From owner-freebsd-arch@FreeBSD.ORG  Tue Mar 18 19:59:42 2008
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To: Brooks Davis <brooks@freebsd.org>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Tue, 18 Mar 2008 13:54:35 EST."
	<20080318185435.GA2853@lor.one-eyed-alien.net> 
Date: Tue, 18 Mar 2008 19:59:40 +0000
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Cc: arch@freebsd.org, Robert Watson <rwatson@freebsd.org>
Subject: Re: Power-Mgt (Was: Re: cvs commit: src/sys/i386/cpufreq est.c ) 
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In message <20080318185435.GA2853@lor.one-eyed-alien.net>, Brooks Davis writes:

>For amusement value, I had a dual P4-Xeon box hooked up to one once
>and found that power consumption with SETI@Home running was about 10W
>_lower_ than idle.

That is a very good indication that the meter is a piece of crap that
does not have sufficient measurement rate to do a relevant job.

Unfortunately, that is the case for most of the gadgets you can buy
in shops.

In general, you are much better off buying the real thing, for instance
a single-phase DIN power-meter like:

	http://www.metermaid.co.uk/din_rail_tech_info.html

They cost less than EUR100/USD150 and have 1% accuracy.

The "SO" output can be hooked up to a parallel or serial port and
you can accumulate and read the number of pulses using the PPS-API,
giving you, in this case, 500mWh resolution.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.