shall be recorded at intervals not to exceed ten minutes, and should be in units of Celsius to the nearest one tenth of a degree. Solar insolation (Wm -2 ) and ambient temperature (C) shall be recorded at least as frequently. Record and report the frequency of attended (manual) tracking, if any. Report azimuth angle(s) during the test. Report the test site latitude and the date(s) of testing. 7.2 Calculating Cooking Power. The change in water temperature for each ten-minute interval shall be multiplied by the mass and specific heat capacity of the water contained in the cooking vessel(s). This product shall be divided by the 600 seconds contained in a ten- minute interval, as: P = (T f - T i )MC v /600  Where: P = cooking power (W) T f = final water temperature T i = initial water temperature M = water mass (kg) C v = heat capacity (4186 Jkg -1 K -1 ) 7.3 Calculating Interval Averages. The average insolation, average ambient temperature, and average cooking vessel contents temperature shall be found for each interval. 7.4 Standardizing Cooking Power. Cooking power for each interval shall be corrected to a standard insolation of 700 Wm -2 by multiplying the interval observed cooking power by 700 Wm -2 and dividing by the interval average insolation recorded during the corresponding interval. P s = P i (700/I i )  Where: P s = standardized cooking power (W) P i = interval cooking power (W) I i = interval solar insolation (Wm -2 )
5 7.5 Temperature Difference. Ambient temperature for each interval is to be subtracted from the average cooking vessel contents temperature for each corresponding interval. T d = T w – T a  Where: T d = temperature difference (C) T w = water temperature (C) T a = ambient air temperature (C) 7.6 Plotting. The standardized cooking power, P s, (W) is to be plotted against the temperature difference, T d, (C) for each time interval. 7.7 Regression. A linear regression of the plotted points shall be used to find the relationship between cooking power and temperature difference in terms of intercept (W) and slope (WC -1 ). No fewer than 30 total observations from three different days shall be employed. The coefficient of determination (r 2 ) or proportion of variation in cooking power that can be attributed to the relationship found by regression should be better than 0.75 or specially noted. 7.8 Single Measure of Performance. The value for standardized cooking power, P s, (W) shall be computed for a temperature difference, T d, of 50 C using the above determined relationship. NOTE: for product labelling and sales literature an independent laboratory using a statistically adequate number of trials shall determine this number. While this value, like the fuel economy rating of an automobile, is not a guarantee of performance, it provides consumers with a useful tool for comparison and product selection.
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