4-20 mA signal

Hello friends,
it is a little while ago I didn't post because of a family
arrangement. But today I need a little help on a off topic not so off
topic theme.
I got a pneumatical a steam valve into my shop that is
governed by a 4-20 mA signal.
I have to repair the positioning mechanism.
No trouble with that, I did it quite a few times on other brands.
But I do not know what is the standard voltage of the 4-20 mA
signal for to make a pot-arrangement to simulate the PLC signal.
There is no book arround to check.
I would appreciate your help very much.

Thank you
Sven Peter