Sprint Day 2:  What developing world substitutions are there for flow meters? - Page 1
Forum section: electronics / projects

author: Enginerding
author_rank: Contributor
timestamp: March 31, 2020, 05:04:36 pm
content: Sprint Day 2 has begun; here's the critical path question:What component substitutions are available to the developing world for flow meters?This could come from off-the-shelf commercial and household items. E.g. automotive Mass Air Flow sensor (bonus: ~$50).This is a dialogue oriented question, the more community the better. Spitball, brainstorm -- let your hair down. If a few options emerge, I'll start a poll.Situation:Ventilators need flow meters that operate at 0-80 lpm. One common item in use is the Sensiron FM3200. (22mm OD input/output). However, there are now less than 100 of these in the supply-chain, according to Octopart. Additionally they are expensive at ~$170/each.======Housekeeping:Analysis of Open Source COVID-19 Pandemic Ventilator Projects by R. Read, et al at Public Invention. (shared by Dave)spreadsheet:https://docs.google.com/spreadsheets/d/e/2PACX-1vTYAfldxoIiO46VAWH1NlhrwFBn9mguqS2bh1spnLEu4AVVN1cj1vaEm6vOp5Z6UnaAbUwd8dslCXdM/pubhtmlhomepage:https://www.pubinv.org/project/ventilator-verification-project/Git:https://github.com/PubInv/covid19-vent-listGit for Inline Ventilator Test Fixture and Monitor:https://github.com/PubInv/ventmon-ventilator-inline-test-monitorGit for A collection of validation test for for emergency covid19 ventilators (clinical usability and suitability as per RMVS):https://github.com/PubInv/covid19-ventilator-validation-testsForum (for Public Inv, not project specific):https://groups.google.com/forum/#!topic/pubinv/This is the principal resource for determining project progress. The author (Read and team), have attempted to quantify each project's progress. I'm a participant in this project, so feel free to make suggestions.Lastly, a few rules that I ask each individual to abide by for success.- This sprint format isn't for everyone. It's fast-paced and goal-oriented.- Please consider using your real first name. So...I'm Dan, nice to meet you!- Before you post, ask yourself how contributory what you're about to post is to thecurrent sprint. Could it be better? If it's obvious, it's probably already been thought of. If you're unsure, DM me andI'll help.- If you have any great ideas that have to do with the future sprints you think we need to prepared for, also please DM.- This goes without saying but, no politics. We can all scroll our social media feeds and get enough of that.- Success will probably include a project t-shirt. That's it. Oh and you get a free ventilator if you build one. So there's that.Previous Day Sprint archives:Sprint Day 1====Hey, all!The previous day's Sprint was a smashing success. There were many wonderful, diverse ideas. Dave even stopped in to share his wisdom. I've archived that thread, and it'll be up on the project Github shortly. Here:https://github.com/EnginerdingDan/Open-Source-VentilatorThus, Sprint Day 1 begins.The next critical path item is:Does a fully operational open source ventilator already exist w/ a suitable BOM for the developing world?At the bottom of this thread, I'll catalog, for reference, existing projects. Be forewarned, most of the "open source" projects touted in media *will be hype*. So I ask that if you post a project; DON'T post a media link. Run it down, and post the actual link to the project's mainpage, for example their Github. Projects that DON'T have a locatable homepage can safely be assumed to be wank. (I'll accumulate media-only links at some point). If they don't have a specifications document, a BOM, a working prototype -- wank!IF, you have other succinct criteria that will define the critical path question, please post it inBOLD. AndI'll update this main post accordingly.Critical Path Criteria(not to be taken as an exhaustive list):Truly open-source?(for example, are all the project files open and available)Working prototype?Design files including BOM?=============Known Open-Source Projects:Motivational Resource: Open Source COVID19 Medical Supplies (in conjunction with Hackaday) <--- 1000s of successful PPE projects being delivered to hospitals.https://www.facebook.com/groups/opensourcecovid19medicalsupplies/Mechanical Designs that Adapt a Bag-Valve-Mask:(BVM:https://en.wikipedia.org/wiki/Bag_valve_mask)MIT E-Vent(shared by floobydust), "Emergency ventilator design toolbox", homepage:https://e-vent.mit.edu/Behind a login. Registration is instant though. The specifications documents are detailed and from a medical providers point-of-view, similar to the UF specifications but less detailed. The electronics work is very basic, enough to essentially "bag" a patient on a timer. Uses aluminium extrusions.OpenBreath(shared by awallin) This Italian project has a notice from March 22d that it's now a 3D printed project due to the economic shutdown in that country.homepage:https://www.openbreath.it/en/Git:https://www.ohwr.org/project/openbreathMotor-driven Designs:University of Florida, Medical School, "Open Source Ventilator Project"homepage:https://simulation.health.ufl.edu/technology-development/open-source-ventilator-project/Engineering Specifications: This is produced by the Medical School, led by Anesthesiology, so the specifications are from the clinical point-of-view of the feedback/control a physician is looking for. They're well detailed, and high-quality. here:https://simulation.health.ufl.edu/technology-development/open-source-ventilator-project/engineering-specifications/BOM: Incomplete. Mostly "home depot" type stuff. No electronics other than specifying the BMP280 (Phillips) and an NXP pressure transducer. here:https://simulation.health.ufl.edu/technology-development/open-source-ventilator-project/bill-of-materials/GitHub: sparse. here:https://github.com/CSSALTlab/Open_Source_VentilatorForum: sparser. here:https://simulation.health.ufl.edu/forums/forum/open-source-ventilator-project/Bellows Design:VentilAid(shared by Dave). (info soon)https://www.facebook.com/groups/ventilaid.community/Not Open Source:Medtronics (info soon, not true open source, seems like wank, "eevBLAB #72"). They say it includes schematics and manufacturing docshttps://www.medtronic.com/us-en/e/open-files.htmlMercedes F1 "Project Pitlane" (cannot find design files or homepage, just media)uncharacterized:https://twitter.com/VentilatorUhttps://twitter.com/Selcuk/status/1244661929777532930

author: coromonadalix
author_rank: Super Contributor
timestamp: March 31, 2020, 05:16:41 pm
content: https://techcrunch.com/2020/03/30/medtronic-is-sharing-its-portable-ventilator-design-specifications-and-code-for-free-to-all/??sorry already posted

author: awallin
author_rank: Frequent Contributor
timestamp: March 31, 2020, 05:19:25 pm
content: this project was posted on ohwr forums maybe a week ago:https://www.ohwr.org/project/openbreath/wikis/homeedit: also check links on this pagehttps://opensource.com/article/20/3/open-hardware-covid19

author: Enginerding
author_rank: Contributor
timestamp: March 31, 2020, 05:22:25 pm
content: Quote from: awallin on March 31, 2020, 05:19:25 pmThat's a new one! Do you think you can run down some of the critical criteria listed above? And I'll add it to the list.Thanks awallin! You're the man!

author: MK14
author_rank: Super Contributor
timestamp: March 31, 2020, 07:30:42 pm
content: Making daily, new threads like this, doesn't seem to be a good idea.This is a forum, NOT a personal BLOG.The various people, "subscribed" to the previous thread will disappear. Ideally, you wanted them to still be in the thread, as they would have been notified and/or could be informed via the "Show new replies to your posts." link, of further progress in the thread.Also, forum users, sometimes need or want to see what has already been mentioned/discussed.If you continue to make new threads like this, you may find it rather counter-productive, and see many forum users lose interest in the thread(s).

author: magic
author_rank: Super Contributor
timestamp: March 31, 2020, 07:43:02 pm
content: Quote from: MK14 on March 31, 2020, 07:30:42 pmThat's not agile. What happened in the past stays in the past. Today we concentrate on the task at handQuote from: MK14 on March 31, 2020, 07:30:42 pmNever

author: Enginerding
author_rank: Contributor
timestamp: March 31, 2020, 07:59:25 pm
content: MK14, don't consider a sprint "day" definitively to be one 24-hour time period. "A task isn't done, until it's done", as the saying goes.The best piece of collaboration would be to identify a promising open-source project OR flesh-out details for one of the projects listed above.Particularly, whether a project has a BOM is going to be a good indicator of its viability.Much appreciated, Dan.

author: EEVblog
author_rank: Unknown
timestamp: April 01, 2020, 02:44:20 am
content: Quote from: MK14 on March 31, 2020, 07:30:42 pmCorrect, we won't tolerate the projects section being filled with a new thread daily.If you want a comprehensive list of venitlator projects and their status, some people have done the hard work for you:https://docs.google.com/spreadsheets/d/e/2PACX-1vTYAfldxoIiO46VAWH1NlhrwFBn9mguqS2bh1spnLEu4AVVN1cj1vaEm6vOp5Z6UnaAbUwd8dslCXdM/pubhtml#

author: Enginerding
author_rank: Contributor
timestamp: April 01, 2020, 03:21:29 am
content: That's a killer list Dave! Who sent it to you? Brilliant. Added above.My intuition was indeed correct; none of the projects are very far along as verified by this 3rd party (R. Read, et al). For instance, the UoF project is ranked in the top 5, but only a 2.9 of 5. Ventilaid is a nice project also, but most of these projects will start hitting the usual project decay bottlenecks (I'd guess 1 standard deviation, if not 2).The biggest problem they're going to have in the developed world is getting the BOM together; it may be critical to identify commercial and household items as BOM substitutes.(Also, it's quite easy enough to modify this thread for new sprints. Doesn't really make a difference to me. Thanks for the input.)

author: Enginerding
author_rank: Contributor
timestamp: April 01, 2020, 03:33:47 am
content: Day 1 Sprint Complete! ✔️Absolute success.The next sprint will begin shortly.

author: EEVblog
author_rank: Unknown
timestamp: April 01, 2020, 07:10:22 am
content: This Aussie one looks great!https://twitter.com/tripleeightaus/status/1244767611365126145

author: Enginerding
author_rank: Contributor
timestamp: April 01, 2020, 05:29:43 pm
content: Bump for new sprint day.

author: senso
author_rank: Frequent Contributor
timestamp: April 01, 2020, 05:44:34 pm
content: Dont edit the first post like that, it ends being confusing AF..Grab the smallest MAF from a car, but remember any name brand like Bosch will run you down 180-250€, and those cheapo crappy ones that cost 50-80€ fail in a couple months and report erroneous values..

author: Enginerding
author_rank: Contributor
timestamp: April 01, 2020, 05:53:38 pm
content: What are "good" OEM brands in automotive? For small cars that would be popular in the developing world? (e.g. Japanese, SK).Do you think you could find an example of a MAF sensor you mentioned?Thanks, you're the man.Quote from: senso on April 01, 2020, 05:44:34 pm

author: AndyC_772
author_rank: Super Contributor
timestamp: April 01, 2020, 06:54:31 pm
content: Quote from: EEVblog on April 01, 2020, 07:10:22 amCool, I like that. Here in the UK the Mercedes F1 team have done one too.https://www.formula1.com/en/latest/article.mercedes-f1-help-develop-coronavirus-breathing-device-for-health-service-use.6Yx9lt3ADUMLwXr7UKDfYT.html

author: ebastler
author_rank: Super Contributor
timestamp: April 04, 2020, 03:47:52 pm
content: Quote from: Enginerding on March 31, 2020, 07:59:25 pmWhat kind of criterion is that?? Where doesyourinitiative stand on the "viability" scale according to your own benchmark?Releasing a BOM is one of the last steps once your design is complete, verified and validated."All unfinished projects are worthless, so let's start another one!"

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: April 04, 2020, 04:02:25 pm
content: Quote from: ebastler on April 04, 2020, 03:47:52 pmYeah lets start more projects that will never be finished

author: Simon
author_rank: Unknown
timestamp: April 04, 2020, 04:38:56 pm
content: If you are buying serious quantities from Bosch MAF meters are like £50, the smallest ones do 0-3l/m. attached is a datasheet of two models that are obsolete but will give you an idea. The other method of measuring flow is to create a small restriction (which by definition any length of hose is) and measure the pressure drop across it but that will need fully characterizing and ideally you want a sensor with the right output curve.Yes if you create too many of these threads you will just loose the people you thought were on board (laughing at you more like), eventually piss everyone off and get banned.

author: Simon
author_rank: Unknown
timestamp: April 04, 2020, 04:49:22 pm
content: Quote from: ebastler on April 04, 2020, 03:47:52 pmBecause in a simpletons mind if he has a list of parts that's it, just attach them all together to the drawings and hey presto. Remember this guy is a medic, not an engineer so he hasn't a flipping clue what he is doing.

author: EEVblog
author_rank: Unknown
timestamp: April 05, 2020, 12:18:46 am
content: And it was smart of him not to mention the open Medtronics specs, I'm sure that's very deliberate.

author: ArthurDent
author_rank: Super Contributor
timestamp: April 05, 2020, 03:06:47 am
content: Dave, That video was a great overview that explains in layperson terms what would actually be required to build a ventilator that actually works. There is also the short-term reliability, life expectancy of the unit, and the whole legal side to consider. Bottom line is this isn't a sprint of days but a long-term expensive project well beyond my, or most people's abilities, and wallet.

author: EEVblog
author_rank: Unknown
timestamp: April 05, 2020, 06:49:19 am
content: Quote from: ArthurDent on April 05, 2020, 03:06:47 amYes, I think almost all of these hack rapid development projects, as well intentioned as they are, will not amount to a usable product.

author: thatguy1988
author_rank: Newbie
timestamp: April 05, 2020, 08:09:53 pm
content: Adafruit sell some very cheap flow meters. Amazon do as well but they don't ship them to all countries.https://www.adafruit.com/product/833https://www.adafruit.com/product/828

author: Simon
author_rank: Unknown
timestamp: April 05, 2020, 08:15:24 pm
content: Those are both liquid flow meters. If they are useful your patient is already dead!

author: Simon
author_rank: Unknown
timestamp: April 05, 2020, 08:25:35 pm
content: The easiest thing to "make", "cheap" is a slight restriction in your tube with a very sensitive differential pressure sensor either side that has a log output. This counteracts the exponential characteristic of the restriction that follows a square law to give a linear output to flow. It would need characterizing. You can get a 125 Pa sensor for 10's of pounds.

author: tkutscha
author_rank: Newbie
timestamp: April 05, 2020, 09:04:28 pm
content: You can get a cheap acrylic air flow meter (<$20) and use a series of optical sensors with an MCU to read it out live. I initially thought of putting a magnet ball in there and using hall-effect senors, but changing the weight of the ball would probably upset calibration. Since you're looking for 0-80 LPM, you could put two in series to get accuracy at the low and high ends.If you have a bunch of them, you could point a cheap camera at all of them to read out the ball heights with image processing software. Maybe a super cheap webcam and some image filters. Make the ball luminescent or glow under an UV lamp to make it stand out more?Looking on Amazon: $16 for a flow meter, $10 for a cheap USB cam, and a raspberry Pi to look at several of them ($35).

author: Simon
author_rank: Unknown
timestamp: April 06, 2020, 07:53:14 am
content: sounds really fail safe and medical.....

author: dietert1
author_rank: Super Contributor
timestamp: April 06, 2020, 08:52:09 am
content: If i remember right, others have been able to measure heart rate of by-passers using video, so video is a powerful tool.A flow meter i built 15 years ago for respiration measurements in a university institute was based on a difference pressure sensor and an "airflow resistor" that can be made with a 3-D printer and/or some tissue. Those meters i made had an instrumentation amplifier to read the sensor bridge, a CMOS analog multiplexer to provide 4 different steps of gain and a 12 bit ADC in a MSP430 microcontroller.Recently i thought a lot about whether such a setup may be enough to control respiration support in a ventilator. In the documents circulating right now they propose monitoring pressure, maybe monitoring flow could be as efficient. Yet again those projects are not electronics, but interdisciplinary.Regards, Dieter

author: Simon
author_rank: Unknown
timestamp: April 06, 2020, 08:59:15 am
content: Quote from: dietert1 on April 06, 2020, 08:52:09 amWhich is what I suggested above. The main issue is getting the pump to work in sync with patient. If you don't their lungs won't survive being on any of these contraptions for weeks.

author: dietert1
author_rank: Super Contributor
timestamp: April 06, 2020, 09:45:21 am
content: Anyway a medical machine needs some redundancy. In fact the proposals include mechanical safety valves and the like.At that time the respiration project was some kind of reuse. Our pulse oximeter modules include logging an ECG signal - again for redundancy and also to determine PTT, a parameter in sleep diagnostics. In those meters i mentioned the pressure signal replaced the ECG signal, so with very few changes we arrived at a solution combining pulse oximetry with airflow meter. It's a USB device with isolation.Regards, Dieter

author: EEVblog
author_rank: Unknown
timestamp: April 09, 2020, 01:58:49 pm
content: Another open source ventilator, in Danish.https://www.dr.dk/nyheder/regionale/nordjylland/danske-forskere-har-brugt-blot-14-dage-paa-lave-en-simpel-respirator

author: donotdespisethesnake
author_rank: Super Contributor
timestamp: April 17, 2020, 07:46:40 pm
content: So did we give up on this?FWIW, if anyone can deliver a hardware prototype, I'd have a go at writing the software. I've no experience with medical, but have worked in other safety critical areas.Edit: The main code for the Medtronics PB560 runs to 53,000 SLOC. That's around 135 programmer-months, a little over 12 years of effort. I hope that gives some useful perspective...Edit2: sorry I miscounted. There are nearer 96,000 lines of code. So double that timescale! (excluding blank, comment only lines but including C source this time)

author: dietert1
author_rank: Super Contributor
timestamp: April 18, 2020, 06:01:02 am
content: At SEAT car factory in Martorell in Spain they needed one week to setup a production of 300 devices per day. They are using on stock wiper motors plus SMPS blocks to operate rubber ventilation bags. The device has about 110 parts. Volume per cycle can be adjusted mechanically, speed gets adjusted by supply voltage. Apparently medical doctors preferred something that doesn't need supervision.Don't know how they control the exhaust valve and i did not read about backup batteries. That virus changes everything.https://www.seat-mediacenter.com/newspage/allnews/company/2020/From-making-cars-to-ventilators.htmlRegards, Dieter

author: magic
author_rank: Super Contributor
timestamp: April 18, 2020, 06:34:06 am
content: Quote from: donotdespisethesnake on April 17, 2020, 07:46:40 pmHave you seen the Project Leader and Scrum Master anywhere recently?I hope it was an early April Fools attempt. If the guy was serious, I kinda feel sorry for himQuote

author: MK14
author_rank: Super Contributor
timestamp: April 18, 2020, 08:59:34 am
content: Quote from: magic on April 18, 2020, 06:34:06 amThe last post(s), they ever made (unless they come back), was dated 1st April 2020.https://www.eevblog.com/forum/projects/sprint-day-0-what-open-source-ventilator-projects-exist/msg2994090/#msg2994090The virus situation, isn't something which people should make April 1st joke threads about, so I hope their apparent disappearance on April 1st (last post, to date), is just a coincidence.

author: MK14
author_rank: Super Contributor
timestamp: April 18, 2020, 09:04:01 am
content: Quote from: dietert1 on April 18, 2020, 06:01:02 amThat is amazing!But they had the needed ingredients, to do it that quickly.The right engineers, in the some building/organisation, who knew each other (presumably or mostly).The test dummy thing, to test out the prototypes.A massive manufacturing plant, with recently suspended production.To try to do it as a thread (here), open source, didn't seem like a good idea. The chap/OP (sorry), didn't seem to have the right skills set, to make it happen.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 09:11:30 am
content: Quote from: dietert1 on April 18, 2020, 06:01:02 amthat still sounds like the type they don't want, no mention of how it interacts with patients without destroying their lungs unless it is used for limited time only.

author: EEVblog
author_rank: Unknown
timestamp: April 18, 2020, 11:57:16 am
content: Quote from: MK14 on April 18, 2020, 09:04:01 amAnd dozens of other teams aren't even close.Hopefully this whole worldwide endeavor shows that going from prototype to a practical working and safe (and approved) medical device is extremely difficult and time consuming.

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2020, 12:10:09 pm
content: Quote from: dietert1 on April 18, 2020, 06:01:02 amI'm surprised that wiper motors have the durability to run 24/7. They run in good airflow under the scuttle panel and normally don't have a high long term duty cycle. Maybe they can get away with it due to reduced mechanical load (?).There again, the only ones I've examined are those that have failed due to exposure to the elements and age.

author: MK14
author_rank: Super Contributor
timestamp: April 18, 2020, 12:36:59 pm
content: Quote from: EEVblog on April 18, 2020, 11:57:16 amThere are so many potential pitfalls, which are not so obvious to the less experienced.I'd be especially worried about getting hold of the correct parts, on time.Typically, you can order a few hundred of a wide range of parts, from the usual electronics parts suppliers, such as Digi-key.E.g. They might have 153 in stock, of the microcontroller, you have used, for the ventilator.But, if you need 25,000 of them (or even a few thousand), you may well find there is a huge lead time, of many months.Substituting other microcontrollers, could well lead to very time consuming software rewrites and retesting again. Especially if the hardware I/O configuration (PWMs etc), is completely different, between the MCUs.Even just a change of case. could mean re-spinning the PCB.Mechanical parts (especially if custom), are probably even worse.I think it usually takes a very long time, perhaps many, many years, for new medical devices, to complete all their extensive safety (critical) tests and approvals.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 12:42:28 pm
content: i recently had an interview with a company that is designing a medical device for virus testing (not covid-19 related). the test involves heating the sample. This fact alone means that their software has to be independently assessed and the device needs non software fail-safes. the risk is that if the container overheats and explodes those nearby could be contaminated. That is all the risk. Rasberry pi's are not qualifiable.It's not as simple as people think.

author: OwO
author_rank: Super Contributor
timestamp: April 18, 2020, 01:00:48 pm
content: You can sense airspeed as a substitute for flow rate, and airspeed is easily sensed with a pitot-static system (2 pressure sensors).

author: MK14
author_rank: Super Contributor
timestamp: April 18, 2020, 01:15:10 pm
content: Quote from: OwO on April 18, 2020, 01:00:48 pmGood idea, in principal.But setting up and developing such systems (properly), can be very time consuming. There are so many different flow rates, pressures, temperatures and ratios of air to oxygen levels.Designing for a high percentage of Oxygen (from what some hospital sources seem to be talking about, as regards treatments), is probably another factor. Potentially making it take a lot longer to sort out the ventilator.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 01:20:52 pm
content: All flow measurement tends to revolve around speed measurement. It's how car mass airflow meters work. there are plenty of differential air pressure sensors that have a logarithmic output to counteract the exponential increase in pressure differential across a restriction.The challenge in the ventilator design is that it correctly interacts with the human body so that it works with the body, the result of working against the body is lung damage.

author: MK14
author_rank: Super Contributor
timestamp: April 18, 2020, 01:39:44 pm
content: Quote from: Simon on April 18, 2020, 01:20:52 pmAlthough it is indeed challenging to make sure the ventilator correctly interacts with patients.You also (ideally), want to make sure that the proposed hardware, performs safely (to specifications), despite changes to various parameters. When you start listing them, you can end up with a huge list.Supply voltages, component tolerances, ambient temperature, mechanical variations, Oxygen percentage, air humidity, operator error, faulty Oxygen supply, etc etc.Covering all those different things, can take years of research and development.Otherwise, the machine could end up harming patients.Presumably some sort of compromise to the usual required safety standards, has been taking place. I don't know how far they have gone, in that respect. Maybe in the future, there will be reports about it.What engineers (electronic/mechanical) really need, is some kind of definitive specification, as to what is required and what will be accepted. Without such information, it can be very difficult to come up with proposals.But it could be argued, that even a spec takes time to write and maintain.I.e. The OP needed to realise this, and come up with such a specification, very quickly.Maybe they did, maybe they didn't. I found it too confusing, with the OP, starting multiple threads, and changing the top of thread(s), on a daily basis. It got too confusing, at least for me.I'm still NOT clear if it was an April 1st joke or not ?

author: chris_leyson
author_rank: Super Contributor
timestamp: April 18, 2020, 02:11:29 pm
content: A small turbine might be one approach as this wouldn't be effected by changes in air temperature or humidity, perhaps a small DC fan used in reverse. Small turbines were used in mines to measure airflow and here is a picture of an L. Cassella air meter No. 312. I think it's calibrated in cubic feet.

author: coppice
author_rank: Super Contributor
timestamp: April 18, 2020, 02:15:53 pm
content: Quote from: Simon on April 18, 2020, 01:20:52 pmMass gas flow sensors don't measure speed. As their name says they measure mass flow. For gases mass movement is a combination of speed and pressure, and for most gas flow measurements its the measurement you really want. The actual MEMS sensors typically used these days for mass gas flow measurement are very cheap. If BOM is your major issue, building the basic MEMS device, from someone like Memsic, into your own tube should be a winner.You could also look at the ultrasonic gas measurement solutions being built for domestic energy meters. You can measure flow quite accurately for a few dollars with those, but the mass flow MEMS sensor is probably a lot quicker to design in.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 02:41:23 pm
content: Quote from: coppice on April 18, 2020, 02:15:53 pmHave a look at vehicle mas air flow meters. They are air speed meters. I use them, we take the meter out of the tube it comes in and into a smaller tube to increase sensitivity because same mass in a smaller tube has to flaw faster. They work on the basis of air cooling a temperature dependent resistor whilst compensating for ambient.Any flow measurement based on pressure differential is a speed measurement, again the cross section of the channel needs factoring in. If you measure pressure drop over a restriction of same mas flow but in different diameters you will get different pressure results.

author: coppice
author_rank: Super Contributor
timestamp: April 18, 2020, 02:46:30 pm
content: Quote from: Simon on April 18, 2020, 02:41:23 pmThe reason you have increased the sensitivity is that the MEMS sensors only sample the mass of the air close to the edge of the tube. If you reduce the tube diameter that becomes a bigger fraction of the total mass flowing through the tube. Its not a linear relationship, because the gas near the middle of the tube typically moves faster than the gas near the edge, but this is fairly consistent over time, so you can calibrate a good mass flow figure for any particular tube.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 02:47:29 pm
content: Quote from: chris_leyson on April 18, 2020, 02:11:29 pmyou would have to account for "stiction" or startup inertia and friction that would bias results. I have just bought a peak flow meter to measure how strong my lungs are. The instructions state to test the freedom of movement of the marker, it's a very simple instrument that relies of free movement with low friction of the needle.

author: coppice
author_rank: Super Contributor
timestamp: April 18, 2020, 02:55:00 pm
content: Quote from: Simon on April 18, 2020, 02:47:29 pmVentillators go back a long way. Before MEMS mass flow sensors there were larger versions of mass flow sensors (there still are in soma applications, like HVAC systems) which may have been used, but I assume if you go back far enough they used spinning paddles, The stiction is a definite drawback, but if you want cheap, and quick to implement it could be a reasonable way to go.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 02:58:36 pm
content: Quote from: coppice on April 18, 2020, 02:46:30 pmWhich is air speed! the air in a smaller tube has to go faster for the same volume. A small unknown portion of the air goes through the sensor but the result is meaningless in terms of volume if you do not know the cross section area you are sampling. The sensor element on it's own will not tell you the mas airflow you have to calculate that yourself. Further back i uploaded the datasheet for a bocsh sensor, the same sensor can be purchased in different size tubes and each one will give different mas flows for the same output voltage. With great ease we took the curve of the smallest tube and recalculated what it would be for a slightly smaller tube. The change being so small that our recalculations were very accurate when we came to callibrate the setup.Put it this way, if you take the sensor out of the tube and run around a room with it will you get a reading? Yes you will, What mass of air did the sensor move through? No one can tell you. Or what if you blew on the sensor, what mass of air did you blow? no one can tell you because the cross section area is unknown. At it's very heart yes t is a mass flow that causes the reading but the sensor itself outputs air speed,Same reason why my peak airflow meter is a tube, it needs a known duct to tell me what the flow was or it would just be a dumb pressure meter. Unsurprisingly the further up the scale you go the closer the marks but then 700 and 800 are further apart again so clearly this was calibrated.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 03:00:19 pm
content: Quote from: coppice on April 18, 2020, 02:55:00 pmI am not saying not to use that type of sensor but to re-purpose a fan is a bad, bad idea. My £15 peak flow meter off amazon is entirely mechanical. But it was designed for this purpose.

author: coppice
author_rank: Super Contributor
timestamp: April 18, 2020, 03:03:49 pm
content: Quote from: Simon on April 18, 2020, 02:58:36 pmTry increasing the gas pressure, while maintaining the same gas speed. Something like a paddle sensor, which senses speed, doesn't increase its reading. Try it with a mass flow sensor, and the reading does increase, because more mass of gas per second is flowing past the sensor.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 03:11:15 pm
content: Yes, and guess what? you have to know the pressure to calculate mass!!!! I am sorry coppice but I work on life support systems using these types of sensor!!! we also take into account pressure. More modern MAF sensors contain guess what?A PRESSURE SENSOR!!!!!!!

author: coppice
author_rank: Super Contributor
timestamp: April 18, 2020, 03:19:30 pm
content: Quote from: Simon on April 18, 2020, 03:11:15 pmYou have this backwards. The MAF sensor gives you the mass which has flowed. You need a pressure sensor if you want to derive the speed from that. For a ventilator you want to know the amount of gas which has flowed, and a MAF gives you that directly.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 03:22:25 pm
content: actually no your right we measure pressure because we want to know the volume not the mass. But as the sensor presents itself for adaption it has to be considered as air speed if you want to recalculate on a new cross section area. But they still do not tell you what the mass measurement of the sensor, they don't tell you what amount of mass is actually going through the sensor and they don't tell you the cross section of the sensor so on the available data it is a speed measurement at nominal atmospheric pressure.

author: chris_leyson
author_rank: Super Contributor
timestamp: April 18, 2020, 04:04:56 pm
content: Good comment about stiction and inertia but once running you would have to overcome very little friction and the load imposed by the gear trains which I guess would be quite low for a well maitained and calibrated intrument. I think the Casella air meter might date from the early 1900's and perhaps eariler if it was purchased second hand. Somewhere around here I've got a much larger version and that only needs a gentle breeze to get the turbine spinning.It's important to note that these sort of instruments were used in free air and the pressure drop across the tubine is tiny. In a CPAP ventillator the pressure range is somewhere between 15 to 25 cmH2O more than enough to overcome stiction and get a turbine started. Turbine flow meters have been in use for over 100 years so why change something if it isn't broken.

author: Simon
author_rank: Unknown
timestamp: April 18, 2020, 05:03:14 pm
content: i am not saying that the little turbine meters are a problem. an actual turbine meter is designed to be light and rotates very easily as it has no brushes. The suggesting though that a cheap DC fan be used as a metering turbine is not a good one. You have a "heavy" rotor to get moving and it has brushes on it that create a lot of friction.A small permanent magnet synchronous motor with very low cogging maybe if it does not have a heavy rotor.