If you do a deep dive into the SFM3300, they use a perforated aperature plate. This is a nice setup as it would help to break up vortices (A.K.A Karman Street) which could present a pressure modulation at your sensor ports and possibly a pertubation in your transfer function. 

Yes!  I did notice the stainless screens myself.  :)  We actually were able to measure that already as when we did our initial tests with a standalone pitot tube, the ports started right on the leading edge.  That resulted in a reading which was skewed downwards and depended on the tube heading into the pickup. 
We have tried to address the inconsistency in the flow by moving the sensing package to the vertical center of the current model, so there it has a built-in lead-in area from both directions.  If this proves to be not enough, I do have a hexagonal print I can drop in both ends that won't require modifying the existing package or starting a print in midair.

There was another suggestion of using a hexagonal print area as our flow restriction for a more targeted orifice type pickup, but the problem with that is every deviation in the printed width of the parts results in multiplying our error.  It gets bad pretty quickly.  So the hexagonal flow alignment parts, if needed, are not going to be part of the sensing area.  That should limit our cumulative error based on imperfect extrusion to a calculated 12%.

Daren, I've had great success using PCBway for quick turn boards.  They're very inexpensive ($5 for 10 small two layer boards) and typically ship within a day.  Located in China, so the shipping is the slowest and most expensive part.  If I'm in a hurry I select fedex International Priority which typically runs about $30.  That gets me the boards in just 2-3 days.

Good luck with the Pitot tube approach.  I'm busily working on my venturi tube version.

Does pcbway really ship within a day? That's much faster than elecrow shipping in 4 days which I usually use.

Yup, for simple two sided boards they normally turn them around in 1-2 days.  Fedex IP is the fastest shipping method in my experience and usually takes about 3 days from China.

Thank you Steve.  We have had less than stellar results with JLCPCB at this point.  Some of our boards had the sensors flipped 180, *within the same order*.  I have yet to document our disappointment here as I'm hoping that they will make it right. 
That was a *huge* waste of time for all involved and resulted in the destruction of 6 of our 10 boards we were going to use for validation/testing.  I'm basically incapable of reworking these tiny components due to nerve damage.. but I sure did try.  That is actually the reason #Arcus-3D-P1 - Pick and Place for 3D printers got started.

The pitot tube is more of a combined orifice/pitot tube approach at this point due to the significant percentage of the tube that the pitot now occupies.  It is looking like it gives us the best of both worlds.  I will admit that a venturi design was on the back burner just in case it didn't work out though, but with the pitot design I get a bit more gain as the leading sensor is under pressure, but the trailing sensor is also under a slight vacuum.

Also, I hate to break it to you but the BMP280 is pressure only.  If you want temp and humidity you need the BME280.  Unfortunately not the same size package close but larger.

Doh.  The difference one letter will make... 
Thank you for pointing this out.  We are actually looking at the Chinese clone of the BMP388 right now and dropping humidity for all the sensors anyway.  

Your sensor tubes are huge compared to the inner diameter of this part.  The tube orifice should be like 1/8 to 1/15 diameter of the inner diameter or they distort the flow too much.  You can streamline the tubes where they enter the wall to minimize the flow distortion.

This is intentional (now).  We will (probably) need higher gain, and making the ports a significant fraction of the tube size gives us some of the gain of an orifice type pickup while still having directional data.

Hi. Very nice project. I'd like to make some of these sensors (with JLCPCB) for prototyping our vent (openvent.org), maybe some small modifications are needed. Can I get in contact? hobodrifterdavid@gmail.com :-)

Replied, worked, met.  Thank you!

Thanks. I am driving valves no vfd or stepper.100% valves.. any res is good. Thanks again.

The data from a new test using the BMP180 sensors looks great. Post is above.

I have confirmed the control operation on my ventilator, beginning pneumatic assembly on bench prior to interning in cabinet. Looking at instrumentation options , your device appears to have the parameters I would want. How is it going?

This thing is coming together now. So any input is helpful.

Thanks.

The sensitivity may be inadequate using the chosen sensors.  We hope to find out today using the real thing instead of the older generation of them.

Thanks, I really only need to set thresholds for signal . I apologize for fielding questions that are likely covered in your material. Thanks for your patience. what is the sensing range for flow and psig operating range?(of your current prototype) 

@flanagan Pressure sensitivity will be fine.  What may be inadequate is the resolution of the flow data from the pitot tube differential.
For the earlier prototype sensor with just tubes we were seeing 1cm of water column difference when I breathed through it.  Based on this and the sensor specifications, we should have had about 1k steps between inhale/exhale.  In reality, this may be turning out to be less. 
Aka... we won't really know until we finish a full prototype.  Everything has been tested in bits.

Could you please publish the mechanical design?  I want to see if your design will suit my needs (ventilator) of if I need to change some things.  openSCAD files would be perfect but I'll settle for STL file.  Anything would be helpful.  Thanks.

Nevermind.  I found everything on the parent project linked to  near the top of this page.  Both source cad and STL files.  Thanks.

I'm wondering how you knew what to do for the positioning of the pneumatic paths.  Did you more or less copy an existing design?  Or have you done these before?  Is there a guide for making flow sensors using pressure sensors somewhere?  :)

I looked at the existing designs for S type pitot tubes and winged it.  Beyond that testing showed it needed to have a lead-in area.  There are three basic types you can look up yourself.  Pitot tubes, orifice plates, and venturi.  Pitot tubes work by stagnation pressure (air stacking up in them), orifice plates by creating a restriction and reading the pressure before and immediately after, and venturi by a similar method to orifice plates except that your pickup is within the restriction.

I am curious on how you are going to seal the sensors to the tubes, I printed one up and have been wondering about that .  How did your testing go?

A touch of RTV on the sensor mount surface, a single screw in the middle.  The sensors are getting reflowed to the boards, and just sealing the board to the printed bit is enough as they are absolute (no air channel needed to the back of them or such).  There is a keep-out specified in the pcb layout to keep the vias out of the area that needs to be sealed.
No graphics of this as two incompatible systems being used, but we generated the outline and everything fits.

Did you also think about a CO2 sensor  - I picked a few up quite cheap, they are rated up to 10000ppm but I am going to see if they can handle 40000ppm. Could be useful to monitor exhaled CO2, for logging or alarm purposes.

I did not. They are huge compared to what we have right now. The whole board is 40x16mm, and the enclosure /pickups for it are like 40mm more than that total. O2 monitoring would be pretty useful, but those are also not small and power hungry. For now, as long as the air is moving in both directions, we have O2 going in, and CO2 coming out.  If we don't, we have bigger problems that a ventilator is not likely to solve..

The sensors I found are 4mm square approx, and the whole I2S EVB  I bought maybe 20mm square. Not excessive. 

Odd I can't reply below hope you spot this here

https://ams.com/documents/20143/36005/CCS811_DS000459_7-00.pdf/

• 2.7mm x 4.0mm x 1.1mm LGA package

@Adam Quantrill Please share then.  I'm used to the huge ones apparently.  Thank you.  As the design is right now we could accommodate probably one more sensor without going over our 1 square in limit or going to the backside.

@Adam Quantrill Yeah, they limit the depth of conversations.  Just do the @username trick and they still get notified if you reply.  Thank you.  I'll look at that.

Is this for point of use monitoring or control?

Both. Our goal is control, but the whole backside of the board is free and the case has a slot for an OLED already. This is step one.

So...are you thinking remote HMI and sensor interface coupled to parent control?

How close are you?

@flanagan This is the remote sensor.  It has a slot for an OLED we may put something on.  It was just easy to do so, so we did.  The plan right now is our MCU will live on the vent, but we have discussed making this the brains of the whole device.  Then we could just supply the signals needed to drive the support devices (motor or valves) from here.  Interface is a bit small for this though.  Our whole board is less than a square in.  We would have room for about 4 capsense buttons, not much else, and that display is too small for quick observation from a distance.  So the short term plan is leave this like it is, move on, get it tested and calibrated, and put the MCU on the vent along with basic controls.  The vent will have its own display.  But it is something we plan to come back to.  The boards are being fabricated now and we should have them in a few days.

Are the PCB files avl?  Also is there any example driver code for reading the sensor?  This is exactly what we have been looking for in our DIY Ventilator.  J

@Jeff Mayes @djeddleman The board just came together today.  Yes, we will be releasing the files, but we would love to actually test one first before someone goes spinning up 1000 of them  :)  If you want to help with that, PM me or @Steven.Carr 

Oh, I did not realize you were still in progress, by all means test, test, test, test!  I will work on printing one up today to be ready for the rest.  Great work!  TX

I have an LTC4316 demo board and two BMP180 boards (similar to the BMP280's) on hand. If you have trouble with the board testing, feel free to let me know.

I notice that you used the 24C01ASM as the EEPROM on your PCB. Make sure you use a 3.3V compatible version of the EEPROM, as the original 24C01ASM is intended for 5V operation.

@Jmayes This project is a whopping three days old.  We are still iterating, sometimes several times a day, and everything you see and can download is still actively in flux.

@djeddleman Thanks!  That would tell us if our design electrically works at least and could help us get a head start on writing a driver.  If you used one of the older pitot tube designs (not an all in one like this) and some kind of cover we could test the basic flow sensing idea using your development boards.  Thus far we have just done water column measurements to make sure we were in the right range. PM me.
Good to note about the eeprom.

Are the Eagle schematic/layout files available? (*.brd/*.sch) I can help review them, and it's easiest to do it directly in Eagle.

What happened to the MPXV7002DP ? At first glance seems like that, with a BME280 or 2 would be a better / easier solution?

it got eliminated.  This works via I2C so I don't need to extend the analog signals from the MPX back to the MCU and is half the cost/higher resolution.  The MCU ADC resolution is 12 bit, whereas I get 24 bit resolution from the barometric sensor.  Granted I'm using less of the range to do it so it probably all evens out.

Fair enough, but I'm worried about drift / relative calibration errors between the units. My spidey sense tells me having 2 (or 4 in this case) independent sensors that you are then doing software tricks for sensor fusion, could be an issue. Especially in mass production? Just thinking about if one gets a little warm, or wet, or contaminated etc, would be hard to tell from a software point of view.

Sorry for playing devils advocate but we're working on our own design at the moment and I was looking at the same differential pressure sensor as a basis (the analogue interfacing and slightly higher cost doesn't phase me, especially in light of potentially more reliable design).
Actually printing out your NEMA23 based design at the moment to give it a try :)

@Rob That is why we have an onboard eeprom in the schematic.  Not sure we are going to need it but if the calibration really differs by that much, we already have somewhere to store those numbers that is permanently tied to the unit.  We also have a couple different comparisons we can do to idiot check the numbers.  Also, the BMP388 actually lists testing lung function as one of its applications.  :)

Good luck on the stepper and let me know how it goes.  BTW variable volume will be controlled by turning 1/4 turn, and then returning back to start. For testing you can just spin it though for full volume.

We got some new numbers from the UK document and at the rates they want the inhale to occur, I'm not sure my motor selection will hack it anymore.  

The old number was 1.5s, and so I targeted 1.0s for some headroom.  

The new number is 0.3s!  That is 3x faster than I had specified and 5x faster than the original spec.  Come on...

If it takes 3x the force, the Nema23 is not going to cut it.  The BLDC I originally specified is also probably doomed.

You'll probably get it turning before I do at this rate, so let me know!