A note on nomenclature - Object/Add/Primitive is a chain of menu commands, (F5)/Sphere means push F5 then select Sphere. It's pretty obvious, really. You'll work it out. And if I give values for a requestor, and don't give all settings, assume defaults. I'll probably get inconsistent by the end, and stop hand-holding so much.
The basic principal is to use a combination of Fog and Particilisation (Particalization for you U.S. spelling types) on an object to create a 3 dimensional cloud of gas.
In practice, do the following as a starting point, then I'll leave you to experiment. This tutorial is the process I used to create a puff from an asthma inhaler (see my home page - the 3M Inhaler in space). Took a while to come up with, but it looks great. The main areas to experiment with are Fog length and Particle size, as well as object shape, obviously. A continuous jet of steam etc could be simulated using the Particle Effect, but I'll leave that for you to explore. Should make great comet trails, too
The Gas Tutorial.
1. In the detail editor, Object/Add/Primitive/Sphere or (F5)/Sphere and set to Radius 15, and click OK. Select the bottom half of the sphere including the "equator" in Pick Points mode, and delete it. Now Object/Add /Cone of Radius 10, Height -30 (just do it!), Vert. Sec. 30 then OK. Have we just learned something new? I know I have :) Join with the hemisphere. Use Set Edge Line and Fill To Edge Line to close the shape. You should now have something like an Icecream Cone.
2. Object/Copy then Object/Paste to create a duplicate, which you need to Scale down to .66 it's initial size. Pick both objects and Join them.
3. Select Functions/Particles and set Pyramids, Specify Sizing, Size 5, Alignment Random, Centering Inscribed. Set a Random seed if you want.
4. Enter Pick Faces mode and Function/Pick Range set the last value to 2. Now delete the selected faces. Do it again. Now back to object mode. (Not sure that simply creating fewer horizontal sections in the initial objects wouldn't have the same effect - this is just how I got here). Move the object axis to the bottom (point) of the 'cone'.
5. Into Attributes, and leaving everything else alone (unless you want pink gas!) set fog length to 1.
6. Create a Default State with Object Props set in State Data Types. Now clone this State and call it Start. Change the Fog length to 200 and create another State called End, remembering to set Object Props. Save the object. That's the end of object creation. You may need to experiment with those Fog values to get the look you want.
7. Create a Project in the Project Editor (funnily enough) the in the Action Editor, make a 25 frame animation. Add our object to Frame 1 of an Actor bar and set the State to Start. Add over frames 2 to 25 with State set to End. Copy the 1 frame size bar to frame two, and set it to cover frames 2 to 25. Set Size bar for frame 1 to 1,1,1.
8. Set the Camera Position to 0,-150,30.
9. Render the animation.
This worked well for my Asthma spray, which only had to fill about a third of the screen, and was faced 3/4 to the camera. You can get artifacts of little black dots, which I suspect is caused by exceeding the EDLE amount (ie, too many transparent (fog) faces for the camera to look through). Set EDLE (Resolve depth in IFW) as high as possible - though higher settings result in slower rendering.
Later Findings : An improvement would be smaller particles in the initial "Start" states, and a greater Fog length in the End state - it doesn't fade out like it was meant to.
And a warning - the render times for this technique can be pretty high, even in Scanline mode. Frame 1 on an 040-40/DX2-66 in lores takes 7 minutes or so.
The timing is all wrong. It's too slow - depending on how big you think the steam jet is I suppose - and it needs to accelerate at the start and ease in to the end.
Let me know if anyone uses this to good effect, and any improvements anyone comes up with.