Pressure Sensor for Flow Bench

[garfield at pilgrimhouse.com]

On Thu, 07 May 1998 21:26:27 -0700, goflo at pacbell.net wrote:

>Motorola pressure sensor part # is MPXL5010 0-1.45 psi range (approx
>41 in.H2O). Output span is 4.5 V temp compensated on-chip.

Oh pennies from heaven; thanks, Jack (and Steve for the spark)! I looked
on the Moto selector guide for low pressure sensors, but somehow
(fractional wits again, alas) I missed this lil beauty.

I guess one can't have everythang; I was hoping for somethin within
+-1%FS accuracy, but in a flow bench, I guess what's really important is
REPEATABILITY, so you can just be sure you are measuring air flow AT the
same diff. pressure drop; so yeah, this gizmo should do very nicely. My
next quest has to be for a calibrated MAF sensor for the flow bench that
will go from 0-600cfm (OK I'll settle for 300cfm if I HAVE to), with
1%FS accuracy. Then move over SuperFlow. Hee hee. I've got my blower
motor controller all figured out AND tested (lobbing it up and down in
my hand as we speak; it's amazingly simple), so I can put a PC in the
loop to read the deltaP and control the blowers, and then use a stepper
motor to open the valve gradually, whilst controlling the blower to keep
the deltaP within range, and then measure the air flow with the MAF
sensor. That way, once I set the thang up, I just push a button and
listen to them blowers go to town, as the controller steps through the
valve lift range, increment by increment. I know, I know, I'm daft as a
loon maybe, but ya gotta dream, dontcha? I want a flow bench that is
nuthin but a controlled air flow and diff. pressure apparatus, and the
entire rest of the smarts is in the PC, so it runs through the
measurement, computes/corrects flows, and plots the results of my latest
plaster model grind, whilst I kick back and have a coffee. Hee.

Hey, anybody up for a DIY FlowBench? If the calibrated MAF sensor could
be found to meet the specs I've mentioned, I'm SURE it could be done.
The major expense are the multiple blower motors, but everybody just
about uses all the same ones, and they're about $100 per 100cfm from the
Grainger catalog, as far as I can fathom. Forget the calibrated
orifices, the manual switching of them, and them huge manometers, and
especially the slant tube variety, along with the manual blower
controls, etc. etc. Let's take a giant leap into the present, and let
our sensors, instruments, and PC control the loop. Washing clothes musta
been a ROYAL tedious pain, just like flow bench runs, before the washing
machine came along. What we need is a simple, straightforward,
present-day-technology flow bench that's computer controlled and
electronically instrumented. Them old-style flow benches do Orville and
Wilbur proud, no doubt, but they're way too big, way too tedious to use,
and, last but not least WAY TO EXPENSIVE!!

There's already a commercial offering like this, available from:
	http://www.quadsci.com
but it's either 200cfm box for ~$2K, or a 600cfm box for $4K+. Argh. Too
low a volume, too high a markup, making it a rich man's sport. Crumbs.
Time to zap it with some DIY-rays.

>Keep threatening to close the loop with servos on the pressure valves!

Hey don't get mad, get even. How bout closing the loop by controlling
the blower motors electronically from the PC. That way you don't have to
mess with them valves, which are just an "orifice" way of controlling
flow, IF the blowers are assumed to be running at a fixed speed. BUT,
just about all the blower motors all them FlowBench guys use now are
AC/DC motors that can be easily thyristor controlled, without
overheating. Thas a better way than choking the poor things! A PC with a
parallel port, and the world of blower motors is at your command.

Like I said, I got the PC-controlled Thyristor motor controller stuff
already worked out (I been plotting about this one for a while, but I've
lacked the "critical mass of fractional wits summation", to pull off the
sensor side of thangs). If any of yous guys can find a MAF sensor that's
available CALIBRATED, and within +-1%FS accuracy over say 300cfm (it
takes a WHOLE lotta blowers to do 600cfm @ 25" H2O, so I figure a 300cfm
flowbench is gonna be fine for most people, bout $600 worth of blowers),
I can supply the rest of the circuitry. Write the interface in Java or
your favorite GUI environs, and POOF, instant automagic flow bench. I've
even got the motor controller stuff for the stepper necessary to turn
the lead screw (you know the valve opening from the # of steps
commanded, and the lead screw pitch) that opens the valve (assuming
you're running flows on a cylinder head). Hook the data logging into
Excel, and presto, instant plots!

OK, I'll go and take my medication now, but just think about it, will
ya? Life is short and uncertain; we gotta think hard about getting to
the dessert FIRST.

Gar