PTS Flowbench Forum

First of all hello everyone, this is my first time posting here, i have recently began a project with my father to build a small flow bench for his engine building buisness, i have offered to help with the sensing and data processing and am planning on using a Raspberry Pi and 15bit ADC to handle both these tasks, the problem I'm having is choosing the correct pressure sensor to hook up, there is so much choice and variation! The flow-bench will be an orifice type with 3 approx 1400w vacuum motors driving the flow. My research so far has lead me to look for something in the 0-5psi differential range to read from the orifice and test piece, does this seem like a good range? does anyone here have experience of similar setups or the correct chip choice hopefully with an approx 0-5v output range?

Freescale has a pressure sensor selector on their website. It's a good place to start.
Most flow is corrected to 28 inches of water. If you're building a bench that will be able to pull 28 inches, on your test piece, then you only need one pressure sensor with a range higher than that.
28 inches of water being around 1 psi would make your 5 psi sensor way too much range.

For the pressure drop across the orfice, Bruce sells a nice set-up with a 0-16 inch range, that flows most heads with only one orfice plate.
You could use less pressure differential, and a more orfice plates if that suites you better.

Unless you are versed in circuit design the trade offs are cost of sensor and ADC versus building an amplifier for a sensor. Most of the ADC will accept +/- 5VDC or +/- 10VDC most sensor components output 0 to 50MV so you will need big multiplication to feed your ADC. That said ADC with built in multipliers are big money and same goes for sensors.

Not sure why you are choosing the Raspberry Pi (neat product) as I think you will end up with both driver issues with the ADC and GUI issues with development of the application interface.

I would use a sensor of 40 to 100 inches of water for the depression and based on your perceived uses (by your motor choices) you may want to use a sensor of say 6” to 8” for your Delta P to save motor power.

Just some thoughts.

Rick

MPX5010DP
http://www.mouser.com/ProductDetail/Fre ... BTh18ovzX0

This is what is used in the FP1
and its good to 40KPa or around 5-psi

IMO you should use a few more vac motors at least 5 to get close to 28. testing at a higher pressure can show some interesting things about a port. just convert back to 28 to get relative #'s

I use 5 but I usually test at 40'' and convert back to 28'' I wish I had more for higher flowing heads

Ok thanks for all the advice so far, could i just clarify that two MX5010DP's would be used to read from atmosphere to the two test points and the difference taken? not to read the relative difference from one test point too the other?

they can be used for both
differential (Pitot or plate)
static (leave one open to ATM and one to your static source)

throw in a third or 4th for pitot probe measurement

4 would be nice because you could have a Int and Exh dedicated pitot

pecracing wrote:Ok thanks for all the advice so far, could i just clarify that two MX5010DP's would be used to read from atmosphere to the two test points and the difference taken? not to read the relative difference from one test point too the other?

What you need to measure is the differential pressure drop directly across what you are flow testing, this is the test pressure, and is usually always held at a constant 28 inches (1 psi) by adjusting the flow bench.

The other pressure measurement is the differential pressure drop across the flow measurement orifice, which can be whatever design pressure you decide to use.
Something between five and fifteen inches would be fairly typical for most benches.
A higher measurement orifice pressure drop will likely give slightly better accuracy, but will proportionally consume more of the available total blower capacity.

Fit as many vacuum motors that you have the available power to run !
No matter how much maximum airflow you end up having, one day you will wish you had just a bit more...

Tony wrote:

pecracing wrote:Ok thanks for all the advice so far, could i just clarify that two MX5010DP's would be used to read from atmosphere to the two test points and the difference taken? not to read the relative difference from one test point too the other?

What you need to measure is the differential pressure drop directly across what you are flow testing, this is the test pressure, and is usually always held at a constant 28 inches (1 psi) by adjusting the flow bench.

The other pressure measurement is the differential pressure drop across the flow measurement orifice, which can be whatever design pressure you decide to use.
Something between five and fifteen inches would be fairly typical for most benches.
A higher measurement orifice pressure drop will likely give slightly better accuracy, but will proportionally consume more of the available total blower capacity.

Fit as many vacuum motors that you have the available power to run !
No matter how much maximum airflow you end up having, one day you will wish you had just a bit more...

Now this last post has me confused: surely it’s back to front?
You use a measuring pressure -28 inches or more if possible, then a differential reading across the orifice to calculate flow?
Or did miss something?

You use a measuring pressure -28 inches or more if possible, then a differential reading across the orifice to calculate flow ?

Yes exactly.
You first set the pressure you wish to test at, across whatever you are testing.
It is usually -28 inches, but it could be some other test pressure.
A certain amount of airflow will result from that.

That exact same air then flows through a measurement orifice, resulting in a pressure drop across the orifice, that increases with airflow.
A second pressure reading is then taken from directly across the measurement orifice, and the airflow calculated from that.

You’re right of course, for some reason the phrase “measure the pressure across the test piece” got me confused.
Thanks.
Huw