Night Drying Doesn’t Work?

I got a call a couple of days ago from Jim S from Wainwright, Alberta.  He said this night drying thing wasn’t working for him, and that he was considering buying a natural gas furnace to add some supplemental heat to the process. Would that work?  I told him I thought so, but I would have to work out the numbers to see; but I’ll save that for another blog. First let’s have a look at why this night drying isn’t working out.

Jim has tough wheat, 17%, that was harvested cold and put in a 3500 bushel hopper bin. Attempts at night drying brought the temperature down, but because of the nasty wet weather they have been having, the moisture has remained close to 17.  Jim thought that water may have been added.  I told Jim that once you got the temperature of the grain down, you have taken the energy out of it that could have been used for drying.  So it is quite possible that now that the grain is cold, that drying will cease.

Here is what I did to get a handle on what was happening in Jim’s case.  I went to the website for BINcast and got the hourly temperature and relative humidity for Wainwright from Sunday Oct 14 til Thurs Oct 18.  The temperature was bouncing around the freezing point, from +4 to -6 C.  The relative humidity was high throughout this time, 90% –  95%.  We didn’t know the temperature of the wheat, but we knew it was cold, so I assumed it was 4 C.   I modeled the temperature of grain, so that it would chase the outside temp in a similar fashion as I saw how the grain temperature changed in our trials at Indian Head. At the end of this simulation, the grain temp was close to -1.5 C.

I now had all the information I needed to calculate the amount of hourly drying. To make a long story short, I used the principle of absolute humidity, and EMC equations to calculate the amount of drying for each hour. For the first few hours we did take out a bit of moisture until the temp of the grain came down, and then we started adding small amounts of water. We did get some drying when the temp went down to -5, but this was short lived.  In the end we did indeed add water, 7.52 kg.  This actually isn’t that much, it would raise the MC from 17% to 17.001% however we can definitely say that night drying did not work in these conditions.   Maybe we should consider supplemental heat?  Maybe Jim should buy his furnace.  Using supplemental heat brings forth a whole host of other questions.  What fuel should I use? How much will this cost?  At what time of day do we apply the heat?  How hot should we get the grain?  Should we have a cooling cycle, or should we just apply the heat continuously. How long should the cooling cycle be?

In my previous blogs, I showed that a 15 deg C cooling of the grain resulted in a 1 percent decrease in moisture.  So let’s say we want to raise the temp of the wheat by 15 C. The specific heat of wheat varies, but it is about 1.36 kJ/kg C. I will spare you the details but to heat 3500 bushels, 15 C, requires 2 GJoules.

I checked with SaskEnergy and used the internet to check out the cost of the different fuels for 1 GJ.

Natural Gas   $6.48 /GJ

Gasoline          $16 /GJ

Fuel Oil (Diesel)  $21.65 /GJ

Propane            $14.66 /GJ

Electricity           $32.81/ GJ

So, we can see there is no decision, if you have natural gas, it is by far the cheapest supplemental fuel.  Also 1 GJ , at least, is the required energy to remove 0.5% MC.  Maybe we can get Jim’s wheat dry for $40 — if we do everything just right? As our benchmark,  and challenge, we will use the same nasty weather conditions above, with the supplemental heat and see what happens?

Stay tuned for the next blog on playing with supplemental heat.

 

BINcast critique

When should the fan be turned out?  There is another calculator — other than mine that tells you when the fan should be operated.  But it is based on assumptions that are not realizable — judge for yourself.

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Hi Ron
Have you seen the BinCast calculator (http://www.weatherwest.ca/bincast.cfm).
Does it work on the same principles as your methods?
Sarah

 

On Thu, Sep 22, 2016 at 3:34 PM, Ron Palmer <Ron.Palmer@uregina.ca> wrote:

Sarah:
I have not seen BINcast as a webpage, but have followed the theory of operation.  It does not work; because they do not take into consideration the grain temperature. Here is the reasoning behind it, and at first glance, it appears to be a rational theory:
If one takes the outside air temp and relative humidity, and inserts these into an EMC equation for — let’s say barley — you will get an Equilibrium Moisture Content. Let’s assume this is 16%.  If I blow this air at my barley, it will eventually reach a moisture level of 16%.  And that is true, if this air is blown at the grain long enough, eventually the barley and air would reach equilibrium.  Reaching equilibrium means that the barley must be the same temperature as the air.  One can not assume that the barley is the same temp as the outside air, in fact it never is.  It takes hours and hours of blowing air (probably a thousand air exchanges) to change the barley’s temperature.  But the outside air is not a constant it is changing hour by hour, and the barley is always playing catch-up.  The outside air and the barley are NEVER in equilibrium.  Sorry but this is not a good calculator.  I can come up with example after example, where this calculator will give the wrong answer.
I use EMC equations, but for temperature I use the temperature of the grain, (because the air becomes the temp of the grain) and I use the EMC equations differently.  You will notice I ask for the moisture content of the grain; using the moisture content and the temperature of the grain, I get the equilibrium relative humidity. That is where the grain wants to put the air’s RH.
If you are really interested, I can give you concrete examples of how the BinCast is totally different than my calculator, and it makes the huge false assumption that the air and the grain are at the same temperature.
Ron Palmer

Convection Current Flow Rates

We have all seen those little diagrams of curved pointed lines that indicate convection currents in a grain bin.  How in the winter the air flows up through the middle and down the sides, and then in the summer where it flows down the center of the bin and up along the warm side walls.  We can also get convection currents right through the bin with air flowing through the fan, vents, and center opening.  But just how fast is the air moving with convection currents? Let’s see.

First of all, the only reason we get convection currents is because of the air being at two different temperatures.  Air is heavier if it is colder.  A column of cold air has more weight than a column of warm air, and consequently will push down with a pressure equal to the difference in weight.

According to http://www.engineeringtoolbox.com/air-density-specific-weight-d_600.html , the weight of air is:

ºC         ºF         lbs/ft^3

10        50          7.786

21.1     70          7.492

32.2      90         7.219

If the outside air and the grain are the same temp, the air in the bin and air outside will have the same weight and there will be no pressure difference and therefore no convection current through the bin.   But let’s say that the grain is at room temperature of 21.1ºC , 70ºF, and the outside air is at 10º C.  That is quite a difference in temperature and also a difference in weight, 7.786 – 7.492 = 0.294 pounds per cubic feet.  But the grain in the bin is 20 feet high, so 20 x 0.294 = 5.88.  A column of air, 20 feet tall,  inside the bin weighs 5.88 lbs more than a corresponding 20 foot column of air outside. But 60% of this column is occupied by grain, and only 40% is air; so this column of air is only 2.35 lbs heavier.  But that is the pressure exerted over one square foot, psi or pounds per square inch is a more common unit of measure for pressure. 2.35/144 = .016 psi  This isn’t very much pressure, when you think of our car tires have 30 psi.   Another common pressure unit is inches of water.  How many inches of water would this pressure support? One psi would support a column of water 27.68 inches.  0.016 psi is equivalent to 0.452 “H20.

To determine the flow, one must have some appreciation for the grain resistance. From our past trials we saw that 3000 CFM required a pressure that would support 6 inches of water. Now if we said that pressure and flow were in a linear relationship, (it is not –it is closer to a squared),  the convection current would be about one thirteenth or 226 CFM.  This is a reasonable flow.  The bin we had was about 200 sq feet. so a flow of 226 CFM would be a vertical speed of 1 ft per second or about 1 kmph; but then again one must remember that this is only for a very significant temperature spread; as the temperature of the grain and the outside temperature become less, so too would there be a reduction in the convection current.

Short Answer for Grain Aeration

I get asked this straight forward question about grain drying all the time.
My cousin Mark recently emailed me:
I was reading an article about grain drying with aeration that you worked on.
 You mentioned that it is best to run fans at night.
We have some peas that are dry but with some green kochia and other weeds.
We will have the same with some wheat that we will combine soon as the Kochia will get swathed but I suspect it will not dry down much as we will combine in probably 5 – 7 days.
What is the recommended drying – cooling times and what concerns should I have with Humidity and Temperature?
Thanks
Mark
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I replied:
Mark:
I don’t have a really short answer to question, but even if you didn’t know anything else, this would be a good rule to follow — even if your grain is dry.
1.  As soon as your bin is full (even before it is full) turn the fan on and run it until 9:00 AM the next morning.  The exception to this rule would be if it is raining or very high humidity.
2. Thereafter, run the fan every night until your grain is cooled down and dry.  This might be for a few nights, or it might be for several weeks if you have tough grain.  The cold clear nights will be the most effective for drying.  Again if it is raining, don’t run the fans.
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OK that’s the simplest; if you want to be more precise about drying conditions, you would only want to run the fan when there are conditions for drying and that is when the absolute humidity of the air outside is less than the absolute humidity of the air in the bin.  Absolute humidity is the actual amount of water in the air.  It is not the same as relative humidity; but relative humidity can be used to calculate the absolute humidity.  And they are kind of ugly equations that are tough to do even with a calculator.  So, I made an internet calculator that does the calculations for you. It can be found at  planetcalc.com/4959/   You can load it onto your cell phone and use it like an app.  You simply enter the moisture content of your grain, the temperature of your grain, and the temperature of the outside air.  It calculates, what I call, the threshold relative humidity.  If the relative humidity of the outside air is less than this you will have drying conditions and the fan should be turned on.  If the outside air’s relative humidity is greater than this, you will not have drying conditions and obviously you should keep the fan turned off.  If you really want to get up to speed on grain drying, read my blog at grain-aeration.com   It explains everything, even why you get condensation in a bin and why the bottom always dries first etc.

Another rule of thumb that mostly works (98% accurate):  You will have drying conditions when the outside air is at a lower temperature than the grain temperature and the relative humidity of the air < 85%   This is not as accurate as the calculator because it does not take into account the moisture content of the grain; however it does not require any calculations or calculators

I hope this helps, let me know if you have any other questions
rjp

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Thanks a bunch Ron.
As long as the relative humidity outside is less than 85 in the evening – I will run the fans and turn them off in the morning
Last night Environment Canada showed the Humidity at 90% so I should not run them at that point
Mark

Grain Drying Questions from Quebec

Here is a question I got from Quebec:

“Mr. Palmer, Last fall we discussed a lot about grain ventilation. I remember how you where generous on this subject. I want to share with you what we do. This year we will do other observations. The producer where we will make observations will get a 4 noodles cable with Stormax. (20’ x 20’ silo). However, control will be executed by water balance at least for the stop time. Re-start will be done manually or by timer followed automatic running. I will try to share you these informations including stormax data if possible. Do you think this approach is good ? Do you have suggestions ?

The producer is supposed to fill it with 2-3 crops (first : rye…temporary storage, second : Oats…temporary storage, third : soya). In the litterature I did not find EMC equation for rye. Is it similar to wheat ? Do you think Opi systems have it ?”


I gave the following reply:

Good to hear from you. I will try to answer your questions. I am not sure what a 4 noodle cable is, I am assuming it is one with 4 senors, that is 4 sensor temp and 4 senors for relative humidity. I am assuming that you are doing a water balance to stop by calculating the absolute humidity of the discharge air and comparing it to the absolute humidity of the inlet air. And you only stop the fan if the absolute humidity of the inlet air is greater than the absolute humidity of the discharge air? Great. To turn the fan on we can’t use the discharge air, but we do want to know what the absolute humidity of the air in the silo is? There are two ways to do this. 1. this is the best way and it can be done if you have a temp and RH sensor in the core of your grain or crop. Use the T and RH as you did before to determine the absolute humidity. 2. If you don’t have the RH sensor in the body of grain, and only the temp of the grain, you can use EMC equations. By plugging in the moisture content and temp of your grain, you can determine the RH to which it will find equilibrium. Use this RH and T to determine the absolute humidity. If this absolute humidity is greater than the absolute humidity of the outside air, then you have a drying condition and the fan should be turned on. You can use the calculator that I made to do this for you. Just plug in the temp and moisture content of your grain, and the outside temp, and the calculator gives you an RH thrreshold, if the outside RH is below this then you will be drying, anything above this, and it will be wetting. Actually the only thing I am doing with the calculator is the math for the EMC equations. And no I don’t know the EMC equation for rye, but with a little digging I am sure I could find it. I think it would be OK to use wheat — it would be close.
My calculator can be found at planetcalc.com Grain Drying Calculator

Grain Drying all comes down to something pretty simple. If the outside air has an absolute humidity less than the absolute humidity of the air inside your bin or silo, then you have a drying condition. The only thing you need to calculate the absolute humidity is the temperature and the relative humidity. When the fan is on, the relative humidity is read at the point of discharge and when the fan is off you get the relative humidity by reading an RH sensor directly in the body of grain, or if you don’t have this RH sensor then you must resort to using an EMC equation (or my calculator) to determine the RH.

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Still thank you Mr Palmer, Yes it is supposed to be an Opi Cable with 4 points reading of T, RH and EMC. When you talk about a T&RH sensor in the grain, what position do you suggest : at the top, at the surface or in the grain and if so how deep ?


The sensor in the grain, the RH & T sensor, should represent as much as possible the entire grain bulk. We know that the bottom has more variation than the top. So I am thinking in the center of the bin or silo, about two thirds to three quarters the way up. You don’t want it too high because shrinkage may lower the grain and leave the sensor exposed to the air, which can be greatly influenced by the open discharge port. We also considered averaging several sensors to get an average T and RH, but one can get into trouble really quickly because the pyschrometric equations to calculate absolute humidity are non-linear, so I think using just a single sensor might be better.
Also the fan must be off for some time to use this sensor, as one must wait for the air in the grain to ‘equalize’ with the grain. I am thinking at least a half an hour or maybe an hour..
Ron Palmer

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Integris of Opi Systems has the capability to start and stop automatically the ventilator by comparison of grain EMC with air intake. To stop ventilator, how this decision is good considering air is not equalized with the grain. To be correct and to take the good decision, ventilator should it be off for a while and then make the calculation after 0,5-1 h ? Do you think so ? Another explanation : Would they correct the reading, considering it is not in a stabilized situation ?

I also ask myself if the decision to stop or to start is depending of one, a combination or the average of all sensors in the bin.

I hope all these questions are not bothering you. Your informations are very usefull to understand.


EMC is equilibrium moisture content, notice the word equilibrium! You must wait some time for equilibrium to occur. Our data shows that you can not correctly do an EMC while the fan is running. You get the wrong value.  The correct way to control the fan is with the absolute humidity, and if you have RH sensors, there is no need to even do an EMC.

 

Excel Column Definitions for Data Collected — for the record

Excel Column Definitions for Data Collected at Indian Head 2007 to 2015 from Reliable System.  This is for the record and is only useful to those having the data.

Column         Definition

A. Date & time stamp of sensor readings on the hour Nov 03/15 17:27:23

B. lowest temperature sensor close to perforated tube, ⁰C

C. Tm, mid temperature sensor in the grain half way up, ⁰C

D.  Tt, highest temperature sensor in the grain ⁰C

E.  T, temperature of discharge air just inside roof opening, ⁰C

F.  RH, relative humidity of the discharge air %

G. Pressure in inches of water or CFM, fans running if pres > 0.05

H. Temperature of outside air, ⁰C

I.   RHair, relative humidity of outside air, %

J.  Not usually used but in bigger bins pressure is sometimes here

K. Net H20 leaving the bin per cubic meter, grams per cubic meter

L.  =IF(G1 > 0.05,750 * K1 * 60/35310,0) if the fan is running, multiply K by the number of cubic feet that are leaving the bin. In this case CFM is 750 and there are 35.31 cubic feet in a cubic meter

M.  MC , moisture content of the grain,%. The initial moisture content is manually put in Row 1 and then we subtract from that using the amounts from L. We need to know the number of bushels and weight of the grain so that the ratio can be calculated.

N.  RHthres % Knowing the MC and Temp of the grain, we can plug these into an EMC equation and get the relative humidity that this grain wants to be at.

O. EMChum , EMC absolute humidity, grams of water per cubic meter that the grain wants the air surrounding it to contain this much water, and we calculate this from the psychrometric equation or the saturation equation factored by RHthres, just calculated in column N.

P.  This is the ratio that shows how much water was added to the air as it passed through the grain and how close it got to the EMC value: (Dchargehum – Airhum)/(EMChum – Airhum). The Dchargehum will be trying to get to the EMChum but will never get there, so this ratio will less than one.

Q. Ƭ, Time Constant for how long it takes the outside air to acquire moisture from the air and be 63% of the way to EMC We know how close it got from P. and we can calculate the time, t, that is was in contact with the grain by knowing the number of bushels and the CFM. The smaller bins hold 2200 bushels, and one bushel is 1.2446 ft3. So the bin is 2738 ft3. But the grain takes about half this space so there is 1369 ft3 of air. At 3000 CFM we would get an air exchange every 1369/3000 = 0.45 min. Or, the air is in contact the grain for 0.45 min. The time constant, Ƭ = -t / ln( P – 1) where t is 0.45 min and P is ratio calculated in column P. This time constant is important in understanding the amount of time that is needed to reach equilibrium.

R. Ratio of t/Ƭ. The bigger this number, the closer we are to equilibrium. It is a measure of the efficiency to which we are using the energy in the grain to dry. The bigger this number, the more energy we are using from the grain to dry.

S. Safe days. See Spoilage Index Report on how safe days and spoilage index is calculated.

T.  Spoilage Index.

Spoilage Index Results from our Trials

The spoilage index is an accumulation of the reciprocal of Safe Days. Safe Days depends on the temperature, T , and moisture content, MC. Safe days is defined as:
Number of days until germination capacity is reduced to 95% (Fraser & Muir 1981)
Safe Days = 10^(6.234 -0.2118 MC – 0.0527 T) wheat and cereals
= 10^(6.224 – 0.302 MC – 0.069 T) canola and oilseeds
The objective for safe storage, is to maximize the number of safe days by lowering the temperature T, and the moisture content, MC. Look at how the number of safe days varies with temperature for ‘dry’ wheat:
• 14.5% @ 30⁰C = 38 safe days
• 14.5% @ 20⁰C = 128 safe days
• 14.5% @ 0⁰C = 1458 safe days
• New Definition: Spoilage Index = Ʃ ( 1/ safe days) x 100
Example, if safe days is six, then after 3 days we will have an accumulated deterioration of: (1/6 + 1/6 + 1/6) x 100 = 50%, after 6 days : (6 x 1/6) x 100 = 100% of the way to 95% germination.
We modified the spoilage index in order to accumulate every hour instead of every day:
Spoilage Index (SI) = Ʃ ( 1/ safe days)/24 x 100
What we got from our trial runs from 2007 to 2015:
File      SI     trial time (hours)
071      22.6   214
072      13.7   259
073      11.6   14.8
0809P  3.1      54
0809W 23.8    290
0810B 60.6     291
0810W 24.9    215
0909B   3.6       151
0909W 149.8  268
1010     250     674
1110     71.6     239
1216      85.3    263 bin had no temp cables, so used discharge T
1217      364.6   335 used discharge T, so SI is questionable
1218      45        417 used discharge T
1219      85.3     405 used discharge T
1309B    3494    1274 certainly had spoilage here, cause high MC
1316      216.9    1724
1317      371.4     1910
1318      72.6       1741
1319      38.7       951
1409      1284      2020
1410     350         1296
1416      315.7     1969
1417     86.5        1969
1418     51.2        1600
1419     109.7      1600
1509     426.7      1940  but using sample tube MC 14.5% gives SI of 72.7
1510     104         1862                                                    15.1                      110.2
1516     242         1941                                                     15.8                     145
1517     276         1941                                                     16.2                      196
1518     399         2069                                                     15.8                      267.5
1519     397         1941                                                     15.3                      113.2
As you can see many of our trials had an SI > 100 and this was mainly due to starting with a high or very high MC so the SI grew quickly in the first few days.

When do we get condensation, dripping and crusting in the bin?

The grain drying calculator gives the precise conditions under which condensation will occur; and that is whenever RHthres is greater than 100. (See the blog on dripping).  In playing around with the calculator today, I can give you a rule of thumb for this without using the calculator.

For oilseeds like canola and flax, that is just dry 10% MC; if the grain is more than 5 deg C warmer than the outside air (roof), there will be condensation.  I tried the calculator with GrainTemp – AirTemp,  30 – 25,  20 -15, 10 -5.  So a difference of 5 C gave me an RHthres close to 100%.

For cereals like wheat and barley, that is just dry at 14.5% MC; if the grain is more than 7 deg C warmer than the outside air (roof  & walls) , you have conditions for condensation.  Tried 27 – 20,  17 – 10, 7  – 0 and they gave RHthres close to 100.

This is interesting, because we see that oilseeds are more sensitive to condensation, and we certainly don’t need much of a difference in temperature before condensation starts to form on the top layer of grain and the inside of the roof —  only 5 deg C  difference!!  For example, if you have wheat that is 28 C and the outside temp is 20 C, you could turn on the fan and you are in for a bit of condensation on the inside of your roof, and on the top layer of wheat.

For years and years we have had spoilage from condensation because we did not turn the fans on immediately to cool the grain with a temperature that was close to the actual temperature of the grain?

The Natural Air Grain Drying Project — a WGRF project

This project was started in 2007 at the Indian Head Experimental Farm as an IHARF (Indian Head Agricultural Research Foundation) project under the supervision of Guy Lafond.  Data was collected every year from two instrumented bins.  I was given the data in 2010 with a vague mandate to find out what was happening in the bins as the fans ran continuously.  Did the fans have to run continuously?  Were there times during the day in which more drying was taking place?

The experiment was set up brilliantly such that readings were taken every hour and most importantly that  RH and T sensors were placed to measure the discharge or exhaust air at the top of the bin as well as RH and T sensors measured the air entering the bin.  This lead to the ability to measure the water going into the bin and the water leaving the bin and thus we could precisely track the drying, hour by hour.  And the Diurnal Drying Cycle was determined.

In 2012 we got a grant from WGRF (Western Grains Research Foundation) and 4 more bins were instrumented to collect more data.  Another three year grant from WGRF was obtained in 2015 to discover more.  It was called:

New Insights into Natural Air Grain Drying  (2015 – 2018)

Objective: To develop a fan control strategy using natural air that results in the safe storage of grain, that is efficient and results in less fan running time, and that results in more uniform drying of grain.

Benefit: Reducing the risk of grain spoilage and preventing revenue loss with on farm storage.

When Should I Turn the Fan On or Off? But Only have Temp Sensors

In the last blog, I talked about the ultimate controller that used moisture cables with temperature and relative humidity sensors.   But what if I don’t have this T/RH sensor cable — I only have temperature sensors?  Well, we can still do a pretty good job of turning the fan on only when the conditions are right for drying.  How, by using the grain drying calculator .  This calculator determines if conditions are right for drying, but it is subject to the accuracy of the EMC equations, whereas the ultimate controller does not require EMC equations other than to determine the MC of the grain.  Also the ultimate controller uses water balance to turn the fan off, whereas again this method requires the calculator to both turn the fan on and off.

Here is what you do; again it is important to get the fan going immediately upon filling the bin.  Then every hour one makes a calculation by entering the moisture content  (MC) of the grain, the temperature of the grain, and the temperature of the outside air.  The calculator returns a threshold relative humidity ( RHthres ) for several grains.  If the outside RH is less than this threshold, then we have drying conditions.  The greater this difference, the more drying will take place.  So we would turn the fan on.   If however the RHthres is less than the outside air RH, then the fan would be turned off.  Because conditions can change rather quickly, this calculation and  a decision should be made every hour or so.  This process would continue until  the average MC (as measured manually with a  sampling probe).  The bottom will as always dry first, but it is not necessary to continue the process until the top is dry, but only until the average is dry.  When the grain is pulled out, it will blend to give an overall dry.  The top, even if it is a bit tough, will not spoil because it will be cooled with this overall process and therefore be safe from spoilage.

After the process is terminated, the temperature of the grain should be monitored, and if the grain temperature begins to rise substantially, then the process should be restarted.  One might not have to do this all winter, but in the spring and summer the process may be used to keep the grain cool.

If one is not sure about the MC of the grain, then the dry level should be entered into the calculator.  For example, wheat may have been put into the bin, some being 14.2%  some at 15.1%  and another unknown quantity at 14.9%.  The dry level for wheat is 14.5%, so that is what should be put into the calculator, and it will do the calculations for drying the grain to this level.

This calculator is not quite as good as the ultimate control strategy, but it is pretty close.  It relies on the accuracy of the EMC equations, and it also requires manual measurements of the MC to determine the average MC and thus when the process can be terminated.