Arduino Air flow meter (low speeds)

[Jeroen Humasol]

Hi guys,

I know it's probably impossible, but electronics has amazed me before so i'm just going to ask. I would like to test the efficiency of a solar dryer for drying coffee beans. I already have a sensor for: temperature, humidity and solar intensity. However it would be great if i could measure the 'hot humid airflow' leaving the solar dryer. Solar dryers work by natural convection. So you have large openings at ground level (if you look closely at the picture below you can see that there's a small distance between the cover and the ground) where cold dry air enters. This cold air gets heated and rises because of natural convection. While doing this it carries the moisture of the beans with it. Once it reaches the ceiling it exits the solar dryer through a small opening close to the roof.

You probably already guessed that the flow speeds will be very low (i've no idea how low, but let's say 0.5 km/h or something). Do you guys know if there exists an affordable sensor that i can hook up to my arduino to measure this? Thanks!

 

[Alec_t]

A hot wire type mass air flow (MAF) sensor may meet your needs.

 

[Kiwi]

https://moderndevice.com/product/wind-sensor-rev-p/

 

[Jeroen Humasol]

Thanks for the replies guys, but for Alec_t: I think this is a bit of an overkill + I don't know whether it will be aible to measure my low airflow speeds and i'm just powering my arduino with a battery so i probably can't get that wire hot enough.

Kiwi: looks interesting, but i can't get to 8V :/ my battery works at 5V, i could buy an extra 9V battery but still, there are no reviews of the product, no datasheet.. And once again there stands 0-150Mph wind speeds but how precise will it be around 0? :s i guess i won't be able to measure this without really expensive equipment..

 

[hevans1944]

Does all the convective air leave through the rectangular aperture shown in your second photo? What are its dimensions? Do you have an outside ambient air temperature and humidity measurement? Can you use that information in conjunction with the interior temperature and humidity readings (which will yield the air density inside) to estimate the mass flow through the aperture? Well, I don't know I if I could without a lot of research. I aced the one introductory thermodynamics course I was required to take in college, but I don't remember enough to make that estimation. So if you don't know how to make that estimate, you are stuck with measuring the exhaust air velocity to calculate mass air flow.

The hot-wire mass air flow-sensor that @Alec_t mentioned is, in principle, a sound approach, but I suspect that it would not be sensitive enough to measure convective mass flow rates produced in your "green house". This flow rate will be much smaller than the air flow rate actively pumped through the intake valves of an automobile engine.

The wind-speed sensor that @Kiwi suggested is also, in principle, a means to measure mass flow. Just multiply the wind speed by the area of the exhaust aperture, correct the air density for temperature and humidity, and account for a whatever edge effect, if any, exists. Problem is, it has a full-scale reading of 150 MPH! The actual reading may be "down in the noise" and not useful if you try to measure the "wind speed" through your aperture with that particular sensor. The device is basically also a hot-wire anemometer like the automobile mass air flow-sensor.

I think if I were to try to measure the air flow through that aperture, I would first try to use a small rotary encoder with a propeller attached to its shaft. A rotary encoder can have very low friction, much smaller than a miniature PM motor with its carbon-brush friction and oilite bearings, although said motor would also (acting as a DC generator) provide a "wind velocity" output as a DC signal once the wind velocity was high enough to overcome friction. You can also use three anemometer cups attached to a shaft in lieu of a propeller, but the only reason that might be necessary is if you don't know which way the wind is blowing. You can calibrate either device fairly easy by walking with it at a steady pace and timing how long it takes to cover a measured distance.

 

[hevans1944]

i guess i won't be able to measure this without really expensive equipment..

Not so! You could even construct your own hot-wire anemometer. All you have to do is provide a current source to a tiny wire you place in the breeze. And you should also have an identical tiny wire shielded from the breeze, but otherwise in the same environment, excited from another constant-current source. Then measure the difference in voltage drop across both wires. This difference is proportional to heat loss from the breeze blowing over the first wire, but not in a linear fashion. Some calibration required. You could breadboard this with a dual op-amp and some fine wire, preferably platinum for best accuracy, but any wire whose resistance changes with temperature will work. Stainless steel wire, perhaps a single strand removed from a larger cable will work. You could also place the "reference" wire perpendicular to the measuring wire so the one that is broadside to the breeze loses heat faster than the one parallel to the breeze.

 

[duke37]

You could get fine wire in a low current fuse. The ends should be fixed and then the glass broken and removed.
A good fuse would provide the comparison.

 

[Alec_t]

Unfortunately fuse wire has a very low coefficient of resistance change with temperature. Instead of a hot wire I wonder if a tiny bead thermistor, having a much greater coefficient, would do the job? Since the air flow would be reasonably constant the thermal mass of the thermistor, even if greater than for a wire, should be of little consequence.
Another option perhaps would be a simple light-weight pivoted vane, whose angle of deflection from vertical would be a function of air speed and could be sensed in various ways.

 

[hevans1944]

We should all keep in mind that Jeroen is a mechanical engineer, not an electrical engineer. While highly competent, imaginative, and willing to learn about electronics, I believe he would prefer an off-the-shelf solution that can either be inexpensively purchased, or modified or built with not too much complication and cost. Hence my suggestion that perhaps he construct his own hot-wire anemometer, just as he successfully constructed a simple solar irradiance measuring instrument from a small photovoltaic panel and an op-amp.

My own personal preference would be a traditional 3-cup anemometer mounted to a rotary optical encoder shaft. I believe this arrangement would be quite sensitive, sufficient to measure the convective mass flow through the exhaust aperture located near the roof of the structure. If the pulses from the optical encoder were counted, the accumulation would be proportional to cumulative mass flow while the pulse frequency would be proportional to mass flow rate.

With some calculations, it should be possible to determine the mass of water removed from the coffee beans as a function of solar insolance, humidity of the air, and temperature difference between ambient air inside the "green house" versus ambient air outside the "green house". From this data, projections of drying time to a specified level of moisture content could be made. It would be most helpful to accumulate at least a year of solar irradiance data during the coffee drying season, perhaps at ten-second intervals, to predict the solar insolance available for drying.

@Alec_t is thinking along the same lines as a hot-wire anemometer with his tiny thermistor approach. I think this is the way this 150 MPH anemometer works that was originally cited by @Kiwi. It might even be possible to "reverse engineer" this commercial sensor to allow it to work at lower maximum air velocities with increased sensitivity, perhaps even with a reduced supply voltage of 5 V DC.

There is a lot of "information" on the Internet with regard to hot-wire anemometers and how to construct them. Caveat emptor on accepting what you read as "gospel". However, here is a reasonably well-written paper by a graduate student that is interesting. He breaks the glass envelope around a miniature incandescent lamp to gain access to its tungsten filament that is used as the "hot wire" sensor. The bare filament is operated at about a third of its rated current so as not to get hot enough to oxidize the tungsten in air. I might try this technique using small-base aircraft instrumentation lamps if I can find my stash of those. The filaments are usually designed for 24 V DC aircraft power, so they will be more fragile than lower-voltage filaments, but also more sensitive to air flow.

More ideas, anyone?

 

[Jeroen Humasol]

Wow, thanks for all the replies guys! I'm sorry for not being that active, but i'm still a student and i have exams in June and as most students i didn't study sufficiently during the year.

To answer your question in one of your first posts hevans: To be honest i have no idea what it will look like. The actual goal of this year is to investigate different options and look which one works best. And if we can find anything that is sufficiently more efficient (and cheap enough for the farmers to make themselves) then we will really start making them in the summer of 2017. So that's a lot of 'if' but i really think we can improve the drying process with these dryers.

Measuring the air velocity would be great and very interesting because like hevans already pointed out i could calculate the amount of moist together with the data about the air humidity and temperature that i already have.

However I spent a year preparing this trip and we mainly focussed on our main goal (making a water turbine to generate electricity) and we're already leaving the 27'th of June. Taking into account that i have exams untill the 23'th of June i fear that i won't have enough time to optimize the air flow sensor enough to get any usefull data out of it. So in fact i'm saying here what you said in the first paragraph hevans of your last post: if i can't buy it or make it easily then i just won't have enough time to make it

If i still decide to build it i think i'll opt more for the three cup anemometer. Mainly because i fear that callibration of the hot wire anemometer will be quite difficult. It will be dependent on humidity and temperature, which will both vary during different stages of the drying process. I also have an electronics store nearby perhaps i could go there and see whether they wouldn't have some sort of 'propellor' or three cup anemometer device.

PS: i'm just a 3rd bachelor student hevans and i'm defenitely not the founder of Humasol, i just took Jeroen Humasol as my electronicspoint name. I'm just a project student

 

[hevans1944]

PS: i'm just a 3rd bachelor student hevans and i'm defenitely not the founder of Humasol, i just took Jeroen Humasol as my electronicspoint name. I'm just a project student

That's a good way to honor one of the Humasol founders. I am so impressed with what your team is accomplishing without the backing of a Monster Mega Corporation. Hopefully we will still be here when you return in the Fall. And I hope the ELC works out for you. Personally, I would have tried much harder to divert some of the water to control the turbine output, but you guys on the site have the advantage of being there, right on top of the problem. That's a lot different from typing comments rectally extracted from behind my computer keyboard in an air-conditioned office thousands of miles away.

Good luck with your exams, and I hope to see you here again later this year. (Hopefully I will be living in sunny Florida by then!)

Umm... one other thing: could you hang a piece of plastic or cardboard from a paper-clip hinge (perhaps) to indicate air-flow from the vent aperture? Nothing big or fancy, maybe 50mm x 50mm. You don't wanna significantly block the air flow, but still have enough area to see a deflection from vertical. This is just to get an idea of how much air flow there is by observing how the flap hangs in the "breeze" during different parts of the day. Knowing the area and weight of this primitive "sensor" and the angle at which it hangs might produce some useful data. If too much flow exists for a good estimate of the "angle of the dangle" you could add one or more paper clips to this low-tech sensor, maybe take some photos against a paper grid background to assist in "calibration" when you get back home.

 

[Jeroen Humasol]

Hey Hevans,

I'm going to have to dissapoint you once again Jeroen is just my first name, and by coincidence that's indeed aswell the first name of one of the founders of Humasol.

I know that it all sounds very impressive, but i can assure you not everything goes as planned. We've had some succesfull projects like this humidity sensor for the coffee beans, but we've had some worse projects aswell. But even if we fail, we will still have a wonderfull experience

If i can't find an anemometer in that shop that i talked about in my previous post than i'll certainly do that, just have to buy a distance sensor then, but that shouldn't be that hard to find.

About the ELC: The problem with diverting the flow is that it is a bit of a 'slow response system'. If someone turns of a load of 500W for example (total turbine capacity is 5kW) then we feared that it wouldn't react fast enough. So that's why we opted for dumping the energy in resistances.

PS: You're moving to Florida for the weather i guess?

 

[hevans1944]

I know that it all sounds very impressive, but i can assure you not everything goes as planned.

It NEVER goes as planned when you get in the field. I have been there many times during a long career. No matter how much you plan, how many contingencies you pack away equipment for, when you get on site there will always be something you wish you had but do not have. So you just make a note of that for "the next time" and do the best you can with what you have.

About the ELC: The problem with diverting the flow is that it is a bit of a 'slow response system'.

That works both ways. The turbines are also slow to respond because of their moment of inertia. A sudden decrease in load can do more than just cause the turbines to over-speed: the output voltage will "surge" too unless the excitation current to the alternator is reduced, which I presume does occur.

So, either dump the water flow or dump the excess electrical power into a load resistance as you are doing. I would probably try to do both: throttle the water flow and dump electrical energy into a "dummy" load, decreasing the electrical power dump as the water flow through the turbine decreases while still maintaining the appropriate shaft rpm. The opposite problem occurs when the load is suddenly increased, for example in the evening when lights are turned on. Then the turbine speed has to play "catch up" and they can be slow to do that.

It is a (complicated) negative feedback control system problem, and not all the disturbing influences are known or accurately predictable. So, maintaining a constant load on the turbines, by dumping excess power, is certainly a simple and workable system with fast response. You can always add a slower "water throttling" outer loop to the control algorithm, if that appears to be desirable after operating in the field for a decent period of time.

PS: You're moving to Florida for the weather i guess?

Partly. We live in a declining neighborhood with increasing crime. Florida is attractive because of the weather and relatively less crime. We will pick a better neighborhood.