Stored grain heats when it is damp, because the grain and the things living in it respire, and you find it with temperature probes on a schedule. Bin 4 has a core at thirty eight degrees. The moisture through that core is thirteen point one percent, and nothing respires at thirteen.
I manage the drying and storage at a grain cooperative and through harvest and into the winter I am the one on nights.
The two measurements, because this trade is the two of them and nothing else.
Temperature comes off cables hung down through the grain with sensors every metre, so you get a profile from the top to the floor of every bin, logged to a system in the office.
Moisture comes off a meter on a sample. You take a spear sample at several depths, you run it through the meter, and that number decides everything: what you can store, how long, and at what price it sells.
The biology is simple. Grain and the fungi and insects in it respire, and respiration makes heat and water, and both speed the respiration up. So a damp patch warms, the warmth makes it damper, and you get a hotspot that spreads and costs you a bin.
The threshold is about fourteen and a half percent. Below fourteen, the moisture is not available to the microorganisms and the grain is biologically close to inert. Below thirteen and a half, stored cool, it will sit there for two years and do nothing at all.
Bin 4 has a hotspot at about three metres down, in the middle.
Thirty eight degrees, in a bin where the rest of the grain is at nine, which is what it should be in November after cooling.
Thirty eight degrees is a serious number. That is a bin heading for a fire in the worst case and a total write off in the normal one.
So the first time it showed, in 2019, I did what you do: turn the fans on, cool it, and if that does not work, move it, which means augering a thousand tonnes of wheat out of one bin and into another at two in the morning.
We did both. The fans pulled it down to twelve degrees over four days and it came back to thirty eight in a fortnight. We moved the whole bin into bin 7 in 2020.
The hotspot stayed in bin 4.
In an empty bin it does not read, because the sensors need grain round them. The season after that, with new grain in it, it was back. Three metres down, in the middle, at thirty eight.
It is not in bin 7. Bin 7 has never done anything in its life.
The moisture, which is the part that makes this impossible rather than expensive.
I have spear sampled that core a great many times. Four depths, three positions, through the hot zone, in four different seasons.
The highest reading I have ever got out of it is thirteen point four percent.
Thirteen point four.
Thirteen point four percent grain at thirty eight degrees is a contradiction. The heat has to come from respiration, respiration needs water, and the water is not there. And if it were respiring at that rate it would be making water, and the moisture in that core would be climbing, and it does not move.
What else has been looked at, because the cooperative takes this seriously and has paid for most of it.
The cables and the sensors. Replaced in 2020, and a handheld probe on a rod put into the grain by hand to the same depth. The handheld reads thirty seven and thirty nine.
The fabric. Bin 4 is the same concrete and steel as the rest of them, built in the same year, in the same row, with the same aeration floor.
Something under the floor. This was my theory for two years: a heating main, a buried duct, a transformer. The drawings show nothing, the ground has been scanned, and there is nothing in that building with more than a lighting circuit in it.
Insects. An entomologist from the levy body came out, which was a good day out for both of us, and sampled the hot zone.
Her report says insect activity in the sample is below the level that would account for any measurable temperature rise and that the population is consistent with a clean store.
She also wrote, and I have kept this, that in her experience a hotspot with no associated moisture increase and no biological load has no explanation in the literature.
What gets done.
I manage round it. Bin 4 is filled with the driest crop we take in, it is cooled harder than the rest, and it is the first bin we empty in the spring so nothing sits in it over a summer.
Graded and sold, the grain out of bin 4 passes on moisture, on specific weight, on screenings, and on mycotoxins, which is the test that would find the damage if there were any.
It has never failed a test.
Eleven loads a year out of that bin for six years. Not one rejection, not one query from a mill, and the mills that buy off us test every load on the weighbridge before they tip it.
What I have changed about how I work.
The temperature round is at two in the morning because that is when the fans are off and the readings are honest.
Bin 4 has a hatch at the top that you get to by a ladder and a gantry, and the point of going up is to look at the surface, because a bin going wrong crusts and smells and you can see it.
I stopped going up in 2022 and I do it off the sensors alone, and if anybody asks I say it is the fall risk, and the gantry is perfectly sound.
In November 2022 I was up there with a lamp looking at the surface, which is flat and grey and absolutely unremarkable, three metres above a spot that has read thirty eight degrees for six years.
A thousand tonnes of grain settles. It makes a noise like rain on a roof, slow, a few grains at a time, and you hear it all winter and you stop noticing.
Under the hatch it was coming up through the surface instead. A little at a time, in one place, in a ring about the size of a dinner plate, the way sand does over something moving underneath it, and I watched that for a while from the top of the ladder, and then I came down and turned the fans on for no reason I could have justified on a form.