World Grain - December 2021 - 68

GRAIN OPERATIONS
Temperature sensors and cables
Number and placement are important to effectively monitor stored grain
Editor's note: This is the first in a series of articles on grain storage management.
by Dirk Maier and Jordan J. Wright
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 and cables
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
(see Figure 1, page 71).
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 rug68
December
2021 / World Grain / World-Grain.com
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World Grain - December 2021

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