MC% is a measurement.
Various methods are used to measure moisture content (MC) in grain. None are direct and none are 100% accurate. Two primary methods — the oven dry and the Karl Fischer titration – are used in laboratories to establish a reference (standard) to calibrate field instruments (bench and portable).
In each case a sample is weighed, ground and the water content is determined by weighing the remaining matter after evaporation in the oven dry method or extracting the water chemically in the titration method. The results differ so ambiguity exists in MC references by default. Both of these methods destroy the sample and therefore are not repeatable. An improved method suitable to be used as a standard is desirable.
Most field instruments utilize the dielectric properties of grain for MC measurement. Electrical capacitance and resistance properties change with different levels of MC in grain. By measuring either one of these values MC can be estimated indirectly when compared to a standard. Adjustment for the temperature of the sample is also necessary because of its influence.
Another way to measure MC is to measure the temperature and relative humidity (RH) of the air surrounding the grain if there has been sufficient time to equalize to the air. An equilibrium moisture content (EMC) formula expresses the relationships among the grain, the temperature and the RH and can provide a reliable MC estimate.
This method makes automation easy which saves time and labor compared to sample and test methods. Another advantage is to support reliable remote operation in near real time with systems that have connectivity. However until recently RH sensors were not available with the necessary accuracy, repeatability, range and ruggedness to economically provide this capability.
An ongoing challenge in MC measurement is whether the sample being measured is representative. This is obvious if a small sample is obtained from a large amount of grain. Does this small sample really represent the whole amount? Mixing samples from various locations or from a flowing stream of grain are efforts to obtain an average MC from a large mass – a representative sample. This is important in the buying/selling of grain when MC impacts discounts, drying charges and shrink factors.
Whether a sample is representative or is a high, low or an average reading is really important when managing stored grain so correct actions can be performed. This becomes more important when different zones in a bin have vastly different MC. Often the trend and pace of MC change is very important in storage management.
Management can be improved with temperature and/or RH sensors on cables in storage bins. However these provide information (data) only from the location of the sensor. These are small sample areas and the readings need to be viewed with caution. This is because air channels around the cables so the readings can be significantly different from grain just a few feet away.
This is especially true when drying or re-wetting is occurring. The differences will diminish as aeration continues because all the grain will tend to move toward the conditions of the air being blown through the grain. The MC lag away from the cable will diminish with further operation, and catch up assuming aeration continues with similar air conditions long enough. Peaks and inverted cones distort this process significantly because air seeks the path of least resistance.
Useful Terminology: EMC — TMC — PMC
Equilibrium Moisture Content (EMC) is the MC of grain when it has equalized to the temperature and relative humidity (RH) of the air that surrounds it (the seeds, kernels). Usually grain in the field is not in equilibrium with the air around it because the weather is always changing and the grain may not yet be dormant.
Seeds come into true equilibrium only after they become dormant. Prior to dormancy bio-chemical changes are ongoing in seeds that affect the equilibrium with the air around them because water is expelled as sugar is changed to starch for example.
The term 'dry down' is often used from a view that water in the seed is not needed. Actually it is a necessary resource and allowing time for 'curing' instead of forcing water from grain prematurely with heat results in better grain and returns higher value. Dry down occurs during curing so there is nothing wrong with using the term. This can occur in the field or in a bin.
Grain in a bin without airflow quickly moves toward equilibrium with the air around it — hours and days depending on differences and conditions. However this is very local so layers and pockets settle with different equilibrium moisture contents that may present high risk for spoilage.
A hot spot demonstrates this. It is desirable to have a uniform EMC throughout a bin for ease of storage and higher value at market.
Achieving uniform EMC in a bin requires a setting (target) and a measurement of outside air so fans can be operated when the air being blown through the grain will move the grain to the TMC:
EMC = Equilibrium Moisture Content
The moisture leaving the grain is in balance with the moisture entering the grain
TMC™ = Targeted Moisture Content — a setting or goal.
This is the desired MC at the point of delivery (POD).
PMC™ = Predictive Moisture Content — a measurement of outside air.
This is the combination of temperature and relative humidity (RH) of ambient air that will produce the TMC when blown through the grain — ideally adjustment is made for fan heat.
Producing a uniform EMC near the TMC is tied to the weather, airflow rate, and hours of operation. The temperature will also be uniform because calculating the PMC incorporates temperature. It is not feasible to control moisture content without synchronizing with the temperature.