ron.palmer@uregina.ca – Page 2

5. Making U Turns

U turn: So far we have described how to do parallel passes that are straight and with negligible overlap; but, when we get to the headlands at the edge of the field we need to turn around and catch the next pass. We need to do a U turn. An optimum U turn would be a turn with the shortest distance in the least time. Manual U turns are anything but optimum. The operator goes deep into the headland in order to be given time to merge onto the desired course line. This is exacerbated by the distance the applicator drags behind the tractor. Also, if the passes are not precisely perpendicular to the headland, extra distance is required to make up for the wedge.

Executing a U turn in principle is the same as staying on course for a straight pass. The difference is that a U turn requires an arced or curved path, whereas the parallel pass is straight. The turning rate for a straight path is zero, whereas the turning rate for a U turn can be calculated, knowing the width of the implement and the speed of the tractor. Assuming that we want the applicator turned off during the turn, it is also necessary to know where the headland starts and the drag-distance of the applicator. Let’s list all the parameters that are required to define the desired course of a perfect U turn: width of the implement or applicator, the drag distance of the applicator, edge of the headland, angle of headland to parallel pass. And to execute the steering to the desired course of the headland we need to know the position of the tractor, the speed of the tractor, and the turning rate. By using all these parameters, it would be possible to automatically do a U turn, by steering the tractor automatically onto the desired predetermined U turn course. Yes, it is more complicated, with more input information required, but it is doable.

A U turn course is much more than a half a circle because some time is required for the tractor going straight, to actually turning to a rate that defines a half circle. We could call this the transition time to get into the turn. The same transition time is required to come out of the turn. The transition time is set by the time it takes to change the turning rate and it could be significant for a larger massive tractor. The transition, makes the calculation of a U turn much more arduous.

The application control is a part of the U turn. We want the applicator turned off in the headland; yet, we don’t want to miss anything. By knowing the delay of the applicator control, it would be possible to precisely turn the applicator on and off at the precise time.

There would be an advantage in doing the headlands at the very end. With manual driving, the headland marks where we are to turn. With auto-steering, there is no need to have a visual mark, as the system knows the location of the headland. In doing the headlands last, it would be possible to cover all the turn marks and ruts made with the parallel pass turns.

Knowing when to turn in a V trap is another interesting dilemma for auto-steering. The system must always be cognizant of having enough room to turn before being trapped; even though it might result in some misses.

4. Headland Alert

Headland Alert: OK so we can do parallel lines; and this would go a long way in reducing the overlap; but what is next? What about the turns, or at least a warning of the turns? We learned early on, when we introduced auto-steering, that the driver’s attention waned, with little to do as the tractor drove itself across the field. One was now free to make a call on a cell phone, or have lunch, or maybe even have a nap. We heard of all kinds of horror stories of farmer’s falling asleep and the tractor faithfully following its line right into the neighbors field, or onto a highway, or into a slough. Consequently, we introduced a headland alert, which was nothing more than a beeper that went off just before the headland. However, it was still incumbent upon the operator to recognize the warning and to make the turn in the headland. The next evolutionary improvement would be to make the turn in the headland. A perfect U turn.

3. Driving a Line & Auto Steer

Auto Steer: There is more to driving a nice straight line down the field than one first thinks. Yes, one needs to judge the distance to the last pass, but just as important is the angle of approach and the rate of convergence/divergence to the last pass. When we drive manually, we combine all these visual cues and make a quick calculation as to how much we should move the steering wheel. We do it without even thinking – it becomes so automatic. It’s not perfect, but one can get pretty good at judging the distance and the angle to the last pass. Can we mechanize the control of driving a farm tractor? The distance to the last pass can be done with a positioning system like GPS, but how do we measure the angle? And we don’t have all day to do this, it must be done quickly, it must be done now – without delay. A delay in a control system becomes the juggernaut Eventually, we used GPS for the position, even though it’s accuracy and delay were questionable. Later we will see how the plant row can be used to improve the accuracy. An electronic gyro would be used to acquire the angle, but it was not without challenge. Electronic gyros put out a voltage that is proportional to the turning rate. One must integrate this to get an angle. And there is drift in the output with age and temperature. Corrections and compensations must be done in real time. Also, the vibration of the tractor, if close to the sample rate of the gyro, could result in egregious angle errors.

In Engineering we are taught that a control system with the least error in tracking is one with critical damping. So, if we are driving and we are off the desired line, we want to get back to where we belong as quickly as possible and this will result in some over-shoot. There will be a little loop in the corrective action taken. When doing the auto-steering trials; I soon learned that farmers wanted to forego this optimum response. They did not want to see any little loops or squiggles – they wanted a perfectly straight line no matter how long it took or the resulting accumulated overlap error. They wanted a controlled correction that was over-damped.

There were three things used in determining the steering correction: the offset, the angle and the tractor’s rate of turn. Let’s look at each of these more carefully.

The Offset: This is the perpendicular distance of the center of the tractor, to the desired track line. It is shown in Fig 1 as the offset. The desired track line is set by the driver doing a strike line across the field. From this strike line a set of parallel lines is calculated one implement width apart. The desired track line is the parallel line closest to the tractor’s position. When the positioning systems coordinate (GPS) is received on the tractor a calculation is made using a point to line formula, yielding the offset. GPS is accurate to ten feet, but it can be improved by using differential GPS, DGPS to get to within about a foot. This can be made even better with Real Time Correction, RTK GPS, to get to inches, especially if we are using relative measurements within minutes of each other. However, all positioning systems do not report the current position of the tractor, they report where the tractor was a moment ago, a fraction of a second ago. This just adds to the difficulty of steering.

The Angle: The tractor is shown on its course line to the * on the desired track line as shown in Fig 1. The angle j is the angle between the actual course line and the desired course line. Some call it the angle of attack. When we have a large offset, we need a large angle of attach, but as we get closer to the desired line, we want to make the angle smaller and smaller so that we don’t overshoot the line. One could think that driving is actually just aiming a few hundred yards down onto the desired line.

Rate of Turn: The tractor could be turning, so the turning will determine what the angle will be in the future. So it is another factor in determining the steering correction. An electronic gyro is used to determine the rate of turn, such as degrees per second. The gyro we used put out a voltage from zero to five volts, with the mid voltage of about 2.5 volts being no turn. A voltage higher than this was a left turn, and one lower than this was a right turn. We called this no turn voltage zero_w. It was not exactly 2.5 volts. It was initially calculated by averaging 32 readings when the tractor was standing still. When the tractor was moving, a reading from the gyro minus zero_w gave the turn-rate. Digital integration of the turn-rate gave the angle j. Angle j is acquired and adjusted by taking the inverse tangent of the difference of past offsets over the distance traveled. The correction to the steering = offset – angle – turn-rate.. This correction was a pulse applied to a hydraulic valve that put a squirt of oil into the steering ram.

2. Measuring Overlap

Overlap: How bad is it? I have heard of stories of how some farmers could drive, straight as an arrow and with no overlap, hour after hour, day after day; but, I knew that I overlapped when I drove, and I had seen many fields in which overlap was evident. But, just how much was this overlap? Could it be measured? Was I trying to solve the problem of eliminating the overlap, if it really wasn’t a significant problem? After some investigation, it became apparent to me that no one had actually measured the overlap of manual driving. We needed a way to measure this and we had to do this without the operator being aware of the measurement being taken. I was well aware of an operator being able to drive more accurately when he or she was aware that someone was watching. I knew first-hand that I always was much sharper, and drove more accurately when someone was watching or where it would be noticed from the road. The overlap measurement must be done without the driver being aware of our intent.

We devised three non-intrusive ways to measure overlap:.

a) Stop and spot and measure the actual overlap with a measuring tape.

b) Measure the width of the field, and the width of the implement. By counting the number of passes made to complete the field, one could calculate the overlap. The number of passes multiplied by the width of the implement would equal the field width. Any excess passes to this would be overlap.

c) I rented a plane and had my passenger take a number of pictures of a farmer driving as I buzzed the operation from behind, without the farmer being aware of our presence. After analyzing the photos for relative widths; it was determined that the overlap increased and was proportional to the implement’s width.

The overall average overlap was 10% of the implement’s width. A ten-foot implement would typically result in one foot of overlap, whereas a 50 foot implement would result in five feet of overlap. Now we knew, and yes it would be a worthwhile project to reduce or even eliminate the overlap.

1. Introduction to Precise Farming

There have been many improvements to crop production over the years. Most of these improvements have originated from the farmers themselves. I grew up on a farm observing many of the changes. When I was ten years old, just a kid, I was driving tractor, a John Deere 830 pulling a 14 foot cultivator. My Dad would do the perimeter of field for headlands and around the sloughs; and after doing the strike line, I was put on the open-air tractor to slowly go back and forth until the field was done. I couldn’t get into too much trouble – nothing to hit. Just wide open, back and forth. Oh, it was so boring. And I had plenty of time to think about how this could be done better. I wasn’t the best driver, getting the line crooked, and the overlap was terrible; but I learned quickly that overlapping was much preferred over the deadly sin of missing. I just kept thinking: There must be a better way; I didn’t know what that better way was, but surely there must be.

I didn’t do much until I graduated as an Engineer and worked in Ottawa on new computerized telephone switches. I learned about computers from the ground up and soon learned what computes could and couldn’t do. I was intrigued by how a computer could control equipment. I also became a pilot and learned about auto-pilots. If a plane could be set up to drive automatically from point A to point B; then why couldn’t a tractor be programmed to drive from point A, at one end of the field, to point B, at the other end of the field. I got my chance to tackle this question.

In 1982, I was hired into the Engineering Faculty at the University of Regina. This was my opportunity to work on the problem of driving farm tractors better – without overlap or missing. A tractor with auto-pilot or auto-steering. To do this, we needed to know where the tractor was at all times; we needed a positioning system. There was no GPS, so a positioning system needed to be developed. With the help of a few students, we developed our own positioning system. In 1985 a John Deere 4020 was outfitted with some crude steering wheel controls to drive and work a small field, including turns. We used an Osbourn portable computer to do the calculations of triangulating to our VHF beacons to determine the position of the tractor. CBC recorded the demonstration and it was broadcast in Canada on Country Canada in 1985. This was the debut for auto-steering; we showed that it could be done; but, did farmer’s really want or need it?

Now that I am retired, I decided to put my thoughts and experience down on paper. Taking what I learned as a boy on the farm, hoeing in the garden and driving tractor as a kid; and my experience with being a pilot and as a professor in electronic engineering, I am in a unique position to bring all this together in what I think are some pretty exciting changes that can be achieved in farming, using existing technology. These are my ideas that I have developed throughout the years and as such I am writing this in the first person. This is the first chapter of my story; there are many to follow.

Sensors to Measure Moisture Content in Head Space

OK, so you want to: “measure the moisture of grain using only a sensor in the plenum and headspace.”
To determine the moisture content of the grain, you would use EMC equations that would require the parameters of the air surrounding the grain in question. These parameters would be temperature and relative humidity. So you need sensors, that would sense the temp and RH of the air surrounding the grain. Now we need to make some reasonable assumptions here. I will assume that you have just been running your fan and as such the top layer of grain will probably be wetter and warmer than the bottom layer. But it is the top layer that we are interested in, why? Because it is the last layer that the air goes through before exiting. So the temp of the exhaust air will be the same as the temp of the air in the top layer. The same can be said about the RH. So whether you have the sensors in the top layer, or in the stream of air exiting the bin, it will be the same. The second assumption is that the top layer of grain in the bin is consistent in moisture content, and as such the air surrounding it will be consistent in temp and RH and that the air exiting the bin would be the same in both temp and RH. And assuming this is the case, then it would not matter if the bin has exhaust vents, because the air exiting the bin, whether by vent or main portal, the exhaust air would be the same in both temp and RH. Now, to get to the question of where to place the sensors. In the experiments we did, we had the temp and RH sensors mounted under the roof, just inside and down a couple feet from the main portal. In your case, I will assume that you will be using a moisture cable with temp/RH sensors every few feet. I would hang this string right from the collar of the main portal. It should be a bit to the side so as not to interfere with the auger’s loading chute. In our experiments, we used as many as 9 moisture cables per bin; but, I think you only need one — one in the center. The layers going across the bin were fairly consistent; but there was a difference in the top to the bottom layers. So, there you have it, my recommendation would be to hang one moisture cable from the collar of the main portal. That all being said, I am not a real fan of the moisture cables because:1. They are expensive2.The RH sensor is not reliable and not that accurate.3. One must use EMC equations by inputting the T and RH to get moisture content, and they are ugly equations, with questionable accuracy.4. EMC equations give moisture content on a dry basis, and this must be converted to wet basis. (This conversion is often over looked)
I like the idea of using only a temperature cable. To determine the actual moisture content, one would sample and measuring it directly. The top layer’s moisture content does not change quickly and is the last to dry.

How Long to Dry Calculations

You asked me how long it would take to dry your grain. I don’t think I gave you a very good answer, so I will try to explain this. I will use the WGRF final report as a reference.
1. On page 6 I explain the black box approach to drying. If one measures the amount of water going in, and out of the bin, the difference would indicate the amount of drying taking place. The Absolute humidity is the amount of water in the air. The absolute humidity can be determined from the psychrometric graph of Fig 4 or from an equation or from an online calculator. www.planetcalc.com/2167/ One simply enters the Temperature and Relative Humidity of the air, and out pops the Abs Humidity. We can get the Temp and RH of the outside air from any weather station. However getting the temp and RH of the air exiting a bin is a little more involved. It will be assumed that the temp of the exiting air will be the same as the temp of the top layer of grain. I think that’s a reasonable assumption; but what about the RH of the exiting air — where do we get it from? We could have a RH sensor at the exiting port; but most of us don’t have that. So, we will again assume that the exiting air will have the same temperature and the same RH of the air in the top layer of grain.
2. What is the RH of the air in the top layer of grain. We will use EMC (Equilibrium Moisture Equations) equations to determine the RH. If we know the moisture content and the temp of a specific type of grain; an EMC equation will give what the RH of the air will equalize to. We can use my calculator to do this at www.planetcalc.com/4959/ Enter the moisture content of the grain, as well as the grain temp. For the outside air temp, DO NOT enter the outside temp, but, rather the grain temp. Then the EMC RH will be the resulting threshold RH for your specific grain. Use this RH and grain temp of the top layer to calculate the absolute humiidity of the exiting air.3. The measurements made on the air entering and leaving the bin are all that is necessary to determine the amount of drying that is taking place. If the absolute humidity of the air entering is 20 gr./m^3 and the abs hum of the air exiting was 25 gr./m^3; then for every cubic meter of air that flows through the bin there will be 5 grams of water removed.4. Let’s say that the fan is pushing 3,000 CFM. For every cubic meter of air that goes through the bin, there are 5 grams of water removed, as above. There are 35.31 cubic feet in a cubic meter, so there are 3000 / 35.31 = 85 m^3 /min or 5098 m^3/hour x 5 gr/m*3 =25,488 gr/hr or 25.488 kg/hr or 56.17 pounds per hour of water removed. (There are 2.204 lbs in a kg)
5. So, how long would it take to remove 1% of moisture. Let’s assume that we have 5000 bushels of wheat at 16.5% moisture and 60 lbs. per bushel. Above we see that we are removing 56.17 lbs/hr from the bin of 5000 bushels. or 56.17/5000 = 0.01235 lbs/bu/hr. But we want to remove 1% or 0.01 x 60 = 0.6 lbs. Therefore, it will, with the conditions and the rate above, will be 0.6 / 0.01235 = 48 hours or two days. The problem is that the outside air is changing in both temp and RH and therefore to give an accurate time, this calculation must be done every hour.
Let’s go through an arbitrary example: The question is: How much drying, in terms of % moisture content per bushel, occurs in one hour given that we have a bin of 5000 bushels of wheat at 16.5% moisture. The aeration fan puts out 3500 CFM. The outside temp. is 0 deg C and the relative humidity, RH, is 66%. The top layer of grain in the bin is 6 C, the middle is 5 C, and the bottom is 4 C.
1. What’s the abs. humidity of the air entering the bin? Using the abs humidity cal www.planetcalc.com/2167/ and enter 0 C for temp and 66% RH yields 3 gr./m^32. What is the RH of top layer? using grain drying calc, www.planetcalc.com/4959/ enter 16.5% for grain moisture, and 6 for both grain and air temp. gives RH of 75.1%3 What is absolute humidity of exiting air? Using the abs humidity cal www.planetcalc.com/2167/ and enter 6 C for temp and 75.1% RH yields 5 gr./m^34. For every cubic meter of air that passes through the bin, 5 – 3 gr = 2 grams of water are removed.5 3500 CFM /35.31 = 99.12 cubic meters per min x 60 = 5,947 cubic meters per hour x 2 grams = 11,894 gr/hr 0r 11.894 kg/hr x 2.204 = 26.22 lbs of water removed per hour6. How much water are we removing from one bushel of wheat per hour? 26.22 / 5000 = 0.005243 lbs of water removed per bu. per hour7. In terms of % moisture, how much is it reduced? Assuming wheat is 60 lbs/bu. ( 0.005243 / 60 ) x100 = 0.00873% At this rate it will take 114 hours ( ~ 5 days) to remove 1% MC ( 16.5- 1 to 15.5%)
I hope this example will help to give one the ability to calculate how long it will take to dry grain.

Can Relative Humidity determine Moisture Content?

A farmer asked me if it was possible to determine the moisture content of his grain, by knowing the relative humidity (RH) of the air being expelled from the grain bin. Here is what I told him:

OK Kelsey, I am behind my laptop and I now can give you an in depth reply. Your question: “a way to determine the moisture content of the grain inside a aeration bin? By measuring the relative humidity of the air exiting the top of a bin?” You need to know two things, the Relative Humidity RH and the Temperature T. Knowing only the relative humidity, is not enough, in fact without knowing the temperature, the relative humidity is meaningless. The amount of water that the air can hold is very much dependent on the temperature. The relative humidity only tells you the percentage of that capacity, at that particular temperature. OK, so let’s assume that you do have knowledge of both the temperature and the relative humidity of the air leaving the bin. There are EMC (equilibrium moisture content) equations in which you can plug in the Temp and RH and out pops the moisture content (MC) of that particular grain. These equations were made by trial and error. Scientist would take grain at a certain MC and Temp; put it into a sealed container and let it equalize with the air for an hour or so and then measure the RH of the air. They would do this hundreds of times for various temperatures and MC and then plot the results on a graph. They then tried to define the points with the best fitting equation. These equations have been around for a long time and are published in ASAE (American Society of Ag Engineering) journal D245.5 “Moisture Relationship of Plant Based Agricultural Products” However they are not for the faint of heart — they are ugly equations. Yes they relate Temperature, RH and MC and you kind of have to know what you are doing with conversions etc. For example we are used to MC given on a WB or wet basis. Whereas the EMC use a DB dry basis for MC. I tried to help some farmers through these calculations, and then it dawned on me that we have computers and smart phones that are really good and fast at doing calculations and conversions. So, I made a sort of app for your smart phone that does these calculations for you. You can find it at planetcalc.com/4959/ called the grain drying calculator. It is normally used to determine if you have conditions for drying; but we can use it also the other way to determine the MC if you know the Temp and Relative Humidity. The calculator asks for inputs of the Temp of Grain, the Temp of the Outside Air, the MC and it gives you the thresholld RH to which any RH below this RH thres — you will have drying conditions. I uploaded this calculator to my Iphone, so that I can use it anywhere.
Let’s make up an example to show how we can get the MC. Assume we have Canola, the RH of the air leaving the bin is 70 % and the Garin Tempi is 20, and Outside Air is 15 deg C. The first thing to do is to set the Grain Temp and Air Temp to 20. We don’t know the MC, but we will guess — put in 10% in the MC; and press calculate. Under Canola we get an RH of 74. This is too high, So guess 9 for the MC and that gives us 68.6. A bit too low. 8 gives us 61% and 11 gives us 79.9%. But we want 70% So, I try 9.2% for the MC and it gives a thres RH 69,96. This is very close to 70, so I will claim that your Canola is at a MC of 9.2% — by a little bit of trial and error. Just make sure that the outside air is set to the grain temp when you are doing this (even though it isn’t)I hope this helps.
//Ron Palmer Ph.D. P.Eng.

Simple Automatic Controller for Aeration Fan

OK I am back, and yes it is that simple:
Turn the fan on if: grain temperature > outside air temperature
I have gone through all the control strategies, and as for a controller, this would be my choice because of its simplicity and ease of use. It does not guarantee the fan will only be on when you have drying conditions, but it does guarantee that you will have the safest , most secure storage with the least spoilage. It keeps your grain cold. The control strategy for only running the fan when you have drying conditions would be what I call the Absolute Humidity Controller.
It calculates the actual water content of the air inside the bin and the air outside the bin. It turns the fan on when the outside air contains less water than the air in the bin. It is much more involved and gives even a micro a hard time with Psychrometric equations, and EMC equations. It also requires the user to input the moisture content and type of grain. It is the ultimate in a control strategy that only runs the fan when there are drying conditions. But it is way more complex, and not as convenient for the user and therefore not as reliable and more expensive.
The strategy of:
Turn the fan on if: grain temperature > outside air temperature also dries the grain except for when the grain temperature is greater than, but only slightly greater than, the temperature of the grain AND when the outside relative humidity is close to 100%.   But once the difference in temperature of the outside air and the grain becomes more than a few degrees, then even with the RH being 100%, you will still get drying. What is the chance of having an RH of 100% and only a slight difference in air/grain temp? Very very small. We could condition our strategy above by saying that we would only turn the fan on if:
grain temperature > outside air temperature, AND the RH < 85% — but is it worth it? This would require a humidistat etc. I don’t think it is worth it because of the probability of having a small differential temp and an RH > 85%
We can achieve almost the same thing by putting in an offset:only turn the fan on if:
grain temperature > (outside air temperature + 2 degrees)
So, what do we need to automate your fans — Are you ready for this??   All you need is a thermostat that is used to control baseboard heaters. You can find these at Lowes or Home Depot. It has a rotary knob that is set to a specific temperature. It is all mechanical ( I believe a bi-metalic strip), that closes a contact (than can handle a pile of current at 110 or 220 volts) and the contact closes when the air temperature is less than the indicated knob temperature. In our case we will set the knob temperature to that of the grain temperature and connect the contacts to the actuator’s (relay) coil. Most aeration fans are wired to have a latching actuator coil. This will need to be modified.   Assuming we will have 220 volts with L1 and L2 power leads.   The following would be wired in series, ignoring the Start and Stop switches.         L1 – actuator or relay Coil – Thermostat – L2 The relay’s coil will only be activated when the contacts on the thermostat are closed, and that is when the temperature of the air is less than the temperature of the grain that is indicated on the knob. Your fan will only run when the air temp < grain temp.
Your grain temp will not change that quickly and maybe once a week you will need to adjust the temperature on the knob down to match the grain temp.
A more automated approach would adjust the temperature of the grain automatically. Thermistors are used to measure the temperature of the grain and the outside air. I used 10 K ohm thermistors.   These are run into a comparator such as LM311, through a pulse integrated circuit and then into a solid state, high voltage relays to simulate the Start and Stop pulses on the fan. I have built these circuits and have wired up a couple of aeration fans with it. I called the system “Cool It” I still have some of these boards around, but I can not sell or even give these boards away as they would have to be certified by CSA or ULC and to go through all that hassle is not worth it.   I suppose I would be willing to share the schematic, but I still would be a bit nervous about the liability.
I hope this gives you a start, with a cost effective, simple solution to automating your fans.

Natural Aeration or Natural Drying??

I wanted to ask about using the terms “aeration” and “natural air-drying.” I’m trying to avoid confusion – I found in a grain aeration spreadsheet by PAMI the following definition:Aeration = grain conditioning/cooling low airflow rate (0.1-0.2 cfm/bu) Natural air Drying = removing moisture from grain high airflow rate (1-2 cfm/bu)
Natural aeration or Natural Air Drying would be using the natural ambient air (no supplemental heat) to condition the grain. I think it is generally accepted that the fan is pumping air into a steel bin. Our research has shown that Cooling is Drying and whether you have an air flow of 1 CFM/bu or 0.1 CFM/bu, you will be cooling and drying the grain. I am not sure where the distinction between airflows for cooling and drying came about, but in fact they are related. Even at very low air flows, you will still be drying — albeit somewhat slower, but; there are advantages to drying slower. 1. Because the higher flows make for more pressure on the bottom of the bin, they also will create more of a difference in top to bottom of the bin drying. Slower flows have a more even distribution of top/bottom drying. 2. You will use more of the heat energy in the grain to push the water out of the grain. 3. You will use much less electrical energy with a smaller fan — even if it takes a bit longer. We found the sweet spot for flow to be about 0.4 CFM/bu. You get the advantages, as mentioned above, and you still get the grain dried in a reasonable time. But back to answering your question: Natural Air Drying and Natural Aeration are really the same thing.

Do you agree with these definitions? Natural air-drying seems to be both a somewhat general term for the two methods and also a specific term for the high airflow rate method – thus my confusion.

Also, would you have any figures on the costs to run a grain aeration fan? You said it would be pennies on the dollar – do you know where I can find more specific numbers?
Here is the math that you can use to calculate the cost. In Sask the cost of a KiloWatt Hour is about $ 0.13 1 HP requires 0.74 KW So to run a 1 HP fan for 1 hour cost 9.62 cents — lets round it off to 10 cents. A 5 HP fan would cost 50 cents per hour or 12 dollars a day. A 10 HP fan would cost $24/day. You can take it from there.

I was looking at your presentation and the diurnal drying cycle graph – it shows drying starting at 6:00 pm and wetting starting at 9:30 am (with best drying happening at 2 am). So, after that initial 24 hour drying period during/after harvest, are those the times farmers should be following for natural aeration?
After the initial 24 hours, the fan should only be run if the outside air temperature is less than the grain temperature. This will probably be at night. And as the temperature of the grain goes down, there will be warm nights that the fan should not be run at all.