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I read occasionally that owners of T&M equipment tire of the noise a cooling fan makes and asks for help to determine if/how a quieter replacement can be found. There are many factors at play in such a decision, none as simple as "it's quieter".

This web site addresses choosing a fan for PC enclosures, and many of the criteria are the same (heat being the most important one for us):

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A quote:

"Over the years, we have observed one clear phenomenon about fans and cooling: The relationship between airflow and temperature invariably becomes exponential at some point. Increase airflow from nothing to something, and the drop in temperature can be dramatic. Keep increasing airflow, and the cooling improvement becomes less and less significant, until at some point, the temperature hardly drops at all. The trick, for the PC builder who seeks both good cooling and low noise, is to find the point where any decrease in airflow (or fan speed) effects a significant increase in temperature, while only a very large airflow increase effects a significant temperature drop. In other words, once you have enough airflow, additional airflow has very little cooling effect, so all you're doing is increasing noise. "Enough airflow" is not a constant, of course, it varies for each system of components."

FYI,
Dave

One have to consider the specs from the manuals about operational
conditions. Most manuals for HP Test Equipment states under "General
Specificatioms - Environmentals" operating temperature 0 - 40, even 0 -
55 dgC for HP8866A/B.

That means that the intake air can be up to 55 dgC, and the beast will
still run cool enough. Imagine how hot the output will be!

Try to measure your input and output temperature under normal conditions
in your shack, and then calculate your margin. You may have a margin of
25 - 30 dg. Then select fans that will deliver adequate air, and enjoy
how
quit it can be.

Best 73 de OZ5TG, Vern

I'm not sure what point silentpcfan were trying to make: the advice is
so vague that you can't make any practical use of it.

For any "box" of electronics which incorpoates a fan, there is the
simple electrical power-in = heat power out balance, and if you make
the pessimistic assumption that no heat escapes from the case (by
convection), you can calculate the air-flow required for a given air
temperature rise (T airin v Tairout) knowing the thermal capacity of air
[=circa 33 joules/cu ft/deg C].

They may be tangentially hinting at the characteristics of finned
heatsinks when force-air cooled. As the air velocity rises, there is a
gradual increase in the temperature drop along the fin (i.e from root to
tip at rt angles to the air-flow). I recall this parameter being
quantified as "fin efficiency": if the tip is cooled to aroud that of
the airstream, it becomes relatively ineffective and the fin should
either be thicker or shorter (or both).

John


--- In hp_agilent_equipment@..., Dave C wrote:

I read occasionally that owners of T&M equipment tire of the noise a

cooling fan makes and asks for help to determine if/how a quieter
replacement can be found. There are many factors at play in such a
decision, none as simple as "it's quieter".

This web site addresses choosing a fan for PC enclosures, and many of

the criteria are the same (heat being the most important one for us):

A quote:

"Over the years, we have observed one clear phenomenon about fans and

cooling: The relationship between airflow and temperature invariably
becomes exponential at some point. Increase airflow from nothing to
something, and the drop in temperature can be dramatic. Keep increasing
airflow, and the cooling improvement becomes less and less significant,
until at some point, the temperature hardly drops at all. The trick, for
the PC builder who seeks both good cooling and low noise, is to find the
point where any decrease in airflow (or fan speed) effects a significant
increase in temperature, while only a very large airflow increase
effects a significant temperature drop. In other words, once you have
enough airflow, additional airflow has very little cooling effect, so
all you're doing is increasing noise. "Enough airflow" is not a
constant, of course, it varies for each system of components."

FYI,
Dave