In Part 2 of the article, the smoke detection performance of the particle-charging method is demonstrated. All three commonly used materials, including ABS, tinned copper and PVC, are subjected to the pyrolysis test presenting highly reliable detection performance for various smoke particle characteristics.
Practical application pyrolysis test
At the pyrolysis stage of fire, the material could be heated to ignition temperatures, such as the overloading of cables and wire sockets. Overloading of electronic components generates a massive amount of heat, increases the temperature of the material and releases pyrolysis particles into the surrounding environment. Because the pyrolysis particles are significantly diverse in composition, characteristics and geometric patterns, it is a huge challenge for the smoke detector to reveal fire threats at the very early stage of fire.
To further investigate the performance of the particle ionisation/charging method at the pyrolysis stage of fire, a test device utilising the particle-charging method (PIM) was subjected to the pyrolysis sensitivity evaluation. For comparison, two other test devices based on laser-scattering technology and particle sizing instrument are included to provide measurement for the optical laser method (OLM) and the particle-counting method (PCM).
Test results
Figures 3, 4 and 5 illustrate the pyrolysis sensitivity evaluation results for three materials commonly used in power system design. They are polyvinyl chloride (PVC), ABS and tin-coated red copper plates (CPs).
PVC is widely used in power transmission applications and electronic circuit design. PVC has a unique pyrolysis property in which the particle concentration decreases against the temperature while the particle diameter increases rapidly after reaching the pyrolysis temperature. Figures 3a and 3b illustrate the sensitivity evaluation results using a PVC plate. By comparing the growing trend in Figures 3a and 3b, the pyrolysis temperature of the test material can be confirmed at approximately 98°C. At the same time, the mean particle diameter obtained by SMPS TSI 3910 is approximately 60nm. We can also observe a linearly growing trend for receiving electrodes A after reaching the pyrolysis temperature. This result suggests that the test device using the particle-charging method can clearly identify the boundary edge before entering the pyrolysis stage. In addition, PIM is found to be very sensitive to particle size variation as the voltage reading on electrode B increases rapidly with the growth of the mean particle diameter, while the smoke particle concentration remains unchanged throughout the entire test. For OLM, the sensitivity measurement shows signs of increase after reaching 135°C as the mean particle diameter increases above 200nm at the same time. However, the particle concentration slightly decreases against the temperature, and the sensitivity measurement of the PCM, referring to the particle concentration of TSI shown in Figure 5b, shows a weak sign of decrease.
Figure 4a shows the pyrolysis sensitivity test results of the ABS plate. The temperature at which the pyrolysis process was started is approximately 136°C. As the particle concentration grows exponentially, the readings on electrode A quickly reach the saturation region. Particle concentration contributes to the main growing incentive of PIM. Similarly, PCM is highly sensitive to ABS smoke particles due to the dramatic increase in particle concentration. However, the optical detector shows no detection sign until the mean particle diameter reaches 150nm.
Tinned red copper, commonly used in circuit system design and power transmission lines, was subjected to sensitivity evaluation as a comparison case to the polymer-based material. It generates a significant amount of nano smoke particles with very small particle diameters. The result is shown in Figures 5a and 5b. Considerable growth in the mean particle diameter and particle concentration at 155°C facilities the voltage rising on the receiving electrode A, confirming a clear sign of microparticle detection by PIM. However, OLM barely detects any signal of the particles until the total particle concentration reaches three times as much as ABS and four times as much as PVC.
Test apparatus
The pyrolysis test apparatus illustrated in Figure 6 consisted of a smoke chamber, a baking plate and a ventilation system. The test materials were cut into round plates (80mm in diameter and 5mm in thickness) and placed in the smoke chamber on the baking plate. The test was conducted for 1 hour, during which the test materials were baked slowly from room temperature to the pyrolysis temperature. That is, the temperature at which the material starts releasing an enormous number of particles or the size of the particle grows significantly.
Harmful gas containing nano smoke particles may be produced during the test; therefore, the smoke chamber is ventilated before and after the test to eliminate the hazards.
Performance evaluation
The particle counting method is susceptible to variation of particle concentration; thus, it obtains a very high detection performance for materials with pyrolysis properties similar to tin-coated copper. In comparison, the particle concentration of PVC smoke particles is insufficient to trigger the alarm threshold value of the PCM. In real-world applications, it could potentially cause a severe alarm time delay and neglection of the particular type of fire threats.
The tinned-copper sensitivity test exposes the limitation of the optical-laser method. Through the entire heating process, tinned copper releases enormous smoke particles with a diameter below 100nm. The particle size restricts the laser diffraction effect by reducing the intensity of scattered light; therefore, there will be a more significant alarm time delay than other detection methods. However, for the particle-charging method, both particle concentration and size donate critically important contributions to analysing and detecting the smoke particle. Therefore, the detection performance of PIM is outstanding for all three materials.
Conclusions
Particle charging and electromobility technology have been widely used in laboratory instruments for particle sizing and particle distribution. Nevertheless, practical application in the very early fire alarm industry is rare.
This work presents the standard PSL nano aerosols sensitivity evaluation of PIM for various particle diameters. The average charging efficiency is 40–50%.
A sensitivity evaluation of pyrolysis particles for various materials was also conducted in this work. PIM has advantages over other detection mechanisms, as it is sensitive to both particle concentration and particle size variation. Therefore, regardless of the material, PIM presents a high performance for sensing diverse pyrolysis particles in the incipient fire stage irrespective of the material’s type. Additional conclusions can be drawn from the pyrolysis test result that significant time-lead for sensing the pyrolysis particles can be achieved by PIM as well as compensating for the drawbacks of existing detectors based on different methods. PIM has been proven to have great potential in the very early fire warning industry.
Obscuration % obs/m refers to the fraction of light absorbed by smoke particles to the overall light intensity. This measurement is mainly used as a criterion in the smoke-detection industry to indicate the sensitivity of a smoke detector. However, the criterion is not adequate enough to reveal the detector’s sensitivity for smoke particles less than 150 nm (microparticles). It is necessary to introduce a new sensitivity criterion to precisely indicate the detection sensitivity for microparticles. PIM sensitivity measurement based on the number of charges on the surface of particles may be a possible solution for giving a new sensitivity criterion to early fire warning systems.
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