Π.Μ.Κ.= Πρόσθετα Μέτρα Κατανάλωσης - Μ3 που προέρχονται από προηγούμενο μετρητή.

or

Additional Consumption Measures - M3 from previous counting.

and

Σ.Δ. = Συντελεστής Διόρθωσης: Ο συντελεστής που μετατρέπει τα συνολικά καταναλωθέντα κυβικά μέτρα (Μ3) σε κανονικά κυβικά μέτρα (ΝΜ3).

or

Correction Factor: The factor that converts the total consumed cubic meters (M3) οτο normal cubic meters (ΝΜ3).

I obtained this information by saving the company's technical 114 page technical manual from the Internet to Acrobat, converting it to Word and then searching for the terms with 'Find". The Greek texts above were copied directly from the technical manual.


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Note added at 51 mins (2016-03-21 13:44:12 GMT)
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Of course, 'Πρόσθετα Μέτρα Κατανάλωσης' could possibly also mean 'Additional Metres of Consumption', but, since all other references to metres are written as 'M', I don't think that it does. However, this is a possibility and so you must decide.

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Note added at 2 hrs (2016-03-21 15:05:52 GMT)
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Sorry, “M3 from previous counting” above, should have been, “M3 from previous counter/meter/indicator”.

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Note added at 3 hrs (2016-03-21 16:47:53 GMT)
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I now think that ΠΜΚ probably does stand for Additional Metres of Consumption, especially as it refers to cubic metres of gas from a previous counter.