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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 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 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. This first article in this series focuses on temperature sensors and explores the number needed on a cable as well as cable placement to effectively monitor the quality of stored grain.

Temperature sensors used for stored grain monitoring consist of either thermocouples or thermistors. According to manufacturers, type “T” thermocouple (TC) sensors are commonly used to manufacture TC-based temperature cables for stored grain monitoring applications. Type “T” thermocouples consist of one copper wire and one wire made of constantan, an alloy approximately 57% copper and 43% nickel.

The junction of the two wires is the actual temperature sensing point, which is rated up to 600°F (315°C). Depending on duty level (i.e., tensile strength) and customer preference, sensor points are spaced 3, 5, 6, 7 or 8 feet (0.9, 1.5, 1.8, 2.1 or 2.4 meters) apart in wire bundles inside heat-shrunk vinyl or nylon sleeves. These are placed along a load-bearing steel rope and extruded together inside a vinyl or nylon outer jacket.

Together with the lead wire they make up a temperature cable that can be installed once it is cut to the correct length and fitted with hanging hardware. Lead wire carries the thermocouple voltage signals from each cable to a multiplexing junction box that can convert analog into digital signals. Lead wire consists of groups that contain a common constantan wire and typically six copper wires that are dedicated to each individual thermocouple within the cable.

Thermistors are temperature sensors whose electrical resistance changes in response to a change in temperature. According to manufacturers, Negative Temperature Coefficient (NTC) thermistor sensors, which exhibit a decrease in resistance as temperature increases, commonly are used to manufacture thermistor-based temperature cables for stored grain monitoring applications. They are produced using powdered metal oxides that determine their electrical characteristics.

NTC thermistor sensors have a non-linear temperature versus resistance relationship and are capable of measuring temperatures up to 572°F (300°C). They are generally very rugged, extremely stable, and highly accurate devices. Depending on duty level (i.e., tensile strength) and customer preference, sensor points are spaced 4 or 8 feet (1.2 or 2.4 meters) apart on a digital sensing cable placed inside a sturdy outer tube. This allows the temperature-sensing element to be retracted if it needed to be replaced in the future.

The outer tube contains two co-extruded load-bearing steel ropes. The cables must be manufactured to the correct lengths and fitted with hanging hardware. Lead wire carries the thermistor voltage signals to a multiplexing junction box that converts analog into digital signals. From there, signals can be transferred wirelessly to digital monitoring and fan control platforms.

Temperature cable manufacturers specify placement of cables in the grain mass as a function of bin diameter. For example, the installation manual of Boone Cable Works & Electronics, Inc. (BCWE) specifies one cable in the center of a 20-foot (6.1 meters) diameter silo, three evenly-spaced cables at a radius of 9 feet (2.7 meters) in a 36-foot (11-meter) diameter bin without a cable in the center, and four evenly-spaced cables at a radius of 14 feet (4.3 meters) in a 42-foot (12.8-meter) diameter bin with a fifth cable in the center.

Read more: Grain operations: Temperature sensors and cables