why is this smoke dissipating?

Basic question, but why is the smoke in the attached file dissipating after a few frames?
What decides dissipation apart from the Smoke Dissipation setting?
explosion_001.zip (24.2 KB)

Ah, this is because smoke is not really smoke - looks like the opacity is mapped to the SPEED grid channel instead, so as the explosion dies down, the speed reduces and the opacity decreases:

One day I hope I’ll be able to say I’m proficient in Phoenix FD. I keep making these silly mistakes.
Thank you!!

Hehe, no probs!