small/tiny flow meter meant for sampling air speed

[coppercone2]

So I am interested in cooling systems and I have been thinking about ducted fans alot.

Lets say you are working on a prototype chassis and you want to see various air flow's through your system.

If you open up your chassis and drill some holes you can position some kind of sensors through out it to measure its actual performance, if you have complicated duct work inside of it.

It seems that for a non-heated test you could just position some calibrated hot wire probes that measure their own cooling in the chassis connected to some bridges to measure things..

So assuming you make these guys, how do you calibrate it? I thought to just make a wind tunnel that I can put some kind of calibrated sensor into, space things out quite a bit, and calibrate my probes that way.

This however is a lot of work.

Questions:

 Are there some kind of small sensors that maybe have little propellers or something you can put inside of a chassis that come with some kind of calibration?

what kind of span and accuracy and drift (with age) can I expect from thermal sensors like made out of old light bulb parts? Like from  the oxidation of the filament.

Is there some kind of work published about designing these sensors with particular wire dimensions for  particular flow rates?

The resistive material is non-linear, so it seems that calibrating for temperature (if you wanna measure flow in a hot chassis) would be a complete pain in the balls and require a heated wind tunnel and complex look up tables, this makes me want to find a tiny propeller type sensor instead that has a tacheometer output.

These sensors need to be small, I am envisioning duct sized from like 5x5 inches, larger electronics chambers, and smaller ducts (down to measuring flow rate through tubing). The hot wire sensor seems really appealing here since you can splice a T in on the tubing and put the wire into the flow path so its all really small.

I would not mind an assortment of different sensors.

I already have a multi-input RTD thats pretty decent, and some DAQ boxes. I am not sure if pressure is a useful metric for measuring thermal performance other then to make sure the fan is not being loaded too hard, flow should provide more useful information for almost all cooling equations i think, unless you can some how predict turbulance based on pressure, flow and temperature.

Also, since I have not played around with solid works flow that much, does anyone who does this alot know how flow can change as different parts of the air circuit are heated? Like for instance if you have two ducts and one of them leads to the middle of a big heat sink, and the other part of the duct blows over a buncha spaced out components that leads into a big chamber, does the flow path change significantly when the big heat sink heads up to a much higher temperature then the other chamber (say the air temp maximum is 85C and the ambient is 30C.

[IDEngineer]

There is another thread on almost exactly this topic, right now:

https://www.eevblog.com/forum/projects/air-fluctuations-indoors-detectingmeasuring/

[coppercone2]

I saw that thread, but I thought other then sprouting this idea it was unspecific and I would be derailing the op by asking questions about sensors in small enclosures when he is interested in his house. He very specifically worded his problem. I am looking for diagnostic probes to put into a small enclosure or even a test system meant for it and for people experienced in troubleshooting such systems.

[Benta]

Try looking into ultrasonic sensors. The idea is a sender and a receiver in the air stream, where you measure the frequency shift at the receiver..

[langwadt]

two temperature sensors, one that is shielded from the airflow one that is in the airflow. Run at enough current to self heat. The shielded sensor compensates for air temperature, the unshielded gets cooled by airflow

[ogden]

Are there some kind of small sensors that maybe have little propellers or something you can put inside of a chassis that come with some kind of calibration?

Google search for "miniature" "anemometer":

https://www.testersandtools.com/uei-dafm4-miniature-vane-anemometer-and-psychrometer.html

hint: check not only google search URL results but image view as well

[coppercone2]

I did but I am greeted by price tags of at least 400$ for a very simple probe.

I mean like little sensor heads I can embed into stuff, like drill a hole in the chassis, run a wire (or run it through a air duct etc) and connect it to a DAQ. I want alot of these things.

After its all closed up and stuff.

The device with a hand held part is just gonna be useful for calibration and taking vent measurements. I wanna see whats going on inside. I was hoping there was a engineered kit for this kind of thing that I can mimic.

Like you know how you epoxy a bunch of thermocouples to various parts/chips in a system, put it back together, then take measurements on them while its running? I want to do that with air.

Or to even embed sensors into my product, so I can have a 'vent blocked' or 'insufficient air flow' or 'change filters' setting, it makes sense to do it with a ducted fan IMO. I don't like axial fans anyway.

I was hoping for some standard solution like with RTD's and thermocouples, where you have a standard form factors, accuracy classes, linearization equations (like calsen van dusen polynomial for RTD or whatever), mounting solutions, etc.

Like a thin-flm class B rtd meant for measuring up to 200C is a 'standard' part between manufacturers pretty much. Once you seen one you know how the electronics and mounting for it are gonna go, and you know there will be standard options like 2wire, 3wire, 4wire, etc.

Or 1/4 inch NPT thread class A rod thermocouple (i.e. you can find similar parts from every manufacturer, like Omega).

I know there are metrological solutions for all this stuff, like a 10ohm 4000$ RTD, but I am just looking for stuff that you would equate to like standard thermocouples, surface mount RTDs, etc... that everyone in the industry is gonna be familiar with.

[ogden]

I did but I am greeted by price tags of at least 400$ for a very simple probe.

You want precise, miniature air flow sensor with calibration certificate for free? Stop dreaming

Besides anemometer there are (already mentioned) MAF sensors - like those in modern cars. Miniature one. Most likely far from cheap:

http://www.directindustry.com/prod/azbil-europe/product-18885-1612478.html

As always you have option to build one yourself - if you think your time & effort does not cost anything.

[coppercone2]

yea, just how free RTD, thermocouples and thermistors are, where you are sold something that can reach a basic accuracy class if you just follow the equation they provide.

Even a shit Chinese RTD is going to be fairly decent.

Where did I say anything about a certificate? I meant a sensor with some accuracy class and provided linearization equation. The circuit for these hot wire things is essentially the same as a high power current RTD, but they use less fine wire and they are less complicated to make since you don't need to account for all sorts of factors or wind many feet of ultra fine platinum on a ceramic bobbin.

guaranteed by design/physical equations to fall within some kind of region with quality assured by the manufacturing process and materials purity.

A little fan I can see giving a problem, but a standard manufactured wire of standard chemical composition with standard mounting is a pretty simple system thats gonna work well. There is little tooling to wear down and stuff, so it should be pretty solid.

[max_torque]

A reasonably small Pitot tube is easy to fabricate, or can be bought, and read with a small differential pressure sensor

[coppercone2]

Do you think standard chip RTD's could be used?

How much heating do you need in this kind of system?

[coppercone2]

A reasonably small Pitot tube is easy to fabricate, or can be bought, and read with a small differential pressure sensor

How thin can a tube like that be practically? How nimble can it be?

[max_torque]

just google "small pitot tube" for the answer!

[IDEngineer]

A reasonably small Pitot tube is easy to fabricate, or can be bought, and read with a small differential pressure sensor

Agreed. If what the OP wants is to "drill a hole and stick something in it", start with an integrated differential pressure sensor. They're reasonably priced (many in the ~$20 range, check DigiKey and Mouser), many have throughhole mounting, and the diff's (as opposed to absolute or gauge style) have two hose barbs. Connect a small hose to the P1 (internal die is protected) side and run the other end into whatever compartment you're measuring. Let the other hose barb just sit in ambient. Voila - you can sense pressure changes relative to ambient and relate them to airflow.

[Conrad Hoffman]

Calibrate by holding it out the window of a car traveling at a known speed. Use a hand held GPS for speed if in doubt.

[langwadt]

steal the insides of something like this? https://www.banggood.com/Wholesale-Air-Flow-Mass-Meter-Sensor-Maf-for-Opel-Astra-G-Vauxhall-Corsa-MK-I-0280217123-p-62109.html?rmmds=detail-left-hotproducts__1

[duak]

If you have access to compressed air, could you use a compressed air flowmeter and a diffusor to provide a standard airflow?  Omega has some +/-5% flowmeters for about $60-80: https://www.omega.com/pptst/FL2000.html  I think a diffusor could be made from a funnel and some open weave fabric.  A round tube with a known inner diameter would attach to the big end of the funnel and your sensor inserted somewhere in the side.  I'm not up on fluid or gas volume measurement but recall the flowmeter reading may have to be compensated - something about "standard volume"?

Cheers,

[coppercone2]

Oh yea I used those flow meters before, they are probably the right tool for the job. Totally forgot you can use them for air.

That would defiantly be a worthwhile acquisition.

Would differential readings need significant compensation for temperature in the -40C +150C range? Then again a system that ends up diverting really hot air where you can't measure the intakes to get an idea of whats going on is fairly insane sounding.

With a flow meter you need to compensate for kinematic viscosity is all I know, but that's dependent on the fluid, and possibly meter reynolds number drag coefficient or something. I think those meters can also cavitate and sometimes they get stuck.

[David Hess]

A differential pressure sensor is easier to use and inexpensive.  It is even possible to make one with just a piece of tubing partially filled with a liquid.

The tungsten filament in a hot wire anemometer operates just below incandescence so operating life is not a problem; they are mechanically fragile however.  The high temperature of the tungsten filament minimizes the effects of ambient temperature changes however a pair of transistors can be used instead with one compensating the other.

[coppercone2]

has anyone ever seen a commercial product with a vent clogged indicator?

I guess its kinda redundant maybe but I kinda like the idea. I thought it might be useful because it would instantly tell you shits fucked up rather then having to wait till its all warm and loaded to decide to have a problem.

Seems like gilding the Lilly from a commercial stand point but I think its really nice. Especially for PC towers.

[ogden]

Where did I say anything about a certificate?

Here:

Are there some kind of small sensors that maybe have little propellers or something you can put inside of a chassis that come with some kind of calibration?

[coppercone2]

i meant useful by design the rest of my posts make that abundantly clear

by the standards of a shipped sensor a rtd is calibrated.

I actually have a variable speed milwakee battery powered air blower that can act as a source for a large tube that can be stabilized. My compressor is too small for a significant standards I refuse to own one of those fucking noise machines.

[ogden]

by the standards of a shipped sensor a rtd is calibrated.

No. Quality platinum RTD's are so good, so well conform to specification and straight temp curve that they do not need calibration for most applications, yet it does not automagically mean that they are calibrated. They are not.

[coppercone2]

yea thats what i mean for practical purposes they come calibrated, the sensor is much better then the required application in many regards, just like the accuracy of flow through a cooling duct is not required to be very high at all. i imagine the simple shit thrown together does the job and someone might sell to a standard.