Thanks Patrick,
you are right, the result of the log is black because the values are negative, it works correclty.. stupid me. So it has something to do with the lookup table. Is there any way to check the content of the 3d texture in opengl? I tried it with this fragement shader:

varying vec2 vTexCoord;
uniform sampler2D tex1;
uniform sampler3D tex2;
void main() {
   vec3 t = vec3(vTexCoord, 0.0);
   gl_FragColor = abs(texture3D(tex2, t));
}

to get part of the content of the 3d texture. Even if the content does not make any sense, I should see something if the content is not 0.0, is this correct? So if I see nothing, the 3d texture is black.