Low cost digital readout of velocity and flow

[Tony]

[bruce]

[115-1172523331]

Tony, I am not sure we should "throw in the towel". You said "It is still entirely possible to build a direct reading digital manometer with a linear readout of pressure, but converting this to velocity or flow directly in analog is not as simple as I had at first hoped."

If I understand your comment, I can use the transducer driven display to give me dynamic pressure, I can input that pressure into my Excel spreadsheet and get my velocity and flow rate. What I am doing now is to read a manometer "display" of "units" of pressure (i.e., inches of water or something), adding correction factors for well drop and, on my pitot bench, atmospheric conditions and elevation, ..., then letting my Excel spreadsheet do something and I get velocity and flow rate. Ask yourself if the digital is more accurate in reading and/or is it easier to use. Add to that that I MAY learn something new (but it is hard to teach an old dog new tricks) and THEN decide if it is time to quit! My job in aerospace was not only to find new ways to do something, but to find new "somethings" that needed to be done. Learning something, ANYTHING, about DAQ is one of the latter! There is still value there for me, even if it is a backup that lets me check on the condition of the bench components or eliminate the second manometer tube on my vertical. (Mine doesn't read the inverse square anyway!). Back into the lurch? -- Doug

[86rocco1]

How about using a sensor with a root square output? I'm thinking of something like the . That particular sensor has a 3500 Pa, not high quite enough for this project but perhaps a similar sensor in the appropriate range could be found.

[Tony]

Doug, you certainly have a point.

Analog readout, such as a meter with a moving pointer, can be extremely useful where you do not need to know an exact number, only if something is high or low, or perhaps increasing or decreasing. Excellent for something like an engine tachometer in a vehicle, where the pointer is constantly moving all the time. Excellent too for engine water temperature or oil pressure indication, where the exact numbers are not that important.

Now probing a port with a velocity probe is probably a bit like like that. You do not really need to know if the velocity is 300 FPS or 301.7 FPS at a particular place in the port. You just want to probe here and probe there and get a general idea of where it is high, where it is low, and where the flow may be unstable.

So an analog display such as a water manometer may actually be just as good as a digital readout for velocity probing.

But flow measurement is rather different. You need to nail down a hard number that is both as accurate as possible, and highly repeatable. It is no good having a digital display that has a readout resolution of 0.1 CFM, if the accuracy of the reading could be out by a +/- 12 CFM error.

If something reads down to 1 foot per second velocity, or one CFM flow, it should be able to read accurately down to that resolution. That can be a very tall order with a digital display for some particular types of measurement.

For velocity probe measurement a water manometer is probably the most straightforward and reliable. Another low cost solution might be a Magnehelic pressure gauge that reads to around fifty inches of water or two psi. These gauges are ideal in that they have two differential input ports and measure differential pressure, and are reasonably large and easy to read. A replacement home made scale calibrated in feet per second would make for an ultra low cost velocity probe readout. These gauges sometimes sell for as little as $10.00 on e-bay, but you need to make absolutely sure it has a suitable pressure range for the intended application.

I also looked briefly at surplus aircraft airspeed indicators. While the quality would be excellent, the speeds are a bit low for the gauges I have seen, and knots are not a terribly useful measurement unit. Something out of a jet aircraft with a digital match indicator would be fun to play with, but I doubt if it would be within my budget!

Getting back to digital manometers. While building one yourself is certainly feasible, it comes down to why would you want to? These days there are measuring instruments made in China that work pretty well for probably less than you can buy the parts for to build your own.

If you look on e-bay under "digital manometer" there are instruments there for $185 that can be used to measure all sorts of things in many different pressure units, beside flow bench pressures. Fuel pressure, boost pressure, manifold vacuum, exhaust back pressure, differential pressure drops across intercoolers and pipework, mufflers and so on. You get a lot of extra features that would be difficult to add to a home built project.

Some things are just not practical or economic to build yourself at home these days.

[115-1172523331]

A quick check, Newark sells the Sensirion 2000 at $93+ per unit (1-9 units) which drops to $66+ per unit (over 50 units). Maybe the square root output eliminates some other components? -- Doug

[106-1194218389]

Does anybody besides me think this will work?



It is a $24.95 data aquisition unit with software. It has analog or digital inputs. Will output to PC and uses their software for read out. You can down load a copy of the software that is included with this to look at it. It also has a remote function that you can hit to start and stop. If we need to go software I think this may work. I just wish I was better with electronics. If we could figure how to couple a transducer to this so it inputs to the software then just maybe it will work.

John

[Tony]

Doug, it would certainly work, but the cost is creeping up. It also converts from analog to digital, then back to analog, so you then have to digitize it again within the display. Two sequential stages of digitizing where the possible errors compound. Once something is digitized, it should stay in digital form.

John, that is a perfect example of being able to buy a an excellent Chinese product far below what you could make it for yourself.

The logical way to do this would be to buy a suitable pressure transducer, and couple it up to a PC (via an amplifier) through something like John's Dataq starter kit. Once it is digitized, you can do almost anything. And you then end up with all the essentials of a proper flow computer.

It really keeps coming back to the Forum flow computer project.

When my original idea fell flat, it really knocked the whole concept out of contention, because it lost it's original simplicity.

[115-1172523331]

Thanks, Tony! It looks like it is back to page 1 for "DAQ for Dummies!" In your library, do you have any books you could recommend for beginners? Who is in charge of the Forum computer project, or is it still early enough in the formulation (forumulation?) that it isn't decided?

As a secondary question, in aerodynamics, dynamic pressure is (as shown in your link on the original text) 1/2*rho*V^2. In using a probe (or a pitot), I thought you are measuring a differential pressure. How were you getting the dynamic pressure from a differential pressure transducer? Inquiring minds still want to know! I always like to read your technical descriptions, but most of the time I get lost! -- Doug

[115-1172523331]

John, Do a search on "dataq" and you will find some other people with that unit, including Bruce. The first place I looked was here:



which turned out to be in the dyno section. As for me, I haven't the foggiest idea of what "4-channel, 10 bit..." means. I do know what a serial port is!

Doug

[Tony]

[bruce]

Ok, so as to avoid any confusion there is a PTS flow computer under development that will be using a card from Dataq posted earlier. It will output to Excel so custom spreadsheets can be made or existing ones used. The "plan" is to offer this unit in various forms ie; plans, kit, fully assembled etc.

It will be offered forsale at a reasonable cost (undercutting the cost of the commercial units* being sold now) along with the rest of my product line. The goal is for everyone who builds a bench to be able to go digital and do it for less than the cost of water gages.

It is not a collective forum project but once it's finished some of the info will more than likely be passed along in this forum. I'm not the one doing the work (I'm sensor stupid) on this project but can say it is in breadboard stage right now and hope to be playing with a prototype on my bench in the near future.

So that being said if the "minds" want to discuss a forum DIY flow computer it's an open subject as it will only enhance the ability of the DIY'er to have various options for a flowbench build. I'd move it to another thread in this category.



* I gave each commercial unit the opportunity to come-on-board when I released my plans and they elected not to, so we are doing it ourselves. It will then be up to the consumer which way they want to go . . . my thoughts on this would be; a 6-8 blower 600-800cfm fully computerized cabinet style flowbench for less than $1500!



Edited By bruce on 1209467616

[115-1172523331]

Bruce, IF I get the pressure transducer samples, would they be of value to this project? Since the low-cost digital display manometer project has "splattered up against a brick wall" (to use Tony's words), I MAY have some extras.

Too many "honey-dos" right now, hoping to get back to my water gauge problem this week and racing starts in less than 3! I need to get things moving again, even if in the wrong direction. I've always loved the phrase "I may be lost, but I'm making good time!" -- Doug

[115-1172523331]

When I resumed my search for "DAQ for Dummies" this morning, I found this site.



I tried the "Design" section first and it may be what I am looking for. If any of the rest of you are as "challenged" as I am, you might give it a try. -- Doug

Try this site too. I haven't looked more than a few minutes, but interesting. Good place for "parts" and it looks like they sell singles.

[R Thompson]