# Study of the Influence of Heat Flow on the Time to Ignition of Spruce and Beech Wood
## 2 Materials and Methods
### Methods  
The experimental device was designed and labelled as an \"electric radiant panel\". The electric radiant panel (Figure 2a-c) was constructed for experimental tests aimed at determining the ignition of the material depending on the magnitude of the heat flux. The design process was preceded by a study of published research papers [53; 54; 55] that dealt with similar devices. The electric radiant panel was powered by the 400 V electrical network, and the panel's power was controlled using three protective fuses. Each of the three fuses controlled one phase with five electric coils. Fifteen coils were used to adjust the panel power to 5 kW, 10 kW, or 15 kW. The constructed device is depicted in Figure 2 as a schematic, and Figure 2b,c shows the real representation. For the pre-experimental measurement, a water-cooled heat flux sensor SCHMIDT--BOELTER SBG01--100 (Hukseflux Thermal Sensors B.V., Delft, The Netherlands) was used (Figure 3). The SBG01 sensor used is designed for measuring flame heat flux in the range of high heat flux levels of up to 200 kW\(\cdot\)m\({}^{-2}\). Figure 3: Water-cooled heat flux sensor SCHMIDT-BOELTER SBG01-100 [44]. Figure 2: Electric radiant panel. (**a**) Diagram of the electric radiant panel with description of individual parts: 1. stand; 2. radiant panel; 3. electrical fuses; 4. frame for sample attachment; 5. exhaust hood; (**b**) example of the electric radiant panel during measurement; (**c**) detail on the electric radiant panel. The SBG01 passively generates an output voltage proportional to the incoming heat flux using a thermoelectric sensor. Assuming the heat flux is constant, the thermal conductivity of the device is constant, and the sensor has a negligible effect on the heat flow, the SBG01 signal is proportional to the local heat flux (W\(\cdot\)m\({}^{-2}\)). Calibration of the sensor is required before measurement. Measurement accuracy depends on sensor properties, calibration quality, and measurement errors [56]. The sensor was calibrated against a reference sensor at NIST [57; 58]. The total uncertainty, according to ISO 14934-4:2014 [59], is estimated to be within \(\pm\)3%, relative to the standard uncertainty multiplied by the coefficient k = 2, providing a reliability level of 95%. To capture and monitor the temperature of the radiant panel, two thermocouples (Ni-Cr-Ni) were used, which were positioned directly on the panel, with their ends connected to a high-performance, multifunctional data acquisition module OMB-DAQ-2416 (Multifunctional Data Acquisition Module OMEGA OMB-DAQ-2416, Omega Engineering, Norwalk, CT, USA). This module was further connected to a computer, where data obtained from the analogue inputs of the thermocouples were stored and displayed using Tracer DAQ-2416 software (Omega Engineering, Norwalk, CT, USA). The accuracy of the measured values during the determination of the heat flux was ensured by using a calibration plate with dimensions identical to those of the tested samples (165 \(\times\) 165 \(\times\) 25) mm (Figure 1). In the centre of the calibration plate, a radiometer was inserted, which was mounted on a stand with the ability to adjust the desired distances (50, 70, 100, 150, and 200 mm) from the radiant panel (Figure 4). The heat flux sensor was connected to the multimeter METEX M-4640A (B&B Electronics Mfg. Co., Inc., Ottawa, IL, USA), which generated an output voltage proportional to the heat flux. The multimeter was interfaced with the DMM MULTIVIEW programme (B&B Electronics Mfg. Co., Inc., Ottawa, IL, USA). The original results were recorded as output voltage values in mV and subsequently converted to heat flux units in kW\(\cdot\)m\({}^{-2}\) (Figure 5). The test was conducted in a laboratory environment without airflow. The results confirmed the general validity of the idea that as the distance from the radiant heat source increases, the density of the measured heat flux decreases.