Method Failsafe for inert gas method?

[busybee]

I was thinking on how to add a failsafe to the inert gas method

If one were to use the inert gas method would it be possible to, via a valve with a programmable timer, have another gas added after a certain amount of time?

Hydrogen cyanide can be absorbed via skin and eyes and when one is already passed out from the lack of oxygen pain wouldnt be a concern. Also the body gasping for air after passing out would have the death really speed up with that inside the hood.
Plus with a with a warning sign rescuers could not remove the hood without personal protection and that would delay the "rescue".

 

[anaerobic_bacterium]

it's better to invest in cylinder's volume than bothering with it. cyanides are illegal to posses in some countries, buying of HCN cylinder is impossible for a physical person without license