Air flow sensor internals - Page 1
Forum section: electronics / general

author: SeanB
author_rank: Super Contributor
timestamp: August 18, 2012, 03:23:36 pm
content: I have had a old dead Siemens air flow meter floating around, and decided to tear it apart before tossing it out. Well sealed unit, basically had to break the unit to pieces to get it apart. Found the reason it failed, the one element had a crack in the end, and fell off when I was trying to measure the resistance. The companion upstream read around 35R, the faulty around 240R before it fell off.Attached are photos from the microscope showing the internal thick film inside. I was expecting there to be nothing, all the control being in the EMU it is attached to. Seeing the insides goes a long way to explaining why they are so expensive as spare parts. Note the trim on some of the thick film units, and Dave's favourite MELF resistors.

author: Unknown
author_rank: Unknown
timestamp: August 18, 2012, 04:23:29 pm
content: nice pics! I know nothing about air flow sensors, I looked for more info on the net, the best I could find was this "measures mass flow by comparing the instantaneous heat transfer coefficient of a slightly heated probe to the moving air to that of still air" is that how this one works?, does the circuit in the pic feedback the result to one of main controller boxes?

author: SeanB
author_rank: Super Contributor
timestamp: August 18, 2012, 06:16:32 pm
content: This one had a flow diverter that took a portion of the inlet flow, straightened it somewhat with a set of baffles and then used a pair of ribbon nichrome elements in line to measure the heat loss to the airflow. The higher the mass of air flowing past the more energy lost and the higher the power applied to keep the temp constant.Attached a photo of the remaining element.

author: Unknown
author_rank: Unknown
timestamp: August 18, 2012, 06:22:44 pm
content: ah i see, clever stuff! thanks for sharing!

author: poptones
author_rank: Frequent Contributor
timestamp: August 18, 2012, 10:05:20 pm
content: These are fun. Years ago, before they were so common, I made one using a 12V light bulb. Just shatter the glass, apply a constant current source across it, and measure the voltage! Amazingly sensitive, just a tiny breeze is all it takes.

author: Unknown
author_rank: Unknown
timestamp: August 18, 2012, 10:10:24 pm
content: hehe, neat trick! - I'm guessing on a car it would need to take into account the air flow temperature eg. extreme winter vs hot summer?

author: poptones
author_rank: Frequent Contributor
timestamp: August 18, 2012, 11:14:49 pm
content: That's the beauty of a MAF sensor - it takes all that into account! As cool air is more dense than hot air, cool air cools the element more. Likewise denser air closer to sea level or pressurized by other means - more air molecules means more thermal displacement across the element.Try it sometime! All it takes is a bulb, a resistor, a battery and a meter. Pull enough mA through the element to heat it above ambient, give it a breeze, and there ya go.

author: SeanB
author_rank: Super Contributor
timestamp: August 19, 2012, 06:05:00 am
content: It responds to mass flow only. It runs at a high enough temperature that inlet tempdifference is not a big error, and altitude means lower mass, so no compensation needed. Other methods like a flap need a temp sensor and an altitude compensation, but this needs none.

author: Unknown
author_rank: Unknown
timestamp: August 19, 2012, 08:04:10 am
content: This was a link from wiki on MAF, very interesting document.http://www.autoshop101.com/forms/h34.pdfDid yours have a thermistor ? (from what I read the thermistor senses the air temp only to help the cold starting. or running maybe?)One interesting thing I read was when the car stops / engine off etc the element heats up to try and burn off contaminants on it, that would prob stress it over time)

author: SeanB
author_rank: Super Contributor
timestamp: August 19, 2012, 12:28:45 pm
content: 3 wire bus, 12V, Gnd and a PTflow signal out. No idea what car it came from, it was in a box-o-junk.