I am trying to render a prt volume, however the scale of the scene is so huge that when I render it renders black. I know it is rendering because when I turn on additive mode it renders the particles and I do have lights on in the scene. I think that the particles are so small that they are not rendering or something. Does anyone have any ideas on what could be the issue here and suggestions on how I could fix this? Thanks!
Could you reproduce the problem with the same scale, but in an empty scene with just a huge box or teapot turned to PRT Volume and the same camera/lights from your problematic scene? If you could post an example of the problem as a MAX file, it would be easier to answer what is going on…
I tested this here with a box primitive with a size of 10,000x10,000x10,000 units turned to PRT Volume with Spacing of 200.0.
When lit and rendered at default Density, the alpha was so low (around 0.0001 or so) that the particles were practically invisible.
This is because each particle participates volumetrically with a default constant value of 1.0 per cubic generic unit. If your scene is huge, you will get one particle with a value of 1.0 per 100000 or so cubic units. (This is quite similar to distribution of matter in outer space 
). In short, Krakatoa calculates the volume in cubic generic units a pixel in the final image covers and scales the densities accordingly. If the pixel represents a vast volume with just one particle with a density value of 1.0 per cubic unit, the final pixel will represent a very very low density and thus its alpha will be close to zero.
The easiest solution is to increase the Final Pass Density Exponent in the Main Controls rollout to 10 or higher (depending on the actual scale of your scene) to compensate.
Hope this helps.
Thanks bobo!
Looks like that was the problem. I ended up cranking the density to 1000 and the exponent to -1 (if you can believe it!) .
Anyway, thanks for your help! 
You are misunderstanding the Exponent I am afraid.
The exponent is there so you don’t ever have to enter 1000 in the Final Density field 
Basically, 1000.0 E-1 means 1000.010^-1 = 1000.00.1 = 100.0
Instead of 100.0, you are supposed to simply enter 1.0 E2 which is 1.0*10^2 = 100.0
In my case with a cube with size of 10,000 and Spacing of 200.0, I had to actually use 1.0 E10 which is 1 with 10 zeros, or 10,000,000,000.0 if YOU can believe it 
Note that the Exponent goes up to 30, so you can express a number with 30 zeros (!!!), or even more if you add more to the Final Pass Density value…
Thanks again Bobo!
I get it, the exponent works in a plus and minus range. I didn’t realize that but now I do, thanks.