Or, one can tape a big plastic garbage bag to the top of the amplifier and
time how long it takes to fill the bag with air. If a 20 cubic foot bag fills
in 20 seconds then you have a flow of 60 cubic feet per minute. Should agree
with the temperature calculations.
I think credit goes to G3SEK for this technique.
73/k5gw
In a message dated 7/23/2005 8:31:32 A.M. Central Standard Time,
david.kirkby@onetel.net writes:
Martin Sole wrote:
> I didn't check with the internal fan only but with both fans running and
> using my cheap Chinese multimeter with the supplied temperature probe taped
> to the top of the case in the airstream I get the following. Normal room
> ambient varies around 27 C depending on how much airconditioning is
running.
> Amp on and in standby, after the 1 minute warm up about 30 C. Normal speech
> at about 1200 W out it goes to around 78 C and with the processor on I have
> seen about 95 C. Second question, can I relate these figures into something
> useful to know if or not the tubes have enough air through them?
>
> Thanks
>
> Martin HS0ZED
With no RF drive applied, but in transmit mode, the dissipation on the
tube should be easy to calculate - it's the total of anode, and filament
powers.
>From the dissipation and temperature rise, you can work out what the
air flow is. I can't recall the formula, but you will find in in one of
the application notes at http://www.g8wrb.org/.
Then look on the data sheet and see if it is sufficient.
If you are at high altitude, you need to be a bit more careful, as the
formula for computing airflow from temperature rise will need to take
into account the lower density of the air.
This is all based around the fact the mass of one cubic foot of air is
known, and the specific heat capacity of air is known. From those, you
can work out everything, but the formula is *really* simple. I just
can't recall the constant.
--
David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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