# Haystack Fires in Australia: Causes and Considerations for Preventative Management
## 5 Monitoring of Agricultural Parameters
### Near-Infrared Sensors
#### 5.1.2 Temperature  
Temperature sensors may also be employed to measure heat emitted by the bale. The four main types of contact temperature sensors are thermocouples, resistant temperature detectors (RTD), thermistors and silicon based integrated circuit boards (PCB), all of which have their advantages and disadvantages (Table 6). Thermocouples  
One of the most inexpensive and robust temperature sensors are thermocouples. Thermocouples can operate over a relatively wide temperature range. Thermocouples are made by joining two dissimilar metals such as iron and copper/nickel (constantan) at one end, thus producing a small open circuit at the other end, which measures voltage which is a function of temperature [83]. The joined end merging two dissimilar metals that directly probes a given material is called the hot end/junction. The other end is referred to as the cold end/junction. A small voltage, generally referred to as EMF (electro-motive force) is created when there is a difference in between the two hot and cold junctions. Generally, the voltage is nonlinear with respect to temperature. However, for small changes in temperature, voltage is approximately linear; therefore, EMF can be measured and used to indicate temperature [84]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{1}{c}{**Resistance**} & \multicolumn{1}{c}{**Integrated**} \\  & **Thermocupe** & **Temperature** & **Theristor** & **Circuit Sensor** \\  & **Detectors (RTD)** & & & **(ICs)** \\ \hline Temperature range & \(-\)100–500 \({}^{\circ}\)C & \(-\)50–250 \({}^{\circ}\)C & \(-\)50–130 \({}^{\circ}\)C & \(-\)40–150 \({}^{\circ}\)C \\ Accuracy & \(\pm\)1–4 \({}^{\circ}\)C & \(\pm\)0.2–1 \({}^{\circ}\)C & \(\pm\)0.05–0.5 \({}^{\circ}\)C & \(\pm\)0.5–1 \({}^{\circ}\)C \\ Sensitivity & Low & Medium & High & Low \\ Linearity & Medium & High & Low & Medium–High \\ Response time & 5–80 s & 1–8 min & 0.2–10 s & 0.5–100 s \\ Self-heating & No & Yes & Yes & Yes \\ Cost & Medium–High & High & Low & Low \\ Power source & None required & External & External & External \\  & & Larger in size, good & & Small size availability, \\ Other & Wired communication only & immunity to electrical & Small size availability & calibrated during \\  & & noise & & production \\ Main uses & Extreme conditions (e.g., & Food and & Household and & Thermal compensation \\ aircraft exhaust gases) & pharmaceutical & automotive & and management of \\  & & other components \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Advantages and disadvantages of the four main types of temperature sensors, adapted from [PERSON] [83] and [PERSON] et al. [49]. Resistance temperature detectors (RTDs) are temperature-sensing devices for which variations in temperature result in measurable changes in resistance. RTDs are usually made from platinum; however, nickel or copper-based devices are not rare with different shapes such as wire wound and thin film [83]. Typically, RTDs are connected to a constant current source. The resistance of an RTD can be measured by estimating the resulting voltage followed by applying a constant current [85]. RTDs exhibit consistency towards a linear resistance to temperature curves; therefore, any nonlinearity to temperature change is highly predictable and repeatable. Thermistors  
Thermistors are similar temperature-sensing devices to RTDs, whose resistance varies with temperature change. Thermistors are made from semiconductor materials. Resistance of thermistors is determined in a similar way to RTDs, but thermistors exhibit large resistance change for a minimum temperature change. Thus, thermistors maintain a highly nonlinear resistance to a temperature curve (Table 6). Due to high sensitivity, thermistors are generally ideal for set-point applications [86]. Silicon Based Integrated Printed Circuit Board (PCB, ICs)  
Integrated circuit sensors are available with a variety of interfaces, such as analogue or digital. In the case of digital, a serial peripheral interface, SMBus/I2C or 1-Wire, is very common.