Hello! my group are designing ofwhat we called a mini-submarine..it is somewat like an underwater surveoyr..but in a submarine form of construction. our adviser told us, that we could use a buoyancy controlling device to address our problem of getting our mini-submarine to float and submerge..but a BCD would cost much..we would just like to know if there is any way we would improvise on buoyancy problem by not using a BCD? Is there a DIY buoyancy controlling device,and thus not buying the real BCD used by scuba divers? We're still deciding and designing what our mini submarine would look like..and it's size. Thank you..any inputs are very much appreciated.