This looks like the manual:http://www.vellemanprojects.com/downloads/0/illustrated/illustrated_assembly_manual_k8067.pdf
It looks like the device is designed to drive 0-20mA output in proportion to temperature. It also looks like you could muck with resistor R9 in the circuit (change to 250 ohms) for a nominal 5V output.
I believe the circuit is relatively insensitive to voltage V+ (within reason) thus the output current (hence calibration procedure) shouldn't depend too much on whether you use 12V or 15V. What voltage source do you have?
Yes, I would definitely limit the output with a 5V zener (really 4.7V since 5V zeners can go quite a bit higher before they actually break down). Also I would use a series resistor of 1k before the zener (but after the load resistance of the sensor) to limit zener current. The zener *shouldn't* ever have to turn on, but......
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