State-of-the-Art Smoke Opacity System for the Steiner Tunnel, 01-R8036

Principal Investigators
David Ewan
Joshua Terry

Inclusive Dates:  03/01/08 – 06/16/09

Background - In the U.S., the Steiner tunnel test is the primary test method for evaluating the reaction to fire of interior finish materials. The method is described in ASTM E 84, Standard Test Method for Surface Burning Characteristics of Burning Materials. The apparatus consists of a tunnel-like enclosure measuring 8.7 by 0.45 by 0.31 m (25 by 1.5 by 1 ft). The test specimen, 7.6 m (24 ft) long and 0.51 m (1.67 ft) wide, is mounted in the ceiling position and exposed at one end, designated as the burner end, to an 88-kW (5,000-Btu/min gross) gas burner. There is a forced draft through the tunnel from the burner end, with an average initial air velocity of 1.2 m/s (240 ft/min). The measurements consist of flame spread over the material surface and light obscuration in the exhaust duct of the tunnel. These are recorded for the 10-minute test duration.

U.S. model building code fire safety requirements for interior finish materials are based primarily on two indices measured in the Steiner tunnel test: flame spread index (FSI) and smoke-developed index (SDI). The FSI is calculated on the basis of the area under the curve of the flame tip location versus time. The FSI is 0 for an inert board, and is normalized to approximately 100 for red oak flooring. The SDI is equal to 100 times the ratio of the area under the curve of light obscuration versus time for the 10-min test duration to the area under the curve for red oak flooring. Thus, the SDI of red oak flooring is by definition, 100.

The codes do not permit the installation of interior finishes that have an SDI in excess of 450. The light obscuration is measured in the exhaust duct of the tunnel test apparatus with a smoke photometer, which consists of a white light source on one side of the duct and a photocell on the opposite side. The photometer is calibrated on a regular basis with a set of certified optical neutral density filters with light transmission values that span the range between 0 and 100 percent. During the most recent annual audit of SwRI's Fire Technology Department, Institute Quality Systems found a nonconformance with the light obscuration measurements in the Steiner tunnel. The photometer calibration was found to be slightly out of tolerance at the low end of the light transmission scale (10 percent and below). The problem was traced to the electronics, which date back to the late 1970s when the SwRI Steiner tunnel test apparatus was built. The issue was resolved by adding a numerical correction of the smoke photometer signal in the data acquisition software. This is a temporary fix as the correction is not in accordance with the standard. Moreover, the existing system may deteriorate further over time. In addition to problems with the electronics, the photometer is also very sensitive to slight changes in the alignment, as the light emitted by the lamp is not uniform and consists of a pattern of concentric rings of varying intensity.

Approach - The objective of this study was to improve the smoke opacity system for the Steiner tunnel. Upgraded components were used to develop a "study system" that would offer more accurate and consistent test data. Providing a basis for comparing the existing system to the study system are frequent optical filter calibrations; test data for client-supplied materials, materials obtained specifically for this study, and heptane pan fires; and standard Steiner tunnel calibrations.

Accomplishments - Through the course of the study, it was found that the newly developed system was capable of providing the results desired. The study system has proven to be more robust than the existing system. Data obtained with the final design is more consistent and accurate. Based on this study, the following has been proposed to the Tunnel Operators Task Group:

• Modify the ASTM E 84 standard to incorporate the option of using the study system.
   
• The new study system is in compliance with five of the six neutral density filters; however, six filters may be excessive. The task group should specify the number of filters required and the frequency in which they should be verified.
   
• As an alternative to requiring a ± 3 percent tolerance for neutral density filter calibrations, allow for the use of piecewise linear calibration curve established based on the values obtained during calibration.

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