Eliminating the effects of refractive indices for both white smokes and black smokes in optical fire detector

Highlights

• Eliminating the effects of reflective indices for fire detection, i.e. the optical detector has the same sensitivities for white smokes and black smokes.

• The detector can measure the volume concentration and surface area concentration for both white and black smokes, where the maximal relative error is only 13.01% in the test of true fire smokes.

• The detector is simple-structure and low-cost, which can be widely applied in fire detection, sensing PM2.5 in environment monitoring and other application fields.

Abstract

Existing optical fire smoke detectors would take more time or even fail to trigger alarms of black smokes because the optical scattering efficiencies of black smokes are much lower than those of white smokes. In this paper, we design an optical fire smoke detector with consistent response to white smokes and black smokes based on dual-wavelength technology and multiple scattering signals. The proposed detector utilizes the incident lights of dual wavelengths to measure the surface area concentration and volume concentration of fire smokes respectively. The particle size distribution of the smokes can be estimated by these concentrations. The refractive indices are deduced by the third scattering signal and more observing angles are helpful to reduce the measurement errors. As a result, a 4-channel detector [140°–950 nm, 45°–950 nm, 140°–450 nm, 45°–450 nm] is developed, which can accurately measure the concentrations of both smoldering cotton (white smoke) and open fire of polyurethane (black smoke).

Introduction

Optical fire smoke detectors are widely used in household and industries fire detection due to their simplicity in production and low cost [1], [2]. Existing optical fire smoke detectors measure the power of scattering light as an indicator of smoke concentration and trigger fire alarms when the power of scattering light exceeds a threshold value [3]. However, the scattering efficiencies (scattering power per smoke concentration) of black smokes would be much lower than those of white smokes, because the black smokes absorb part of incident light, while the white smokes scatter most of incident light [3], [4]. Thus, existing optical fire smoke detector will underestimate the concentration values of black smokes and may take more time or even fail to trigger the alarms. Therefore, it is an emergency to develop an optical fire detector with consistent responses to both white smokes and black smokes.

Efforts were made to balance the response of detector to white smokes and black smokes. Comparing with the forward scattering (the observing angle from emitter to receiver larger than 90°), the backward (the observing angle less than 90°) is helpful to mitigate the difference of responses to white and black smokes in a certain degree [5], [6]. However, it cannot solve the problem of lower scattering efficiency caused by black smokes essentially and the difference between the responses of white smokes and black smokes still exist. Some works reported that optical smoke detectors with empirical multiple observing angles or/and multiple incident light wavelengths can distinguish a few different types of smokes by pattern recognition, but the reasons to select wavelengths and observing angles are not explained [7], [8], [9], [10], [11], [12]. According to Mie theory, the intensity of the light scattered by smoke depends on the wavelength of incident light, the observing angle, the refractive index and the particle size distribution. These works do not derive the refractive indices and particle size distributions of fire smokes from the scattering signals with multiple wavelengths and observing angles. Thus, they are not general design methods for fire detectors with consistent responses to both white smokes and black smokes.

In this paper, we propose a general design method to develop optical fire smoke detectors with dual wavelengths and multiple observing angles to measure the surface area concentration and volume concentration for both white smokes and black smokes. By applying dual wavelength technology [13], the surface area concentrations and volume concentrations of fire smokes can be obtained, and the particle size distributions are estimated. After that, the third observing angle is utilized to confirm the refractive indices of the fire smokes, the measurement errors would be reduced by the scattering signals from more observing angles. A 4-channel fire smoke detector with dual wavelengths and two observing angles is designed in consideration of performance and cost, which has simple optical structure and only a few optical components. And then the design scheme is verified by 5000 kinds of random generated fire smokes via simulation. A sample optical fire detector with the four channels is manufactured and used to measure the concentrations of the smoldering cotton (white smoke) and open fire of polyurethane (black smoke) with the maximal deviation only by 13.01%.

The following paper is organized into four sections. Section 2 introduces the general design method of fire smoke detector with dual wavelengths and multiple observing angles. Section 3 shows the simulation results for optimization in fire detections to both white smokes and black smokes. Section 4 shows our sample optical fire detector with four scattering signal channels and the experiment results for smoldering cotton (white smoke) and polyurethane open fire (black smoke). Section 5 concludes the whole paper.