I found this solution in a old Hans post  (
http://ntg-context.ntg.narkive.com/g0lT1D2A/scaling-mpgraphics)

2016-05-25 18:30 GMT+01:00 Wolfgang Schuster <schuster.wolfgang@gmail.com>:

> DesdeChaves <desdechaves@gmail.com>
> 25. Mai 2016 um 19:09
> For educational purposes I try to simulate a ammeter for reading practice.
> I would like create six or more ammeter with different readings, but i'm
> not able to do that because metapost create the same random number every
> time I call the buffer that defines my ammeter.
>
> How can I fix that?
>
> Is there a reason why you use \externalfigure because the width can be set
> with \scale, e.g.
>
>     \scale[width=8cm]{\getbuffer[ammeter]}
>
> Wolfgang
>
>
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-- 
Atentamente

DesdeChaves