Gassy Ballast Systems
No, it's not a case of bad indigestion. Gas ballast systems have been employed in RC submarines for decades. They are very simple, light, and take up very little room in a cylinder. They also make really cool bubbles!
Basically, the energy required to displace water from the ballast tank is stored chemically in liquid state rather than electrically as it is in the case of a pump or geared ballast system. This has the advantage of using far less onboard power during vent and blow cycles. Of course, the downside to the system is that when you're out of gas, you're out of gas, hopefully not at the bottom of the pond.
Today's systems typically employ battery powered ballast solutions, or, in the case of our series of SubDrivers, a hybrid pump/gas solution.
Regardless, I am still a huge fan of gas ballast systems, and finding the liquid gas needed for these systems has been harder and harder to do. Fortunately, a few enterprising individuals have found an alternative, or at least brought this alternative to light after many years of it going by the wayside in favor of airbrush propellants.
The following was penned by a gentleman by the name of Ed Tordahl, who took the time to do some research concerning the different types of gas propellants that can and have been used in RC subs. This is great reading if you're a engineering type of person. If you could care less about the math, skip to the end to see his summary of effectiveness and cost of the three different gases he analyzed.
A while back I did some research on the expansion ratio (Liquid to gas) of gases that we use. Because I'm kinda nerdy here and always love peeking under the hood to get a better understanding rather than the usual conjecture and hearsay we come by in this hobby. I heard once it was like 300:1 (from Skip himself), so this seems kinda right. BTW, I know the average hobbyist doesn't care, but I'm NOT average!
A VERY difficult subject to research! Many numbers, many variables, terms like Heat of Vaporization at Boiling Point kJ/kg. But after some time and cross referencing: I kinda came up wit the following, which is close enough for us:
Expansion Ratio of Common RC Submarine Gases
CARBON DIOXIDE (CO2)
Carbon dioxide (CO2) is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor. It is slightly toxic and has a threshold limit value of 5,000 ppm. At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.5 times that of air. At atmospheric pressure and a temperature of −78.51 °C (−109.32 °F), carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseous to solid through deposition. Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 518 kPa at −56.6 °C. The boiling point of the liquid is -70°F to +88°F, depending on pressure.
When vaporized at 60°F, the expansion ratio is 535:1.
CO2 exists as a gas or solid below 60 psig. In liquid form, carbon dioxide weighs 4.9 pounds per gallon. In gaseous form it is slightly heavier than air.
1,1-Difluoroethane (C2H4F2) R-152A, aka Propel
Density of gas as Liquid .91g/cu CM
Density of gas as NTP .00297g/cu CM
When vaporized at 75°F, the expansion ratio is 306:1.
1,1,1,2-Tetrafluoroethane (CF3CH2F) R-134A, aka Freon
Density of gas as Liquid 1.24g/cu CM
Density of gas as NTP 0.0045g/cu CM
When vaporized at 75°F, the expansion ratio is 275:1.
So Theoretically......
2.5cm x 10.0cm reservoir @ 75% capacity to empty an 8.9cm x 20.0cm tank volume BT.
Flask Volume: 36.82 ccm
Ballast Tank Volume: 1244.23 ccm
Ratio of volume is 33:1
So in this case if we timed EACH blow to the millisecond:
Co2: ~17.42 blows
Propel: ~9.27 blows
Freon: ~8.33 blows
Taking all this with a huge grain of salt, as I'm calculating the density of the liquid form (60 psi/ 75F) vs the density as a gas (0 psi/ 75F). It's all out of my field and I'm making assumptions because you can't just Google "Expansion Ratio". Well you can, but see what you get. I got more confused as I dove deeper lol.
So if my numbers are close to "correct". It kinda shows us this, in TODAY's prices.
Freon (doubled in two years) Walmart: $8.88/ 12 oz Can (340g) 9.23 fills of the above tank
Propel toolsource.com: $20.76/ 13 oz Can (384g) 10.42 fills of the above tank
Co2 $23.99 Crosman® 23140 - 12 g CO2 Powerlet Cartridges, 40 Pieces Above tank = ~3.06 Cartridges. For the record s single Co2 cartridge yields 5.96 surfacings
So....per Cost per blow of that hypothetical tank
Freon ~76.8/blows per Can = $0.12 per surfacing
Propel ~96.6/blows per Can = $0.21 per surfacing
Co2 ~227.1 blows per 40 cartridges = $0.11 per surfacing
Someone once said Freon yields to cracking or crazing of the cylinder, I think that may have been acrylic, not Lexan. I have had NO crazing of the cylinder. I seem to think it feels "colder", but that may be in my head.
I hope the above didn't bore you too much, and I'm NOT 100% positive of the numbers, but it's a LOT better than hearsay and I hope allows us a better understanding.
Freon beats Propel in overall cost ."
