Re: [T3] Dual 40 DCNF Carbs

[Jim Adney <jadney@vwtype3.org>]

On 23 Sep 2002, at 7:40, Jens Vagelpohl wrote:

> > Aren't air flow and air mass metering the same thing?

> they are not. air flow just measures air volume with the help of the 
> spring-loaded flap. air mass metering also takes the ambient air 
> density/pressure and temperature into account. it uses a small heated 
> wire in the intake air stream that gets cooled down by the air moving 
> through the intake unit. the air mass is calculated from how much this 
> wire gets cooled down by the air.

Even the flap still measures air mass (per unit time), because the force on 
the flap is a function not only of the air velocity, but also of the air density. 
It's a momentum transfer problem, thus taking into account both density and 
velocity, giving a result in mass per unit time.   

The heated wire approach is just a different (and better!) way of doing it, 
because it doesn't introduce any significant impedance to the airflow. I think 
it is also more accurate over a wider range of flow velocities. This is a 
standard mass flow technique, but it requires certain knowledge about the 
specific heat of the gas being measured. Fortunately, the gas in this case 
does not change in composition, just density, so this is not a problem in this 
case.  

I have no experience with these systems, but I like the idea of there being no 
moving parts. OTOH, I wonder how many of these wires get destroyed by 
someone who sticks a screwdriver down there. I assume the wire is thin and 
somewhat fragile. Anyone here ever worked with one of these?  

I used to work with a device that we made which incorporated a tungsten 
wire that was either .0005 or .001" dia. I have to admit that it was quite 
strong in spite of being nearly invisible. So maybe these systems really 
aren't particularly fragile.  

-
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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