Air fluctuations indoors - detecting/measuring? - Page 1
Forum section: electronics / projects

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: September 22, 2018, 08:12:10 pm
content: Funny headline, eh?What do I mean?Well. Inside a living room, say, somebody opens a window and now there is a little draft. Or you have the windows open all the time, but it's getting windier. I really do mean air drafts, not anything with necessarily windows as a part of it.The device in question is, obviously, required to be directly in the path of the air movement, and that's fine.Now I wonder: Is there a cheap way to detect such things? Because it would be for a toy project only. It mustn't cost muchI have some non-EE (I'm not one) ideas floating around, and no clue how realistic they are...Thinking out loud here...E.g. having 2 temperature sensors, one which is more hidden from the air and within some more isolating material, so that only long-time room temperature is captured by it, as a reference, and then another sensor which is more exposed, and in some way, reacts quickly to its local temperature changes. And if the "quick sensor" has a significantly different value (more than the very slow day getting cooler in the evening, which the quick sensor sees first), there is movement?Or, it could be heated a little, in a controlled manner, and when there is a little draft, it takes slightly more average current to keep it at a defined temperature? So it can't store much energy, it needs to react quickly, and the heating on/off will probably also be a rather quick switching action.The fact that I can sense air drafts on my skin tells me this isn't, in principle, a totally stupid start, or is it?Does this smell like something where some sources of noise are going to be a party pooper, eh?Something like that?Something totally different?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 22, 2018, 08:19:14 pm
content: The typical sensor work about the way combining the two ideas. There are 2 temperature sensors heated by a certain amount and the one with more air draft stays colder. Such a system is also used in some cars to measure air flow and in some industrial vacuum cleaners to make sure they actually work and the filter is not clogged.Another way, though usually a little more expensive is using a kind of ultrasonics - it gets faster with tail-wind.

author: Nusa
author_rank: Super Contributor
timestamp: September 22, 2018, 08:26:06 pm
content: Make a DIY Anemometer. Could be as as simple as some paper cups, sticks, and a pivot point.Add small magnets and a hall-effect sensor and you have a speed output, which you can then calibrate.Alternately, have a cheap DC motor on the shaft and measure its output as a generator.Many variations on the theme.

author: Cerebus
author_rank: Super Contributor
timestamp: September 22, 2018, 08:28:10 pm
content: Search for "hot wire anemometer". You'll need a length of resistance wire, a constant current source and some way of measuring the voltage across the resistance wire and reacting to it, a microcontroller with an ADC is the obvious thing to do that with.

author: Tangent_Tracker
author_rank: Regular Contributor
timestamp: September 22, 2018, 08:34:00 pm
content: Break the glass of a filament blub in a vice. There you have your hot wire!

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: September 22, 2018, 08:41:34 pm
content: Hm! Breaking bulbs, eh? Ha. Some old spherical bicycle bulb could just be the right thing, you would say? (It has a nice form factor)You guys have been so quick, I need to look those things up, thanks already!

author: hlavac
author_rank: Frequent Contributor
timestamp: September 24, 2018, 05:25:13 pm
content: Yep, light bulb filament. Just dont put normal current through it, it will burnLight bulb filaments have a nice property of increasing their resistance as they get hot.Try putting small constant current through it (it shouldn't glow at all) and measure the voltage drop...

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 24, 2018, 05:30:27 pm
content: Yup, an exposed light bulb filament in a simple bridge circuit will detect air motion you can't even feel.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 24, 2018, 07:04:49 pm
content: You can also buy nichrome wire to use for it - you can sometimes get it in craft/hobby stores as replacement cutting elements for hot wire foam cutters for a few dollars for a meter or two.If you have a pretty high sensitivity, low thermal mass temperature sensor and a fairly high resolution voltmeter, you may even be able to detect drafts passively, if their temperature differs at all from ambient. It's not low cost unless you have one, but if you have a 5.5+ digit multimeter and a resistor with sort of a large tempco (a few hundred ppm/degC), you can get some information out of that too in resistance mode (it's effectively the same concept as the aforementioned circuits, but very easy to setup and see if you have the meter).

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2018, 11:10:18 pm
content: Get a sensitive differential pressure sensor and through tubing, terminate one port outside and one port inside.For the hot wire anemometer, use small low voltage bulbs. Jim Williams published a bunch of examples in the application notes he wrote.

author: IDEngineer
author_rank: Super Contributor
timestamp: September 24, 2018, 11:51:03 pm
content: I picked up a handheld anemometer - a little shrouded fan in a case with an LCD display - for about ten bucks a while back. Some site like Monoprice or TMart. Included a nice padded zipper bag. Works great, detects the slightest breath of air. I like DIY projects as much as anyone but when you can get all the basics, done, in a package for ten bucks... your time is better spent working on your preferred interface rather than fabricating the mechanicals.Scan around on the web for handheld anemometers. They're surprisingly commonplace these days and remarkably inexpensive. Not lab grade, but doesn't sound like you need NIST traceability.EDIT: I found this photo on TMart. Mine is slightly different in that the fan is larger relative to the enclosure and more integrated into the overall enclosure, not sticking out like this one. Current price on this one was under $20. I bet you can find one even cheaper.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: September 25, 2018, 07:38:17 pm
content: Hehe, that last thing there is about 20x as big as I want it to beBut interesting to know it anyway, might be useful for other scenarios.Interesting mentions so far in this thread.So before Jim Williams was mentioned, I was going to try this:First, using a simple current source like this, to CC-feed the filament of a glassless bulb:https://en.wikipedia.org/wiki/File:Const_cur_src_113.svgThen, use a voltage divider as a second "sorta constant current source", adjustable with a trimmer, to get the same voltage as the filament end at the collector at "no air flow", and amplify the difference between those two voltages with a single opamp.Now, I found this, AN5-7, Figure 11:http://www.analog.com/media/en/technical-documentation/application-notes/an05f.pdfThat one also uses a small lightbulb, but regulates it to constant temperature (indirectly). Then the apparently non-linear resistance changes are not only amplified, but also bent to linearity.Is that approach better, I mean the constant-temperature vs. const-current?Also... given that this is not for scientific measurements but to roughly sense different kind of air currents like you can feel them on your skin - could the analog linearization be omitted, could I just do that in software, when I have a 12bit ADC?

author: cat87
author_rank: Regular Contributor
timestamp: September 26, 2018, 12:20:18 pm
content: Aside from the hot wire method, you ca also try something to see the actual air flow. You basically need a light polarizer and a source of light. There's a name for this thing but for the life of me, I can't remember it.If automatic detection is needed, perhaps a camera looking through the polarizer and some computer vision python library? Not too practical, I'll admit, but waaay higher on the coolness scale

author: tecman
author_rank: Frequent Contributor
timestamp: September 26, 2018, 07:03:09 pm
content: Quote from: DaJMasta on September 24, 2018, 07:04:49 pmNichrome has a relatively flat tempco over small ranges. Copper would be better, but I would agree with tungsten

author: IDEngineer
author_rank: Super Contributor
timestamp: September 26, 2018, 07:18:29 pm
content: Quote from: DaJMasta on September 24, 2018, 07:04:49 pmFYI, eBay also sells nichrome wire as replacement element material for vape devices. (I've purchased it that way to use for firework fuse igniters.)

author: IDEngineer
author_rank: Super Contributor
timestamp: September 26, 2018, 07:23:53 pm
content: Quote from: TinkeringSteve on September 22, 2018, 08:12:10 pmOften that sensitivity is due to the tiny little hairs on your skin being moved by the airflow, not solely due to temperature changes caused by evaporative cooling. If the latter were the only sensing mechanism, you might not feel air movement that was the same temperature and humidity as your skin. It's an evolutionary defense mechanism.Which raises an interesting point: A mechanical sensor (such as an impeller) detects air movement directly. Most of the other schemes are measuring secondary effects (temperature changes, evaporation, etc.) which, since they are at least one degree removed from what you REALLY want to measure, will be subject to errors in the secondary mechanism being used. It's probably possible to come up with a scenario that would "fool" such secondary mechanisms, but something that is actually measuring real, honest to goodness air movement would be more resilient.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: September 26, 2018, 07:51:58 pm
content: IDEngineer: yes, it was clear to me from the beginning that there are very probably scenarios that would fool this detection. Like perhaps suddenly opening an adjacent window in winter with freezing temperatures outside - even if there was currently zero wind, the sudden drop in temperature would probably fool the thing, if I'm not mistaken.Luckily, that's not really important for a cheap toy projectBut good to know about, certainly.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: October 10, 2018, 01:10:09 pm
content: Quote from: cat87 on September 26, 2018, 12:20:18 pmMaybe you mean schlieren photography? That doesn't usually use polarization, but it does usually use lenses/mirrors and a knife-edge. (There's also synthetic schlieren, in which you just put a known pattern behind the flow and analyze the change in the observed image of the pattern. It requires computation to produce the flow image, though, which regular schlieren imaging doesn't.)I've also been wanting to measure indoor air currents recently. A hot-wire anemometer would work for me (and I even found an apparently intact lightbulb on the sidewalk the other day), but how do you detect flow direction with it? I guess you could put a pipe around the sensor, and rotate it while taking readings, but that still leaves you with two possible directions.

author: edigi
author_rank: Regular Contributor
timestamp: October 10, 2018, 01:30:11 pm
content: While I don't know what's the point of keeping the window(s) open/checking air flow but one idea (beyond the obvious anemometer) is based on good CO2 sensor(s) + maybe sensitive temperature sensors.I use MH-Z14A based (NDIR) CO2 measurement devices (one of my projects) and noticed that it's very sensitive to the air flow changes.I don't know if it's cheap enough for you.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: October 11, 2018, 03:37:47 am
content: Another question occurred to me: For many applications (such as finding draft sources), an uncalibrated anemometer of whatever type is fine, but if you want to calibrate it (in, say, m/s), what's the best way to do that?I've heard of people driving a vehicle while holding the anemometer out the window and recording the measurement vs. the vehicle's speedometer reading, but a) I don't have a vehicle with a speedometer, b) there might be wind outside (which you can compensate for somewhat by driving in two opposite directions), and c) speedometers are not always the most accurate things. (AIUI, they're calibrated such that they never read even a little bit below the actual speed, but reading higher by quite a bit is accepted.)You could move it by hand at not-guaranteed-to-be-constant speed (but in a straight line) between two points a known distance apart, and divide the distance by the time taken to get the average speed. But I expect most anemometers will respond nonlinearly to wind speed, so you can't just find a constant of proportionality between the calculated average speed and the average anemometer reading over the same interval.I suppose you could mount the anemometer on some kind of CNC machine (mill, laser cutter, 3D printer, camera slider, …) and run that at several known speeds, but that just doesn't feel like a good calibration to me for some reason. It's the best I've thought of so far, though.I did have a thought just now about this, though:Quote from: PointyOintment on October 10, 2018, 01:10:09 pmMy thought just now is that you could add an intermittent, known-direction airflow to the measured airflow. For example, put a speaker behind your anemometer, and drive it with a low-frequency sine wave. When the anemometer reading increases, the speaker is moving in the same direction as the measured airflow, and vice versa.This would still be very laborious if you wanted to map the air currents in a whole room.

author: AlfBaz
author_rank: Super Contributor
timestamp: October 11, 2018, 03:48:31 am
content: An LM399 with uncut leads and a 6.5 digit multimeter

author: cat87
author_rank: Regular Contributor
timestamp: October 11, 2018, 06:20:22 am
content: Quote from: PointyOintment on October 10, 2018, 01:10:09 pmA, yes, that's the one. The name was on the tip of my tongue when I initially wrote that.Thanks for adding that in

author: BradC
author_rank: Super Contributor
timestamp: October 11, 2018, 07:06:44 am
content: Quote from: TinkeringSteve on September 26, 2018, 07:51:58 pmThe "sudden drop in temperature" would be effected by air movement, so it'd be detecting valid air movement albeit perhaps with an error in magnitude. If there was no air movement then the temperature change caused by *only* conduction of the air would be very slow. That doesn't happen. Convection is air movement.

author: ynfo
author_rank: Contributor
timestamp: October 11, 2018, 08:51:09 am
content: You could make a variation of Lovelock's ionisation anemometer (http://ynformatics.com/wp-content/uploads/2018/10/lovelock1949.pdf).An alpha particle emitter between charged plates streams ions across to a detector. Any "cross-wind" will reduce the amount of ions arriving at the detector.Maybe hack a smoke detector?

author: IDEngineer
author_rank: Super Contributor
timestamp: October 11, 2018, 09:21:39 pm
content: Quote from: PointyOintment on October 11, 2018, 03:37:47 amAgain, I think having a hand-held anemometer such as the type I discussed earlier in this thread would be really helpful if you're building anemometers. If I were building an oscilloscope I'd probably have an oscilloscope. If I were building a DMM I'd have a meter as a reference. They're not that expensive and you might learn something unexpected, in addition to having the ability to quantify your own designs.

author: floobydust
author_rank: Super Contributor
timestamp: October 12, 2018, 01:50:11 am
content: You can easily detect a slight breeze with a thermistor.Put enough current though it that it self-heats, and any convection cooling changes will lower its resistance.Seen this used before on candle simulators and a motion-sensor for a halloween gag.Using a 100 ohm PTC thermistor and 100 ohm resistor to 5VDC, its about 4% R change per degree, lots for an A/D to look at.