film thickness using the following relation: e temperature coefficients of electrical resistance, a, of copper and silver thin films were determined as illustrated in Figure 4. e very high electrical resistivity of the corro- sion product allowed us to neglect the effect of the parallel resistance path of the corrosion product. e sulfur concentration and the relative humidity were fixed by the chamber temper- ature and by the choice of the saturated salt solution, respectively. An indirect means of determining the air composition is by mea- suring its corrosivity towards copper and sil- ver foils inserted through narrow slots into the front door of the chamber and exposed to the chamber environment, typically for a day. e total thickness of the corrosion prod- ucts per day was determined using coulomet- ric reduction [15] . Corrosion rates of the copper and silver foils on a per day basis are shown in Figure 5. e effect of the chamber tempera- ture is clear: 40°C chamber environment is less corrosive than 50°C environment. On the other Figure 4: Method for determining the value of temperature coefficient of resistance a. Table 1: Electrical and temperature test conditions for the FoS chamber at 40°C.