I have heard both sides of this issue. This is, again, an issue where you need to evaluate the transmitters performance against the process or application tolerances. If the unit will drift out of an acceptable tolerance limit and still be operating properly (other than the tolerance drift), then there is certainly value, as you need to be sure that the readings it gives you are accurate enough for your need. There are applications, also, where transmitter accuracy is not important, and an inaccurate reading is not important to the correct performance of the process being monitored. Those questions will need to be answered before making such a decision.
The other issue is unstable operation. I certainly won't be the one to give someone an excuse to save moneyt by not doing correct maintenance. It could well be (in some applications) that accuracy is not an issue, but unstable operation is still important. In those cases, maintenance is still important, but for other reasons. I don't believe there is a single correct answer to this. Each application needs to be evaluated to determine how much change in the transmitter's readings and feedback is acceptable. The implications of an incorrect reading from the transmitter must be evaluated, and determination made as to what amount of error can be tolerated. How often, and what type of maintenance is performed must be based on the answer to that evaluation.
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