Calibration of Instruments

Hire someone to do it.

Quit kidding yourself.

Are you an instrumentation engineer or tech?

With plenty of controls experience?

If not, be prepared to read a lot.

Calibration is comparison to a reference standard. By eliminating "external sources" you have no reference standard to compare against.

Given that you've missed the essence of the task at hand, just buy some self adhesive cal stickers, fill them out and use them.

Then the execution matches the intent.

So, what's wrong with the original calibration certificates that the suppliers sent with their instruments?

Perhaps calibration is not what you seek. Perhaps you are looking for assurance that the devices are operating correctly and as specified. In order to calibrate some of the devices you mentioned, you would have to remove them from the system in which they are installed. This alternative may not be the best choice unless the device is critical to some process.

A calibration station, setup, and procedure would have to be developed for each device. This could amount to a substantial investment.

Perhaps your investment would be protected more effectively if you reviewed the software running on your PLC. Most installers only write the PLC code necessary to do a specific task. That is like wiring your house without any circuit breakers or fuses. But you need to realize that you usually get what you ask for, not necessarily what you need.

So, perhaps the next task is to put together a list of things you can test without destroying anything to see how the PLC program handles it. Chances are that it will be unable to interpret an undesirable condition because it was never part of the original specification. You will probably be able to determine the loss of flow or pressure by several methods. Having those methods as a backup method of detection is actually better than a "calibration program"

Find profesionals/laboratory with calibration equipment in mentioned fields and arrange calibrations. Probably this will be the cheapest way.

Doing Yourself may take very much time. And You will have to have calibrating standards /references to do it.