Of course I'm biassed here in favour of my own answer in the eng-spa pair...

But I like David's 'process measurements' also - provided that the context is one that actually involves a 'process' of some kind. That might be the case, for example, in a biscuit-making plant, or a refinery in the petroleum industry, but in areas of engineering that I have worked in - such as the operation of broadcast transmitters - there is no 'process' as such (there is no tangible 'product' being processed) and we always refer to 'operational measurements'.

Note also that 'operational measurements' is commonly used when there is a need to make a distinction vis à vis measurements taken while installing or commissioning systems, or while diagnosing fault conditions: in these circumstances additional measurements would normally be taken, desides referring to the normal operational measurements.

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Note added at 9 hrs (2008-06-24 15:38:55 GMT)
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oops: missed out a word:

would normally be taken, desides referring to the normal operational measurements.
-->would normally be taken, *besides those* referring to the normal operational measurements.

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Note added at 10 hrs (2008-06-24 16:39:21 GMT)
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Restricting ourselves to the operational / operating problem:

‘operational’ is an adjective meaning ‘related to an operation’. So we have ‘operational staff’ meaning employees involved in operating something.

‘operating’ is the gerund of the verb ‘to operate’, and it can be used as a noun (in the same way as Spanish can use the infinitive as a noun). So we have ‘operating the machine is easy’, where ‘operating (the machine)’ is a noun-phrase forming the subject of the verb ‘to be’.

Other examples of correct usage include:
“We closed the factory for operational reasons.”
“Our operational requirements are making excessive demands on our resources.”
And, in the context of María Eugenia’s question:
“Operational measurements on the system were out of tolerance so we called in the experts.”

Then we have a ‘grey’ area where popular usage has distorted the previously clear origins and meanings of these words. We have ‘operating instructions’, which should, perhaps (?), be ‘operational instructions’ ...