Date: Sat, 5 Jul 2014 10:42:42 -1000
Reply-To: "SDF ( aka ;jim lahey' - Scott )" <scottdaniel@TURBOVANS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "SDF ( aka ;jim lahey' - Scott )" <scottdaniel@TURBOVANS.COM>
Organization: Cosmic Reminders
Subject: Re: Update on AFM testing procedure
In-Reply-To: <vanagon%2014070516145541@GERRY.VANAGON.COM>
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the ecu doesn't read the temperature of the wires in the MAF sensor.
it reads the change of resistance that occurs as more or less air flows
over the wires. The ecu is sending a small current through the wires.
( and a 'burn off' cycle is a common strategy )
There are some MAJOR advantages of a MAF Sensor over an Air Flow Meter.
- no moving parts ( ! ! ) no contacts to get dodgey or worn either
- changes in humidity and barometric pressure are recognized by the ecu
by how the Resistance of the wires change. Very 'smart' that way.
I gather there have been attempts to make a MAF sensor for Digifant
waterboxer fuel systems, without success.
Scott
On 7/5/2014 9:15 AM, David Beierl wrote:
> At 02:16 PM 7/5/2014, Neil N wrote:
>> usually clean it when I replace the air filter. David's comment below
>> makes me think that "yes", it burns off any oil during use. There is a
>> protective mesh on this MAF.
>
> The one that I read up on long ago would heat the filament to at
> least red hot, possibly white hot for a short time at some point in
> either startup or shutdown cycle, possibly both. Being platinum it
> was much more resistant to that sort of insult than to anything
> touching it.
>
> In use it would maintain the wire at a specific temperature and
> monitor how much current was required at that moment to do so. That
> would allow calculation of how much heat the wire was losing to the
> flowing air and thus what mass of air was contacting the wire in each
> measurement cycle. And finally from that the mass flowing through
> the intake. I think maybe they kept the wire hot enough that
> differences in intake air temperature were swamped - just short of
> glowing tickles the back of my mind somehow. What I'm not sure I
> understood at the time and certainly don't remember now was how they
> measured the temperature of the wire so they could control it. I
> have a bit of a feeling that it was something diabolically clever
> that I thought would have to conflict with what else they were doing
> with the wire,
>
> The wire needs unobstructed airflow (and hence heat transfer) on its
> surface and because it's so thin very small particles on the surface
> can have considerable effect. I think it really has to run squeaky
> clean all the time.
>
> Yrs,
> d
>
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