[AMPS] Re: [HomeBrew] Fan->make correct and 8877 vs GS35

[Ian White, G3SEK]

John Kirk wrote:
>
>Hi Xu:
>In general, the square "muffin" fans will be inadequate for cooling large
>external anode tubes.  The type you want to look for are known as "squirrel
>cage" because they have inside, a round blade assembly that looks like those
>things that caged hamsters and rats exercise on.
>Even with them, I don't think power consumption equates real well to air
>movement - best to try to dig up the manufacturers specs, or conduct a test.
>I think G3SEK's website and books have some material on how to measure air
>flow.

Nothing on the website or the "DX Book" but it's very simple: you time
how long it takes for the outlet air to fully inflate a light plastic
bag. The idea comes courtesy of George, K6GT.

The volume of the bag can be a bit tricky. In Britain a standard kitchen
swing-bin liner is 50-55 litres, but you can measure it by pouring water
into the bag. (The really clever trick is to support the bag in a barrel
of water while you're going it. The guy in GJ who told  me that said he
learned it from Archimedes.) 

The other slightly difficult bit is to work out how to start with the
bag crushed almost flat, and whip it quickly over the output - because
you have to make the whole measurement while the blower is already
running at full speed. I usually tape two sides, and very quickly pull
the other two sides tight around the case.

With a bit of practice you can get repeatability better than 10% which
is fine for this application.

It's a very good idea to make a mock-up with the blower, the tube and
the important parts of the air-flow circuit.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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