Por que meu extintor é esvaziado após um uso?

If it's a dry chemical extinguisher (seems likely, most common, particularly with a pressure gauge) the simple answer is that the valve has got dry chemical dust in it and no longer seals properly as a result. When refilling the valve or valve parts will either be cleaned or replaced as needed, the dry chemicals will be placed in the container, the valve installed, and clean gas will be added to pressurize the tank, which will not contaminate the sealing surfaces of the clean valve. When you discharge the extinguisher, powder and gas both flow out through the valve, and some powder stays in the valve. Leaks ensue.

The instructions on the side of the (refillable, servicable) ones I have state that they should be fully discharged and then professionally refilled immediately after any use. So the above would not be an issue.

If you want an extinguisher you can partially use (and particularly if you are building a CO2 tank) look for a CO2 extinguisher. They don't typically have gauges (since a gauge would only tell you what the temperature was) - you pesar them to see how much is left. The instructions will still want you to refill after any use (because that's how fire extinguishers are always labelled, to be ready in the event of a need at their rated capacity) but practically speaking most CO2 extinguishers will reseal (there is no dust, just gas) and if you have a clearly labelled "normal" extinguisher and a CO2 that you label as "not for use as an extinguisher" you could do that - or you could just get a small CO2 tank that isn't an extinguisher at all, which would be even more clear.

Fire extinguishers (here in the US anyway) are REQUIRED to not be usable after being discharged, partial or not, because you can never know HOW MUCH extinguishing material was discharged by just looking at the pressure gauge. So the valves are designed with breakaway seals that, once broken, will not hold the charge for very long, forcing you to replace it or have it recharged and re-certified by the manufacturer. These are devices used in saving lives, it's all very serious from their perspective.

While not strictly speaking "single use", I would consider a typical consumer-grade residential fire extinguisher (I have 2 - one on each floor, with the upstairs one near the kitchen) to be a single-use item. This is for a few reasons:

• Even a moderately sized fire could make good use of the entire extinguisher, so if it is "half used" it is already in the mode of "possibly not enough for when you really need it again".
• While the main case is pretty simple and sturdy, the firing mechanism has more small (and possibly plastic) parts and is therefore likely to have problems after use. Remember, that one use in a fire involves pushing a lot of chemicals at high pressure through a small mechanism very quickly - easy for that to cause damage to the mechanism.
• The price ($15 - $50 typical in a quick search) doesn't allow for super-high-quality mechanism, so it is "just good enough to do the job".
• Because the price is low (which encourages people to buy an item that they will hopefully Nunca use!), it is not cost-effective to professionally recharge these fire extinguishers, which means the mechanism does not need to be designed for multiple uses.

You may ask:

If this fire extinguisher is only going to be used once, why bother to have any pressure gauge on it at all?

The answer is that because there can be leaks for other reasons - e.g., fire extinguisher dropped but did not activate, corrosion of parts, etc. - the pressure gauge provides an easy way to tell very easily whether the fire extinguisher is likely to be functional when it is really needed.

Fire extinguishers with plastic heads cannot (or should not) be refilled (recharged)...only metal heads.

Plastic heads can split, crack, etc. over time. Either they won’t take a charge or they’ll loose a charge over time.

Apenas um aviso:

An accident with kilogram amounts of pressurized CO2 in a constrained space could turn that space into an actual "gas chamber" - there could be injuries from losing consciousness and falling, or even death or permanent damage from choking.

There are (at least) two types of fire extinguishers, ones that are preloaded and ones that you must load before use. The ones preloaded have the chamber, that contains the extuingishing medium preassurized. The ones you must load have a small gas tank and when you want to use the extinguisher, you press a button that releases the gas in the tank with the medium. Possibly you've got an extinguisher of the latter kind and the medium tank is not made to keep the preassure for a long period. Hint: if the extinguisher says one has to load it by pressing a button or the handles prior to use, this is probably the case.

Source: fire protection training in Germany, possibly fire extinguishers in the US work differently.

Can't help you with arts and crafts reuse, as that's off topic here. Do hydrostatic test the tank to 225% of working pressure. Better to go "tink" than "BOOM".

For use in a home,

Take the extinguisher to get it refilled

There are shops which do this, because commercial enterprises have their extinguishers refilled every few years. Extinguishers expire, after all.

Independentemente, you should refill it because you partially used it, and extinguishers are designed to be used on a single fire. The on/off isn't so you can use it on later fires, it's so you can use it efficiently to attack the same fire.

Some low-end extinguishers are not refillable, and are indeed one-shot devices. If that's so, the refiller will tell you that.