# An OCARI-Based Wireless Sensor Network for Heat Measurements during Outdoor Fire Experiments
## Appendix B Heat Sensor Conditioning for the OCARI Platform  
With respect to the Zigbee commercial solutions, a large set of usual instruments is usable while working in the range 0-10 mV/5-20 mA (i.e., sensors for heat, gas detection, and air pollution). With respect to the OCARI platform, an adaptation is required and in the present case, for the K-type thermocouple (TC) and the Medtherm Schmidt-Boetler as Gardon fluxmeters (HFM). The voltage scales of the values on analogue inputs range between 0-3.3 V and are not suitable for the sensor's measurement range (0-40 mV for the TC/0-10 mV for the fluxmeters). Both thermocouples and radiative fluxmeters produce microlevel voltages, and it is necessary to use signal amplifier circuits. An experimental measurement calibration of the signal was performed. In order to acquire data from thermocouples and fluxmeters, an adaptation of electronic interface is required to turn the output millivoltage from the transducer into voltage input of the ADC node (Figure 5). A commercially available AdaFruit 8495 K-type thermocouple amplifier is used to collect low voltages and to amplify the signal strength while mitigating the effect of heat and electromagnetic noise. The sensor signals are sent to the Atmel analogue-to-digital converters ranging from 0 to 3.3 V (ADC_1 and ADC_0) through the thermocouple amplifier. Subsequently, we highlight methods to use the AdaFruit amplifier, since it plays the main role in enhancing OCARI open-source architecture for a wireless data acquisition system. The component provides both amplification of original millivoltage from transducers and a voltage compensation of the cold junction, thereby ensuring that 0 coincides with the ambient temperature or a null heat flux. In order to understand how the amplifier behaves, we modelled its answer by injecting a pure sinusoidal 0-40 mV voltage in input. The input-output voltage and the corresponding temperature are shown in Figure 10. Based on datasheet from which the model is derived, if the amplifier is used as recommended, the range 0-40 mV is amplified into a 1.25-6.3 V interval, which corresponds to a temperature range that spreads from the ambient one (if considered between 0 and 50 \({}^{\circ}\)C) to 1000 \({}^{\circ}\)C. Hence, the default set-up of the AdaFruit amplifier can introduce a cut-off of large temperature fluctuations that are representative of the field scale conditions [22]. With respect to covering the full range offered by the component, we decided to switch the Vref voltage off (1.25 V in the model), and this set the temperature compensation of the cold junction. In the aforementioned conditions, the amplifier returns output voltages in the range of 0-5.5 V. Subsequently, the resulting output is adapted to the 0-3.3 V range of the OCARI module with a voltage divider bridge. A precalibration procedure of each sensor-node couple leads us to match the input voltage of 0-40 mV with a 0-4096 digit range (in differential input configuration) to ensure that the transducer spreads over the full range of temperature (i.e., up to 1360 \({}^{\circ}\)C at 40 mV). The cold junction temperature is considered equal to the average between the one inner to the Campbell Scientific and the local ambient temperature as measured by an external thermometer. This choice makes us replace the local cold junction temperature of each TC provided by each AdaFruit amplifier by a close nonlocal reference temperature. However, this makes it possible to avoid the cut-off of temperature maxima in the surrounding area of the 1000 \({}^{\circ}\)C level. Specifically, with respect to both strategies (i.e., maintaining a 1.25 V reference voltage or not exhibiting different outlooks), the original cabling of the amplifier must be preferred when a large size WSN is used to detect a local temperature threshold between 0-1000 \({}^{\circ}\)C, because it guarantees that discrepancies between cold junction temperatures do not exist between a large number of nodes.