Project DIY Flowbench (part 1)

Remember how a few years ago the biggest problem in building stuff was finding the right sensors and microcontrollers? Well, now that literally every soccer mum's station wagon is high boost turbodiesel packed full with electronics, finding the right sensor isn't much of a challenge anymore.

But let's start at the beginning: A flowbench is used to measure flow through (all sorts of) orifices. As such in order to build a flowbench, we need a means to measure this flow. In the past this was achieved with all sorts of barometers and pressure transducers, which only indirectly measured the changes in flow, because actual mass airflow meters (MAF), where highly specialized tools only used in labs.

Below you see the two standard MAFs used in a plethora of VAG corporation TDI-models. Why choose one of these over (more modern) alternatives? These are fully analogue and just put out a 0-5V voltage, which can be referenced to airflow, based on a set of charts widely available on the internet.

Bosch VW 028906461

 
Pierburg VW 074906461

The pinout:

Pin	Bosch	Pierburg
1	Temperature sensor output, not used in ALH engine	5V reference input voltage
2	12V supply input	Signal ground
3	Ground	12V supply input
4	5V reference input voltage	-
5	Output voltage	Power ground
6	-	Output voltage

So basically what we'll need to build the flow bench is a stand for the head to clamp it down, a MAF a trusty shop vac and either an Arduino (or a clone) plus a little display or maybe an older Raspberry pi.

All we have to do on the electronics side is to have a means to read the voltage (hence the Arduino or Raspberry Pi) and reference that to the airflow following a rather simple kx^2+d formula (based on the graphs I've seen so far.

If you extend the curve all the way to 5V, you get somewhere close to 700kg/hr, which should be plenty enough for any given head one may encounter on a motorcycle. I hope to find some more graphs as I doubt that it really starts at 1V at zero-flow. More on this, when I make some progress.

(Special thanks to whoever archived the old Tech4tdi-page as the pictures and graph have been taken from there.)