For measurement of the temperature of grain in bags, a bimetallic thermometer can be used but it is slow and measures an average of temperature over a large proportion of the stem. A thermocouple or thermistor system incorporated in a metallic spear is preferable for bags or for taking spot measurements in bulk grain at depths up to five metres. Monitoring of the temperature within a grain bulk over a long period is best carried out with either a thermistor or thermocouple system. In both cases the leads should be placed in situ before the silo is filled with grain. If monitoring at a small number of points is required, then the thermistor system is most suitable, and thermistors are essential if long-term monitoring by automatic recording is required.
Thermocouples are cheap and convenient to use where large numbers of points are to be monitored.
Monitoring systems requiring many sensors also need a switching device, which should be located, if possible in an air-conditioned room. Otherwise, in hot humid climates, they may fail. Temperature gradients near the surfaces of a bulk of grain can be severe in tropical and subtropical climates. Such gradients are best measured with thermocouple or thermistor systems, because their sensing heads can be made small enough to take meaningful measurements of temperatures at points in close proximity to each other.