Co2 / gas bulb handheld prop

[TomHoward]

Hi

 

I've been asked to help make a handheld prop using a threaded or non-threaded CO2 or NOS bulb, which the end result is essentially squirting liquid on a button press.

 

I remember some old LM mini confetti cannons that used a 16G threaded CO2 bulb, and had a little level or something to actuate.

 

I've been given a rough sketch that includes three chambers - one at the bottom that holds the cartridge, one in the middle that has a button-press operated valve on it, and the top one that holds a liquid and squirts it out on the button press.

 

I'm not sure about the safety of this being handheld, the connections to use on the threaded bulbs, or what type of bulb to use.

 

Has anyone made anything like this, or knows any appropriate parts, suppliers or references?

[Brian]

I remember some old LM mini confetti cannons that used a 16G threaded CO2 bulb, and had a little level or something to actuate.

 

The Le Maitre mechanism comes from aircraft life jackets. Pull the lever and the whole 16g of CO2 is instantly released. Probably not what you want if you want fine control.

 

I'm wondering if a sprayed whipped cream N2O unit might be more controllable?

 

 

[EDIT]

Chemical formula for Nitrous Oxide corrected.

[TomHoward]

Controllable is not really an issue - it's all deployed at once, to squirt fake blood (it's a Sweeney Todd razor).

I was thinking of using a reservoir that could be pumped up with a bicycle pump instead of the cartridge, but I don't think the same pressure can be reached.

 

I'm trying to find a push button valve or similar to actuate, then I might test with a reservoir from a bike pump, then move on to the cartridge if needed.

[cedd]

These cylinders are used in education to power rocket cars (take a look at the F1 in Schools website). Some searches along those lines might find you some ready made solutions. A lot use a firing pin driven by a solenoid, which I know isn't what you're necessarily looking for, but there might be a crafty way of still doing it and getting most of the co2 out of the cylinder and in to the liquid compartment to propel it.

 

Personally we had a set of co2 razors on our production of Sweeney and the biggest issue was they pretty much ruined the set after the first use. Often they'd draw a line along the wall following the path Sweeney made with the razor around the victim's neck.

I've been working on my own design for a local production that's a bit more subtle, but the production has just been cancelled. My solution used a sprung syringe to hold and drive the blood, with a mechanical catch to hold the plunger. The catch acted in multiple positions so each razor could do 3 separate cuts without being reloaded.

 

 

 

[the kid]

To be honest I would worry about the pressure behind a bulb. We used to do a film set up at college with a plant sprayer, it worked pretty well and had a good effect, the issue was it was more of a gunshot than a squirt but I think with regulation and better work, it would work.

[TomHoward]

Were the CO2 ones you had hired?

 

The design I’ve been given shows the bulb discharging into a chamber, then into a liquid from there, but it would make more sense to hold in the bulb and dispense direct from there.

 

How did you get on with the syringe? Did you drive it with a mechanical spring or air pressure or similar?

[Jivemaster]

The gas pressure in a CO2 or N2O cylinder will be enough to propel fake blood too far and too quickly, plus the cylinder gets cold when suddenly emptied. 10ml of fake blood travelling at air rifle speed and making a splash elsewhere seems a bit OTT when 2ml of fake blood dripping down the victim would probably be more effective. Yes do account for the work to re paint the set if blood hits it.

 

One suggestion from the www is that the pressure in a CO2 cartridge is about 70bar, that deserves careful engineering.

 

Cycle shops sell CO2 cartridges and tyre inflator devices. A triggerable inflator device may be a good start.

[ImagineerTom]

So essentially what you're trying to create is a water pistol.... so why not use a waterpistol and just extend the squirting tube?

 

you definitely definitely don't want to be messing around with compressed gas when you don't even know the basics of how it works and when it's a completely overengineered sledgehammer being used to crack an already de-shelled nut.

[Dave m]

Cant't you use a squeezable razor handle?

[alistermorton]

I've been working on my own design for a local production that's a bit more subtle, but the production has just been cancelled. My solution used a sprung syringe to hold and drive the blood, with a mechanical catch to hold the plunger. The catch acted in multiple positions so each razor could do 3 separate cuts without being reloaded.

 

Beat me to it - I was about to suggest a spring syringe rather than a gas cartridge.

 

 

 

[TomHoward]

To be fair I’ve been presented with drawings of a unit that uses gas, and asked to look into the workings of it - gas is more of a starting point than a conclusion, which is why I asked as it sounds a bit over-engineered to me and handheld sounds a bit risky.

 

Water pistol pump pumping fake blood sounds like it is worth investigating, and it doesn’t need consumables either.

[Dave m]

[cedd]

The units we had were indeed hired, although I've not got the foggiest idea where from. I was on sound and the company who staged it don't exist any more.

 

The syringe idea was still being worked on but I had options for either a sprung version (but was struggling to keep the tension up throughout the entire stroke of the syringe as the spring extended).

The idea was that the handle would contain the actual mechanism and the blade would be hollow (probably 3d printed) with passages throughout to distribute the blood to the blade edge. To load it you'd remove the handle, stick it in the blood (or attach a tube dipped in to it) and then screw a piece of 4mm threaded rod in to the end furthest from the blade, screw it in to a captive nut attached to the end of the syringe plunger and then pull it back to "######" it. Once latched the rod would be removed and the unit was then armed. Attach the blade again and it's good to go.

 

The other option was to produce a motorised driver which would then have allowed for better control of flow but would have been relatively slow due to the gearing required for such a small motor.

 

In short, definitely a work in progress!!!! I never properly started as the production got started, and whilst I toyed with building a set anyway to have in stock for hire, it's not high up my list of priorities.

 

 

 

 

[alistermorton]

Could the talent not operate a thumb slide on the handle to eject the blood, I wonder. Easier than faffing around with springs and motors, fun though that is.

[Malcolm Gordon]

 

The syringe idea was still being worked on but I had options for either a sprung version (but was struggling to keep the tension up throughout the entire stroke of the syringe as the spring extended).

Would a constant force 'Tensator' type spring such as by Spiroflex work here? One could perhaps adapt a brush spring as here. I've no idea if singles are stocked and can be bought.