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I agree with what George is saying. If the process is such that it can tolerate some variation in temperature, say +/- 5 degree C, then a simple ON/OFF control would certainly be adequate. These types of processes typically have a pretty slow response, meaning if you can tolerate a reasonable deadband so you’re not constantly switching (not as big a deal since you are using solid state devices), and then when the temperature drops below your programmed limit call for heat and drive it to the positive allowable programmable temperature limit and let it drift back down.
To do a true Time Proportioned or Split-time Time Proportioned PID control in the micro/SLC environment may pose challenges. However, since as George says you’re using an ON/OFF or a discrete Final Control Element anyway, any “cooked up” time proportioned PID control will be reduced to an ON/OFF type anyway.
Hope that helps.