Why does refraction + volume scatter cause a much darker result? I've noticed that glass objects tend to turn really dark when adding volume scattering.

It seems the volume scatter shader gives a much different result when a refraction shader is used in the surface, even if the aforementioned refraction shader is theoretically doing nothing.

Here's a test, rendered with 1000 samples:


On the left is a refraction shader with the color and IOR set to 1, essentially just an un-optimized transparent shader.
Naturally, it looks like nothing is there (though some extra noise can be observed if you look closely at the ground where it should be)
On the right is a volume shader setup with a true transparent shader. Looks fine.
In the middle, the volume setup from the right is used with the "transparent-refraction" from the right.




As you can see, the middle looks much different from the right, despite the refraction shader making no change to anything in the left.

Exact setup for the middle Suzanne:



What's going on here?
Solution

Set Properties panel / Scene / Bounces / Volume value to at least one.

Explanation

Probably your volume bounces is set to zero because it is the default setting. That means only direct lighting on volumes are allowed, however caustics¹ are not. The same goes for the diffuse bounces value: If you set it to zero, caustics will not be present² and the perfectly transparent refractive object will cast shadow.

¹ Caustics is actually simply indirect lighting, but is handled specially due to its noisy nature.

² No recursive indirect sampling (also known as bounce, hence zero bounce) allowed.

Alternative Solution

You could also disable shadow casting on the refractive object and leave the volume bounces zero. The result will have less noise, but will lack proper caustical shadows. Or you could set up fake caustics like this one: https://urchn.org/post/fake-caustics-in-cycles