What is something you have in your house that you're never going to use, but you have it just in case?

@ninalanyon
Where in your house are they located?

@swirlie One at the bottom of the stairs and the other at the other end of the building. Both on the ground floor. Fire extinguishers and smoke detectors are required by law in all Norwegian homes. I only have two extinguishers because one is so old that it no longer qualifies under the regulations. The fire inspector nonetheless advised me that it would still be worth keeping. Now that I think about it I should give both of them a shake to make sure the powder is loose.

@ninalanyon

I was just reading where a dry powder extinguisher only has a shelf life of 10 to 12 years before it must be recharged.

@swirlie They have a little pressure gauge on them. The old one is still at the top end of the green and it's quite a lot more than twelve years old. Of course I won't head for that one first if there is a fire unless it is something very, very, minor. The fire inspector was quite definite that it was worth keeping though and said that it was simply that the regulations err on the side of caution which is quite understandable.

@ninalanyon
The pressure within the tank is not what's in question if the gauge is still in the green band.

The problem apparently arises if the powder has started to 'cake' within the cylinder OR the powder has started to disintegrate, at which point it becomes solidified around the inner edges of the cylinder, leaving only the center core of the total dry volume available for use if the extinguisher was ever activated.

This will occur because metal is porous which means air and humidity will pass through it.. even if the cylinder is pressurized, despite the cylinder being a sealed container.

It's the humidity that gets trapped inside that causes the problem with the powder over time, hence the need to re-charge it at least once every 10 years.

@swirlie I agree about the powder settling, but because of air and water getting in. The idea that air and humidity (water) will get into a container pressurized to ten atmospheres of nitrogen doesn't sound very likely to me. After all if air (roughly 80% nitrogen) can get in then the high pressure nitrogen would also be able to get out.

Edit 20250424T1010: I missed out the word NOT in "but because of air and water getting in.". It should say "but NOT because of air and water getting in."

@ninalanyon
Why do car tires eventually go flat despite being filled with 100% nitrogen then?

@swirlie Because the nitrogen leaks out of course. And when they are flat the internal pressure is the same as the outside, the pressure difference is then zero.

@ninalanyon
..but why would an alternative like nitrogen leak out no differently than would ambient air leak out of a tire?

@swirlie A leak is a leak. It's the same process for a fire extinguisher as a tyre. If anything could leak in then what is already in could even more easily leak out. But the pressure inside the extinguisher is still ten bar which indicates that there is no leak in either direction.