When all three temperature sensors are operating normally, the values
T1, T2, and T3 will be nearly equal
to each other, so these three differences will be very small. If any
of them is not small, then we may want to discard one or more of the
temperature values in our determination of an official USCRN
temperature. Because the thermometers have different degrees of
accuracy at extreme temperatures, we use two different tolerance
levels in determining whether a temperature difference is small
enough: in comparing two temperatures that are both between -50C and
50C we use a tolerance of 0.3, and in in comparing two temperatures
where at least one of them is less than -50C or greater than 50C, we
use a tolerance of 0.6.
To formally state this, let i and j be the indices of the two T values being compared (i.e. i,j = 1,2,3, i ≠ j). Then define: tol(i,j) = 0.3 if -50 ≤ Ti ≤ 50 and -50 ≤ Tj ≤ 50For each of the three differences Dij, we say that Dij is "within tolerance" if Dij ≤ tol(i,j)The algorithm also takes into account the speed of the fans for each temperature sensor; if a fan speed is too low then the value from the corresponding sensor may be suspect. To precisely define how we decide whether a fan speed is too low, let: FNi = normal fan speed for fan #iFNi is determined for each fan by examining a history of fan speeds for that fan and keeping a running average of that fan's speed. FCi is referred to as the "critical fan speed" for fan #i. We say that a fan is "functioning normally" if Fi ≥ FCi.
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