A 3ft balloon uses quite a lot of helium to fully inflate it however you don't always need to fill them to bursting. Another consideration is that if you are filling them in a room where they aren't going to be displayed then you have to consider the widths of the doors between where you are and where you need to get to! I learnt my lesson the hard way when decorating in the Sheene Mill in Melbourn which is an old 17th century mill. Having inflated the balloons in the conservatory I went to take it through to the function room on the other side of the building and came across a really narrow door which couldn't have been more than 27" inches wide. Luckily there were french doors in the restaurant and the bride hadn't arrived yet so I was able to transport them around the outside of building.
Having said that I usually only ever inflated to about 30" anyway unless particularly requested otherwise because of the amount of helium used inflating just those few extra inches and if inflated to 30" they will still float for 3-4 days where as inflating to 34" will probably give you an extra day, 3-5 days.
A 3ft balloon inflated to 30" will use 8cu ft of helium (.2266cu m)
A 3ft balloon inflated to 34" will use 15cu ft of helium (.4249cu m)
A large disposable helium tank holds .42cu metres so there is only enough helium to fill one 3ft balloon if inflating to 34" but inflate to about 29" and you will get 2 from it.
If you are looking to inflate more that 1 or 2 then you need to be thinking about renting a tank from someone like Airproducts. You will get about 7 (30") or 4 (34") from a 10litre tank and 15 (30") or 8 (34") from a 20litre tank.
3ft balloons are quite tough to tie by hand. It can be done but you need strong fingers, Hold the inflated balloon in your left hand (if right handed) then pull down on the neck and wrap it half way round the back catching hold of the lip between my 3rd & 4th finger of your left hand. Let go with you right hand and then bring your hand to the front and grab hold of the neck and place it between your thumb and forefinger you can then pull the neck over the lip to tie a knot. To make it easier you can tie a small piece of ribbon to the neck first then you can use the ribbon as leverage to help you pull the lip through.
In the video below the guy is using a balloon seal with ribbon to secure the balloon. A balloon seal folds over the neck and then nips it to stop the helium escaping - not quite as secure as tying a knot but works well.
See how he also uses the cabinets as a measure of the size of the balloon - just measure the space from the nozzle to the cabinet and when the balloon touches the cabinet it is to your desired size. However don't forget to inflate where the handles won't affect your measurements or take them into consideration when measuring if they are unavoidable.
The guy uses a 260Q (modelling balloon) to secure the balloon to the nozzle. Personally I have never found this necessary I have always held them on with my hand but if you want the extra security and don't have a 260Q you could use ribbon, string or a cable tie.
If you do use the balloon seal then make sure you put it on before inflating so that you don't have to fiddle about trying to get it over the neck once filled with helium.
Hope this helps but if you have any question please ask!
