[time-nuts] 5070B once more.... (actually 5370A fans)

[Didier Juges]

Part of the problem is that in some equipment (not all, but based on my
experience, most), the noise comes from not from the fan itself, but from
poor air routing inside the equipment itself. The air moving around
encumbered passage makes noise. As someone pointed out earlier, take the fan
out of the equipment for a few seconds, in most cases the noise will be much
less. So if the noise is a function of the air flowing over components, the
fan type makes no difference, the same air flow will produce the same noise.

In some cases, the fan itself may be the primary cause of the noise, and
when that happens, the fan is usually deffective (worn bearings, or
damaged/dirty blades.) In that case, replacing the fan with the same model
but new will fix the problem.

I recently replaced the fan on an HP power supply. With the fan out of the
enclosure, you could clearly tell the fan was bad, and the new one (same
model) was almost quiet. Once in the chassis, it did not make that much of a
difference. There was less whine from the fan itself, but the noise from the
air flow over the heat sinks and the grill was dominating the noise from the
fan.

Part of a good design is to design the cooling for the expected environment.
Therefore, a well designed piece of equipment will have adequate cooling,
but not more than is needed to meet the requirements, otherwise it will be
wasted money. And while much equipment is designed to operate at up to 50
degree C ambient, many only do so at degraded performance. Nominal
performance on most precision instruments is only achieved within a
typically narrow range around 25 degree C or thereabout, which is where the
equipment is calibrated.

So, think twice before reducing the air flow, regardless of other
considerations (reliability) justly pointed out by others.

Didier

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Poul-Henning Kamp
Sent: Thursday, May 21, 2009 12:12 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] 5070B once more.... (actually 5370A fans)

In message <4A15808F.4090002 at karlquist.com>, "Richard (Rick) Karlquist"
writes:

>"Modern" fans obey the same laws of physics as the original equipment.
>They don't magically produce more airflow for less noise.

Agreed: there is no magic to it.

But a lot has happened in aerodynamics since Hermann Papst invented the
external rotor motor and had to add fans to keep it cold.

In recent years noise from air transport have become a competition
parameter, and these days you can buy standard fans that move twice as much
air at the same dB level, as you would have found five years ago.

As for reducing air-flow in old kit: I wouldn't do that without a careful
session with a thermovision camera.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
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Never attribute to malice what can adequately be explained by incompetence.

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