Buffered metapost function freezes random numbers

[DesdeChaves <desdechaves@gmail.com>]

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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?

Thanks

Jorge

My code:


\startbuffer[ammeter]
\startMPcode
   r := 4cm; len := 10bp; min_thickness := 1.75bp; hour_thickness := 3bp;
      path cadran; cadran = fullcircle scaled (2r);
     numeric escala;
     escala := 0;
 for i = 50 upto 70:
        if i mod 5 = 0:
          j := i div 5; angl := 90-30j;
          freelabel("\tfb\bf" & decimal escala, (r+len)*dir angl, r*dir
angl);
          draw ((r, 0) -- (r - len, 0)) rotated angl withpen pencircle
scaled min_thickness;
          escala:= escala + 0.5;
        else:
          angl := 90 - 6i;
          draw ((r, 0) -- (r - .5len, 0)) rotated angl;
        fi
 endfor
pickup pencircle scaled min_thickness;
numeric  minute;
%randomseed := uniformdeviate infinity;
minute:= 50 + uniformdeviate(20);
 pair A, B, C;
A:=1.6r*right+.5r*up;
C:=1.6r*right+.8r*up;
B:=1.1r*left;
 drawarrow origin -- r*dir(90-minute*6) cutends (0, 1.5len);
fill fullcircle scaled .75len;
draw unitsquare xscaled 3.2r yscaled 1.5r shifted B;
label("\tfb I (A)", 1.2r*up+1.6r*right) withcolor red;
draw fullcircle scaled 2len shifted A;
draw fullcircle scaled 2len shifted C;
\stopMPcode
\stopbuffer



\startbuffer[programa]

\placefigure[center,nonumber]{}{
\startcombination[2*1]
{\externalfigure[ammeter][type=buffer,width=8cm]}{\_\_\_\_\_\_\_\_\_\_$\pm$\_\_\_\_\_\_\_\_}
{\externalfigure[ammeter][type=buffer,width=8cm]}{\_\_\_\_\_\_\_\_\_\_$\pm$\_\_\_\_\_\_\_\_}
}
\stopcombination

\stopbuffer


\starttext

\dorecurse{3} {\getbuffer[programa]}

\stoptext

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