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I have worked with both Balco and platinum sensors in various applications.  I like that a high resistance Balco sensor costs less than a platinum one, but I find that the resistance of the Balco sensors seems to shift more over time.  Is Balco more sensitive to environmetal factors such as humidity and pressure? What else could be causing the shift?

Balco is a nickel iron alloy that tends to oxidize with sustained exposure to 200 degree C, while platinum is fairly inert to as much as 1700 degree C. Resistance will increase with oxidation. Because of the substantial difference in oxidation temperatures, these sensors are frequently built in different packaging materials that may increase strain within the sensor. Balco is generally packaged in organic materials with a higher thermal coefficient of expansion. This mismatch in packaging places new stress in the element and shifts resistance. Balco is commonly used for strain gauges because it is going to change resistance a lot with induced strains from TCE or pressures. Humidity makes corrosion even worse.

Most platinum sensors take much more care to ensure the TCE of the packages matches the platinum metal because of the wide temperature span it must cover.

You might also see some effect of "annealing" with a balco sensor that is brought above its normal operating temperature. The annealing will drop the resistance but oxidation begins to increase the resistance until the metal ultimately fails open.

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