W7CW Jay E Ostrem wrote:
> This may be an over simplification, but I think Rich is alluding to
> it;
>
> Let's say you are measuring back pressure on an installed tube,
> utilizing a
> manometer. It seems to make sense that if the back pressure is correct
> for
> the altitude, then the cfm should be correct also.
>
> It should be possible though that you can reach a point where the tube
>
> cannot pass anymore air, but the back pressure will increase.
>
While its probably true that if the pressure is okay, then the flow is
too *if you use the correct bases*, you can not apply this if you make
your own bases, which many amateurs do. Neither can it be applied in
the case of some of the triodes (3CX5000A7, 3CPX5000A7, YC156,
3CX15000B7) and others, where you need to cut some slots in the chassis
for air flow. Eimac dont tell you how big or how many slots to cut on
the data sheets for any of those tubes mentioned, so although they state
the back pressure, its measured under conditions that can't be repeated.
This is not helpful, but from the data sheets I've seen, is not unusual
either.It is possible to calculate flow, by measuring velocity pressure,
which is different from static pressure. In effect you make a pitot
tube, and measure the pressure caused by the velocity of air. Hence the
pitot tube is aligned along the air flow, not at right angles to it, as
should be done when measuring the static pressure. You then need to
make lots of meaurements, since the velocity wont be constant across the
face of the pipe down which the air is flowing. There is some formula,
that then gives flow. However, the method requires that measurements
are made well away from discontinuites, which is no mean feat with a
tube/valve. Hence I dont think its practical for measuing tube/valve air
flow. It might be possible if you put a long pipe on the anode, but then
process of doing this will probably effect the measurement.
The pressure drop along a pipe at fixed flow, is, if memory serves me
correct, inversly proportional to the 4th power of the diameter. Hence
doubling the diameter, reduces the pressure drop by a factor of 16.
Hence the space allowed for air flow in bases *might* possibly have a
quite large effect on the pressure drop. Most of these theories only
apply well away from discontinuitites, in laminar flow, so its anyones
guess what will happen close to discontinuities, with lots of
turbulance.
The flow should not stop increasing as you increase pressure, but it is
approximatly a square law (for turbulant flow), so large pressure
increases are needed for a modest increase in flow.
G8WRB
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