Reliability of an NTC Thermistor

The reliability of a thermistor can be a major factor to consider when a warranty is going to be used for the product that the thermistor is in. For example suppose that a thermistor is used in a motor drive that will go through on-off cycling 15 times per shift @ 3 shifts per day and has a warranty for 10 years. Then to find the product life we use:

(15times/shift)*(3shifts/day)*(365days/yr)*10yrs = 164,250 cycles needed

To insure that the thermistor can last that long we use a calculation called the Mean Time Before Failure (MTBF), which is calculated as follow:

MTBF = m = Ta /r

Where Ta = ∑ Ti + (n-r)td

Where: Ti = cycles to failure or time to failure prior to completion of the test
              r = # of failures
             n = total # of samples tested
             td = duration of test