Steve Thompson wrote:
>Sugiyama wrote:
>> Hugh,
>>
>> We can see it in here.
>>
>> http://www.g8wrb.org/data/Eimac/3CX1500A7.pdf
>
>As well as a data sheet with airflow figures, you can also find an
>article describing how to estimate what flow/pressure a blower will
>produce. In general, CFM ratings are not a useful guide - you need to
>know if a blower will produce enough pressure.
Umm, beg to differ. What actually provides the heat transfer is the mass
flow rate of air (grams per second) and its specific heat. Unfortunately
the mass flow rate of a gas is very difficult to measure.
One step away from the true mass flow rate is the volume flow rate
(cubic metres per second, cubic feet per minute, or whatever). If the
temperature and the atmospheric pressure are also specified, then the
mass and volume flow rates are accurately proportional. Volume flow rate
is much easier to measure, so most tube manufacturers specify this
parameter.
The tube manufacturers will always have derived the pressure-drop
specification from a volume flow rate measurement, so pressure drop is
one more step away from the thing that really matters. But more
important than that, the tube manufacturers don't tell you that
*accurate* measurements of small pressure drops are much harder than
they seem; or how they themselves did it. The measurement depends on
inserting two probes into the air stream, but different shapes and
orientations of the probes can give widely varying results.
When I worked in a lab that had all the equipment for measuring volume
flow rates and very small pressure drops, it was easy to see how much of
a crap-shoot the pressure drop measurement can be. Volume flow
measurements are so much better... if you have the equipment to do it.
Having worked in the lab, I was hung up on the "need" for expensive and
specialized instruments like miniature vane anemometers. Then on this
mailing list K6GT described how to measure volume flow rate quickly and
easily, by timing how long it takes to inflate a thin plastic garbage
bag. Problem solved!
Although the accuracy still isn't better than 10-20%, it is plenty good
enough for amateur purposes - and please believe me, it is FAR more
accurate than attempting to measure a low pressure drop with a home-made
manometer.
There are two tricky parts. One is to devise a way to place the squashed
bag very quickly over the air outlet of the amplifier, *after* the
blower is running up to speed, and to seal it around the edges while
starting a stopwatch with your 5th hand. This part can be done with
play-school engineering involving cardboard and parcel tape.
The other minor problem is to determine the inflated volume of your
standard garbage bag. This involves pouring a known volume of water
into the bag, while the bag is supported in a barrel of water.
Fortunately it only has to be done once (or take it that a standard
European swing-bin liner is 50 litres).
--
73 from Ian GM3SEK
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