I was talking to a friend the other day about the MAF sensor problem discussed on this Forum, and he was most interested - he needs to measure mass flow in another automotive application - this time a steam-powered land speed record car he is designing (yes, this is straight up, it's not the start of April...).

Can anyone offer thoughts on how the MAF sensor in a Scooby works? He's most interested in understanding the principles behind it and potential sources of such items.

Thanks in advance for all help! But sorry, no, you can't have a drive when it's finished :) I can recommend a visit to the car museum in Coventry, though.

In the older scoobs, it's a hot wire. The current required to keep it at a set temperature is measured. Air blowing over the sensor tends to cool it down - thus more current is required to keep it up to temperature.

Sorry - not a very good explanation - the link is a bit better - more general than just MAF sensors though :)

thermal anemometry

If it's anything like my old MAF ....it doesn't work at all !!

May be completely wrong, but as far as I understand there are three types:

- Early Imprezas used a hot-wire MAF

- Later ones changed to hot foil

- MY02 onwards, I think, use indirect hot foil


All operate in much the same way:

Take the first one as the easiest example. A piece of basically bare wire is extended across the air-intake within the MAF sensor and a voltage is applied across it. This causes the wire to heat up considerably. Due to the increase in temperature, the resistance of the wire reduces - which can be, and is measured (indirectly) by measuring the current in the circuit.

As the air-flow increases across the wire, the cooling affect cools the wire, increasing the resistance and increasing the current.

The current in the hot-wire circuit is therefore directly proportional to the air-flow across it.

Hot wires are prone to false reading through contamination - especially when oiled air-filers are used, as the oil hits the hot wire and sticks - changing the cooling characteristics.

Hot film works the same way, though bigger surface area - and I guess the foil is perpendicular to air-flow (and hence risk of contamination is reduced).

MY02 onwards use a (damn, can't remember the name for it) - anyway another pipe mounted perpendicular to the airflow, which houses the hot foil airflow sensor. The xxxxx effect causes a proportional airflow in this perpendicular tube, which the hot foil measures by virtue of current etc. As the foil is in now way in the primary air-flow it is very well protected from foreign object contamination.

May have got my increases and decreases the wrong way round in places - but hopefully you get the idea, and hopefully it's close to being correct ;)

Shades

Brilliant, thanks, guys! I'll pass that info on.

Slightly off-topic, thinking about record-breaking cars and my Coventry museum comment, how about a Scooby gathering there in the New Year? I'm willing to organise, and could get us a better tour than the usual walk around. If I get a few 'interesteds' on this thread, I'll post another and get something sorted.

Mr Shades

is it the Venturi efect ??

Midlife.....