Every Constant Humidity Controller (CHC) is supplied with an equilibrium chart, and it’s one of the most useful tools you’ll have in the drying shed. It tells you exactly what set air relative humidity (%RH) to dial in on your CHC to reach a specific moisture content in your crop. Get this one setting right and the rest of the drying process takes care of itself.
What is the equilibrium chart?
The equilibrium chart shows the relationship between the relative humidity of the drying air and the final moisture content the grain will settle at once it’s in balance with that air. This relationship, known as equilibrium moisture content, is different for every crop, which is why the chart lists wheat, barley, oats, rape, peas, ryegrass and beans separately. Grain left in contact with air of a given humidity for long enough will always dry down to (or pick up moisture up to) the same equilibrium point, regardless of how it got there. That’s what makes the chart such a reliable reference for setting your controller.
Moisture content / relative humidity equilibrium chart at 15°C
| Set air %RH | 55 | 60 | 65 | 70 | 75 | 80 | 85 | 90 |
|---|---|---|---|---|---|---|---|---|
| Wheat % mc | 13 | 13.5 | 14.5 | 15.5 | 16.5 | 17.5 | 19.2 | 20 |
| Barley % mc | 12 | 13 | 14 | 15 | 16 | 17.8 | 19 | 19.5 |
| Oats % mc | 11.5 | 12.5 | 13 | 14 | 15 | 17 | 19 | 20 |
| Rape % mc | 7 | 7.5 | 8 | 9 | 10 | 12 | 14 | 16 |
| Peas % mc | 12 | 13 | 14 | 16 | 18 | 21 | 24 | 27 |
| Ryegrass % mc | 11 | 12 | 13 | 14 | 15 | 16.5 | 20 | 24 |
| Beans % mc | 12 | 13 | 14 | 16 | 18 | 20.5 | 23 | 26 |
Why is the chart geared around 15°C?
Equilibrium moisture content doesn’t just depend on humidity — temperature affects it too. Harvest Installations’ chart is calibrated for an ambient air temperature of 15°C, which reflects typical daytime conditions during a UK harvest. This gives a consistent, dependable reference point for day-to-day drying rather than a chart that needs constant adjustment as conditions shift through the day.
For colder conditions, such as winter drying at an average of 5°C, the chart includes a simple correction: set the %RH around 10% lower than the chart shows to reach the same moisture content you’d get at 15°C. For example, wheat at 55% RH gives 14.5% moisture content under the winter correction, rather than the 13% shown in the main 15°C table.
How do you read the chart?
Reading the chart is straightforward once you know your target moisture content. Find your crop along the left-hand side, then move along that row until you reach the moisture content you want to achieve. The %RH heading at the top of that column is the setting to dial into your CHC.
For example, if you’re drying wheat and want to bring it down to 14.5% moisture content, the chart shows this corresponds to 65% set air RH. If you were drying rapeseed and needed 12% moisture content, you’d set the CHC closer to 80% RH, since rape reaches equilibrium at a much lower moisture content than cereals for the same humidity level.
It’s worth noting how differently each crop behaves on the chart. Rapeseed, for instance, equilibrates at far lower moisture levels than wheat, barley or beans at every humidity setting, which is why it needs a distinctly different approach on the controller compared to cereals.
Using the chart in practice
The chart is designed to be used alongside your fan and CHC as part of a straightforward, staged process.
Start by levelling the crop surface in the store to get an even airflow of 15 to 25 cubic feet per minute through the grain. Run the fan and CHC together, 24 hours a day, set to a high humidity of around 80% RH, until the grain reaches roughly 18% moisture content (around 12% for rapeseed). This first stage removes the bulk of the surface moisture without over-drying any part of the bulk.
Once the crop is down to that point, switch the CHC setting to match your actual target moisture content using the equilibrium chart — for wheat destined for 14.5% moisture content, for example, that means resetting to 65% RH. Leave the system running at this setting until the whole bulk has equalised at the target level.
Finally, once the desired moisture content is reached, turn off the CHC and run the fan alone to cool the grain, choosing a time when the ambient temperature is suitable for cooling without re-wetting the crop.
Why does this matter for grain drying?
Setting the wrong %RH is an easy way to lose money on a drying run. Too low a humidity setting and the grain will overdry — cracking the seed coat, increasing the risk of insect damage and mould in store, and in the case of milling wheat, damaging the gluten structure that millers pay for. Every percentage point of moisture lost beyond target is also a percentage point of saleable weight gone, which adds up quickly across a full store.
Set the humidity too high, and the air won’t dry the crop fast enough, or may even add moisture back into grain that’s already at target — extending drying time, burning more fuel than necessary, and leaving the crop vulnerable to spoilage while it waits. AHDB’s grain storage guidance sets out safe moisture and equilibrium relative humidity targets for exactly this reason.
The equilibrium chart removes the guesswork from this balance. Because it’s based on the known physical relationship between air humidity and grain moisture, rather than trial and error, it lets you set the CHC with confidence that the crop will settle at exactly the moisture content you’re targeting — no more, no less. That’s the difference between a drying run that protects both grain quality and fuel costs, and one that quietly erodes your margins.
Getting the setting right for your crop
Every farm store is drying a slightly different mix of crops and targeting slightly different end moisture contents depending on the buyer or end use. If you’re ever unsure which %RH setting to use for your crop and target moisture content, the equilibrium chart supplied with your CHC is the first place to check — and our team at Harvest Installations is always happy to talk through the right setting for your specific harvest.