Having used both regular ice and dry ice, I will confirm that dry ice is better at cooling the fog to a lower temperature, and lasts longer. For the price, to me, it's worth using it. You can also use less of it than ice because it's colder, so in the case of putting 10 lbs of ice, perhaps 5 lbs of dry ice would work.
Keep in mind, as has been said on various posts, there are a few important factors to always remember for the ultimate low lying fog.
1. Outside temperature. If it's cold, chances are even with dry ice, your fog will be warmer than the outside ambient temperature and rise fairly quickly. One way to counter this is to read #3.
2. Duration of warm fog in contact with ice. There is a trade off obviously of the power of your fogger, the type of fog juice, and how long it is in contact with the ice before it exits the final whatever it is traveling thru. I had the Chauvet 1250 last year and despite the debacle of the units having only 800 watt heaters, my fogger was MUCH too powerful for the 60qt igloo vortex chiller I built. The fog came out immediately even with packed dry ice and while it laid low.. it rose fairly quickly. This may have been partly due to #1 above, and partly because the fogger was just too powerful for the little chiller, hence.. this #2, not enough contact time with the cold ice/dry ice. I do believe in this case dry ice is better.. if you plan to use a more powerful fogger with smaller chillers, definitely look at either turning down the fogger output (which also helps keep reheat cycles to a minimum and possibly giving you a more continuous flow of fog), and if possible use dry ice given its much colder characteristics.
3. Spray water over the land/grass/whatever, cold water if possible, to help keep the area the fog is traveling over colder, longer.
4. Type of fog juice. As stated above, the best juice by popular opinion is Froggy's. It has a great hang time and is about the same price as any other gallon of juice. That said, experiment a few days before your show... lay down some water, try some regular ice if you can make enough for testing it, and see if you can, grab different qts of juice. I have some Black Label I think it is, and I don't think it's that great. However, I've yet to really be able to test it, so I could be wrong. It's gotten pretty good reviews on the net. Froggy's is still the best and if I didn't have almost 2 gallons of this stuff, I'd have Froggy's this year. It also depends on the realism your going for. Real fog doesn't stay around for a long time, it moves and disappears. Fog juice that says it evaporates faster is better for low lying chillers because it should evaporate before it can warm up and rise. That said, if you're trying to cover a large area and your output is 30 feet away from the end of the area you're trying to cover, chances are the fog will be gone before it can spread that far. To achieve that result, you either need a 2nd fogger and chiller, or perhaps a more powerful fogger and do the long pvc/abs tube with perforated holes along the path, and route it around the areas you want to have fog. Then, load it up with dry ice (so that it lasts all night) if you can, and go to town. That will give the fog longer travel time and will give you more coverage.
BTW, if you and a few others in the area are all looking to do dry ice, and don't have anything near by or want to drive, but have a little money (combined perhaps), for about $300 or so you can get a dry ice making kit, and then you fill up the cartridge with CO2 every so often. The kit plus 1 tank makes quite a bit of dry ice. The costs of filling up the CO2 is far less than buying the dry ice in stores.. but this is only useful if you have enough people interested in pitching in. Heck, if I lived in a remote place I'd consider it, but not many I know do this chiller stuff and its only .89 a lb at Safeway up the street. I only use about 15 or so lbs I believe. I am that crazy tho, that I might actually go buy the kit anyway, so I can make my own dry ice whenever... but not this year.
