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The Design and Construction of a Flame Conductivity Device to Measure Flame Penetration through Floor Systems

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The Design and Construction of a Flame Conductivity Device to

Measure Flame Penetration through Floor Systems

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The Design and Construction of a Flame

Conductivity Device to Measure Flame

Penetration through Floor Systems

IRC Research Report

#223

Date of Issue: June 2006

Author: George Crampton

Published by

Institute for Research in Construction National Research Council Canada Ottawa, Canada

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The Design and Construction of a Flame Conductivity Device to Measure

Flame Penetration through Floor Systems

George Crampton Abstract

The purpose of this apparatus is to measure the linear flame penetration along the length of seams in a floor assembly. This is used to indicate when the floor system has failed and to determine the time of floor failure in the fire tests conducted in the Fire Performance of Houses test facility. The principal of operation is the electrical

conductivity present in open flames due to the ionization of the fire gases. A conductor wire is stapled to the joint in the floor seam and a porous metal screen is positioned over the conductor wire. When flame penetrates through the floor seam electron current flows between the two conductors and the amount of current flow is directly proportional to the length of flame passing through the seam.

Introduction

Three flame conductivity devices were constructed and installed on the test floor over the 3 centre seams in the floor. The test house contains three levels, a basement, first, and second storey. The floor of the first storey was the test floor and fire was in the basement. The conductivity wires were installed along the 3 seams running across the floor joist directly over the fire area of the room below. The joints or seams were spaced 1.22 metres apart based on typical floor construction.

Circuit Design

The circuit design was a simple series resistor chain incorporating the ion field between the 2 conductors as one resistor and a fixed resistor of 2 megohms as the other with 9 volts DC applied across them. A 0.1 microfarad capacitor was connected across the fixed resistor to average the fluctuations and provide a smooth signal DC signal for the data collection system. The voltage drop across the 2 meg resistor is buffered by a CMOS operational amplifier to reduce noise and strengthen the signal for the data collection system. This design provides an output voltage of approximately 40 millivolts per centimeter of flame penetrating through the seam.

The circuit diagram is shown in Figure 1. Nichrome wire ( 18 gauge ) was stapled along the crack in each of the three floor seams. A 5 cm high cover cage conductor made from galvanized steel lathe, 14 cm wide folded in half to an angle of 90 degrees, was positioned over the conductor wire. This was held above the floor with insulating glass plates, 3 cm wide x 12 cm long x 6 mm thick, to make sure no current flow was present unless there was flame. Teflon insulated wires connected the 2 conductors to the electronic circuit outside the test facility. As the flame penetrates the floor seams and passes over the wire and through the porous cage the circuit begins to conduct current proportional to the length of flame penetrating the seam. The flame must be at least 5 cm high in order to contact the upper cage.

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2 megohm 0.1 microfarad capacitor 9 volt battery +

-Figure 1: Schematic of Flame Conductivity Device +

-CMOS op amp Buffer

Output to Data System Nichrome wire in floor seam

5 cm high steel mesh cover cage conductor

3 cm x 12 cm x 6 mm thick glass insulator plates

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Calibration

The flame conductivity devices were calibrated using a bench scale box with a plywood top containing a seam. A raw propane flame was directed at the underside of the seam and was allowed to burn through. The length of the flame was compared to the output voltage from the device resulting in a 40 millivolt per cm of flame calibration.

Installation Setup

The installation setup is shown in the two photos in figure 2. The floor assembly in the photos is weight loaded with concrete blocks and the floor seams must be

uncovered so that no part of the conductivity device’s cage touches anything except the glass insulators. Note that the cage is caulked to the glass insulators with silicon caulking and the insulator is then caulked to the floor. There can be no bridges that allow the caulking to contact both the cage and the floor. They must be separated by glass. This caulking arrangement stabilizes the device so that floor deflection will not move the assembly. The electronic portion of the device is located outside the test facility and is shown in figure 3.

East Seam

Top View

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Figure 3. Electronic signal conditioner for the three Flame

Conductivity Devices with the cover plate removed.

Results

Flame penetration through the floor can be one of the floor failure modes. Figure 4 shows the results from a test. The centre joint was located directly over the fire and failed first showing about 0.5 metres of rapid flame growth along the seam before the joint closed. This reduction of flame along the centre seam could be char build-up or downward floor deflection pinching the joint closed. There is a very fast flame presence on all seems starting with the east joint at 470 seconds followed by the centre and west seams at 485 seconds. This occurred 5 to 10 seconds before the floor collapsed as observed by cameras and temperature data.

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0 50 100 150 200 250 Limitation

This device will measure the flame penetration length until the operational amplifier reaches its supply voltage saturation. This is approximately 8.5 volts which represents about 1.7 metres of flame. If it is desired to measure longer lengths either the 2 megohm resistor must be lowered or the supply voltage increased proportionally.

Summary

This report describes the design and function of three flame conductivity devices used to measure the linear flame penetration along the length of seams in a floor

assembly to indicate when the floor system has failed. This instrumentation provided important information relating the amount and timing of flame penetration through floors in the test house. These devices will provide useful comparison data between a variety of different floor systems presently being tested in the Fire Performance of Housing Project.

Figure N. 31. Flame Sensors

Time (s)

East Joint Centre Joint West Joint

0 100 200 300 400 500 600

Figure 4. Graph showing flame penetration along 3 floor seams.

Flame Length

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Acknowledgements

The author would like to thank staff of the Fire Research Lab for their assistance in this project.

References

1. Sears, Zemansky, and Young, “College Physics, fourth edition”, Addison-Wesley Publishing inc.,1974.

Figure

Figure 1:  Schematic of Flame Conductivity Device+
Figure 2.  Photos showing installation of Flame Conductivity Device
Figure 3.  Electronic signal conditioner for the three Flame  Conductivity Devices with the cover plate removed
Figure N. 31.   Flame Sensors

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