6. The apparatus of claim 1 , comprising a first comparator configured to compare an impedance determined by the processor circuit against a specified range; and in response, wherein the processor circuit is configured to declare an alert if the impedance falls outside the specified range.
6. The apparatus of claim 1 , comprising a first comparator configured to compare an impedance determined by the processor circuit against a specified range; and in response, wherein the processor circuit is configured to declare an alert if the impedance falls outside the specified range. 7. The apparatus of claim 6 , wherein the specified range is determined using information about one or more previously-determined impedances.