Not sure, I'll have to look into it a bit deeper.
I just have a photo in a book called 'Vietnam, the Australian experience' that shows Aussie engineers using a small portable pump ( looks like a small petrol engined generator or compressor) hooked up to three or four 55 gallon drums and pumping a dense white gas into a hole in the ground. (there is a small cloud of gas around the hole that I presume is a tunnel entrance)
The caption says they are using CS gas and pumping it into VC tunnels before sending in tunnel rats.
Damn sure I wouldnt want to be the bloke they sent in that hole after filling it with CS, i know that much.
Pretty sure Charlie got wise real quick and started adding water filled S bends and such to defeat gas attacks.
Apparently they allso used coloured smoke in an attempt to locate tunnel exits, airholes, chimneys etc, pumping it in the same way they did with CS.
I'll search around a bit and see what I can find on the net.....would just scan piccie in book, but I dont have a scanner at home.