KANSAS CITY, MISSOURI, US — Monitoring the temperature, moisture content and carbon dioxide (CO2) levels in stored grain is essential to maintaining quality. Aeration with ambient air is the most common method used to manage grain temperature, moisture content, and CO2 levels. Still, the complexities of stored grain ecosystems make effective monitoring and optimal management challenging, especially as storage bins (silos) increase in size.
Temperature and relative humidity sensors installed on cables and placed in the stored grain mass are routinely used to track grain mass temperature and moisture content. Carbon dioxide sensors can be placed in the plenum and headspace below and above the stored grain mass, respectively, to track the onset of spoilage due to biological activity from molds and insects.
The number and placement of sensors, and the interpretation of sensor readings, are key to effectively monitoring conditions in a stored grain mass and managing optimal windows of aeration based on real-time weather data.
The second article in this series focuses on the placement of temperature cables to effectively monitor the grain quality during the aeration and storage periods.
Temperature monitoring during aeration
Temperature is the key factor for monitoring the progress of an aeration front advancing through a grain mass during the cool-down period. In a cored and near-level surface grain mass, airflow is pretty much uniform and thus a cooling front will move uni-directionally upward (or downward). When the grain mass remains peaked, airflow is no longer uniform and thus a cooling front will move more slowly through the uncored and deeper center and faster through the shallower periphery of the grain mass.
Nevertheless, airflow moves uni-directionally upward (or downward) and not sideways through the grain mass. For a typical design airflow rate of 0.1 cfm/bu (0.11 m3/min/MT), a cooling front will require a minimum of about 150 hours to advance through the grain mass. At a typical temperature sensor spacing of 6 feet (2.1 m) along a cable, the advancing cooling front would move past a new layer of sensors about every 15 hours if grain depth is 60 feet (60 feet/150 hours = 0.4 feet/hour; 6 feet/0.4 feet/hour = 15 hours) and every 10 hours if grain depth is 90 feet (90 feet/150 hours = 0.6 feet/hour; 6 feet/0.6 feet/hour = 10 hours).
Thus, sensor spacing along a cable should consider airflow rate, grain depth, and the stored grain manager’s preference for tracking cooling front advancement through the grain mass. For example, if a stored grain manager wanted to check cooling front progress twice a day (every 12 hours), then optimum sensor spacing along cables in a 60 foot and 90 foot deep grain mass would be about 5 feet (5 feet/0.4 feet/hour = 12.5 hours) and 7 feet (7 feet/0.6 feet/hour = 11.7 feet), respectively.
On the other hand, if temperature sensors were spaced 3 feet (0.91 m) along a cable, and airflow rate was as high as 1 cfm/bushel (1.1 m3/min/MT), as is typical in an on-farm drying bin, the cooling front would move through a 30-foot deep grain mass in a minimum of 15 hours and past a new layer of sensors about every 1.5 hours (30 feet/15 hours = 2 feet/hour; 3 feet/2 feet/hour = 1.5 hours).
Thus, even though it may seem intuitive to place temperature sensors more closely together along cables in shallower depth bins, when the cooling front advances rapidly, a large number of temperature sensors could be overkill as they will not yield more useful information to the stored grain manager.
No explanation or supporting documentation is provided by companies that manufacture and/or sell temperature cables as to why one might select one or the other spacing of temperature sensors along cables.
Spacing of temperature sensors
In terms of the number of cables and their placement in a grain mass, a year-long aeration and storage period was simulated using one of our computer models. The model used weather and grain data to predict temperatures at more than 2,000 sensor locations in the grain mass.
Results were compared to sensors on three cables, one placed in the center, and two at two-thirds of the radius. Sensors were spaced 4.5 feet (1.4 m) along the cables for a total of 30 sensors and 9 feet (2.7 m) along the cables for a total of 15 sensors.
During the cool-down period while grain was aerated with ambient air, the number of nodes — whether 15, 30 or 2,000 — and thus the number and placement of cables did not affect the predicted grain temperature the stored grain manager would have observed as the cooling front advanced upward through the grain mass.
Thus, spacing of temperature sensors throughout the depth of a cored and leveled grain mass provides useful information during the cool-down period while placement of cables along the radius less so. The exception applies when monitoring the advancement of a cooling front through peaked and un-cored grain, given it will take substantially longer (perhaps as much as two to three times longer) to move a cooling front through the peak of a grain mass compared to the shallower periphery layers.
Temperature monitoring during storage
The general recommendations for temperature cable placement throughout the grain mass, particularly in the center (or not) and at what distance from the sidewall needs further analysis. Grain is an excellent insulator, and if un-peaked and cored will stay cold after aeration cooling for many months, especially when aeration fans are sealed (covered) when not operated.
Warming primarily will occur as a function of ambient weather conditions, especially solar radiation on the sun-exposed bin roof and wall surface areas. This cyclically warms and cools the outer layers of grain near the bin wall and headspace during the day and night, respectively. Thus, the question arises where the placement of temperature sensors (and thus cables) would provide the most useful data during the grain storage period. Unlike the cool-down period, substantial differences are observed in the predicted average temperatures during the storage period.
The 15 temperature sensors spaced 9 feet (2.3 m) apart along four cables provided as much useful information as the average temperature predicted by 2,000 sensors. This contradicts the intuition that more sensors will provide more useful information to a stored grain manager.
In temperate climates, grain temperatures in the periphery are lowest during winter and highest during summer but are generally not captured because of the lack of cables placed near the silo wall. The silo wall and about a 3 foot (1 m) layer of grain closest to the wall experiences the greatest temperature fluctuations during a typical fall to late spring storage period. This demonstrates that stored grain managers essentially have no temperature management control over the periphery layer of a grain mass.
The question arises as to the purpose of temperature monitoring when the grain mass in the periphery (and near the surface) where most of the temperature fluctuations occur is not equipped with temperature cables.
Cable supply companies have similar recommendations for placement of temperature cables as a function of bin diameter. However, upon closer examination, the placement of cables nearest the wall shows little consistency as bin diameter increases (see table l, bottom of page 48).
For example, when grain mass depths are assumed the same as bin diameters (i.e., 36 to 105 feet; 11 to 32 m diameter), distances of the periphery cables to the outer wall range from 7 to 10 feet (2.1 to 3 m), resulting in radii fractions of 1/2 to 5/6, periphery grain volume to bin wall surface area ratios of 5.8 to 8.7, and periphery grain volumes per cable increasing as bin diameter increases.
For the largest commercially available bin diameters (135- and 156-foot diameter) at maximum level grain depth, values were extrapolated by assuming periphery cable distance to bin wall was 8.5 feet (2.6 m) and periphery grain volume per cable was the same value as for the 105-foot (32 m) diameter bin. This would result in placing 19 and 22 periphery cables at 59 feet (18 m) and 70 ft (21.3 m) from the center, respectively, and result in the same periphery grain volume to bin wall surface area ratio of 8.0.
As the ratio of volume to surface area increases, the dampening effect of grain insulation to the cyclical temperature changes of the grain nearest the bin wall increases. This reduces the potential for managers to obtain useful information from temperature cables placed too far of a distance from the bin wall and grain surface during the storage period.
On the other hand, if the primary value of cables from the center along the radius to the periphery is gained from monitoring the advancement of the cooling front during aeration, then fewer cables with optimally spaced sensors should suffice. Given that grain often is left peaked and un-cored (especially during the bin filling period), there is little doubt in the value of always placing a temperature cable in the center of a bin for monitoring the progress of the aeration cooling front compared to shallower periphery layers, especially as bin diameter and height increase.
Monitoring grain during the cool-down (aeration) and storage (non-aeration) periods using temperature sensors is critical for managing and maintaining quality. However, the documentation for recommending the number of sensors on a cable and the number of cables and their placement in a grain mass is not readily available. Nor do the current recommendations reflect what may suffice for stored grain managers to extract useful information from temperature data recorded.
Therefore, temperature cable manufacturers and suppliers should consider revising (and justifying) their current recommendations for the optimal number and placement of temperature sensors and cables needed. This would make monitoring of stored grain quality more effective while substantially reducing the costs of cable-based temperature monitoring systems for end users.
In the next article in this series, we will evaluate the effectiveness of temperature cables for detecting increasing temperatures in the grain mass due to biological activity that results in self-heating and spoilage.
Dirk E. Maier is a post-harvest engineer with the Iowa Grain Quality Initiative at Iowa State University. He may be reached at firstname.lastname@example.org.