Elsevier

Fire Safety Journal

Volume 41, Issue 6, September 2006, Pages 459-475
Fire Safety Journal

Round robin study of total heat flux gauge calibration at fire laboratories

https://doi.org/10.1016/j.firesaf.2006.04.004Get rights and content

Abstract

Total heat flux gauges are widely employed in fire research and fire testing laboratories. Several fire laboratories have developed systems for calibrating these gauges. There are major differences between these calibration facilities, and prior to this work there had been no systematic check of the degree of agreement between calibrations from the different laboratories. This report describes the results of two round robins in which five fire laboratories performed independent calibrations of two sets of Gardon and Schmidt-Boelter total heat flux gauges (i.e., a total of four gauges). One set of the gauges was also calibrated by two additional laboratories whose primary focus is measurement and calibration. Even though systematic variations between calibrations from the different laboratories were identified, the degree of agreement is viewed as being satisfactory for most fire testing and research purposes. Additional experimental findings concerning gauge angular response and linearity are presented. The need to develop a better understanding of the relative response of this type of gauge to radiative and convective heat transfer is emphasized.

Introduction

Many types of experimental measurements are important for fire science and engineering. In recent years there has been an increased emphasis on improving the quality of these measurements and the need to quantify their uncertainties. In 2000 the Building and Fire Research Laboratory (BFRL) at the National Institute of Standards and Technology (NIST) hosted a workshop on this topic entitled “Measurement Needs for Fire Safety: An International Workshop” at the request of the FORUM for International Cooperation in Fire Research [1]. One of the recommendations generated by the workshop participants was that a collaborative effort among the member laboratories should focus on characterizing the uncertainties associated with heat flux measurement in fire environments.

As a result of this meeting the FORUM Heat Flux Measurement Working Group was formed in July, 2000. During initial discussions it was discovered that a variety of different approaches were being employed in the laboratories to calibrate heat flux gauges. Schmidt-Boelter [2] and Gardon gauges [3] were typically used for total heat flux measurements. To the knowledge of the participants, the various calibration approaches had not been directly compared previously, and it was decided to conduct a calibration round robin in order to assess the consistency of the different approaches. Ultimately two independent round robins using Schmidt-Boelter and Gardon gauges were completed with five fire laboratories participating. Additionally, the four gauges were calibrated by the manufacturer, and the two gauges used during the first round robin were calibrated by two additional calibration laboratories, LNE in France and the Physics Laboratory (PL) at NIST.

The need for improved calibration facilities and procedures has been generally recognized in a variety of engineering fields requiring accurate heat flux measurement. A workshop held at NIST in 1999 summarized the needs of several industries, along with those of fire science [4]. The workshop proceedings include a summary of an effort within the International Standards Organization aimed at standardizing heat flux gauge calibrations for use in fire science and engineering studies. Much of the work supporting the development of this new standard has been performed as part of a European cooperative project known as HFCAL. Even though some laboratories have participated in both the HFCAL study and the current round robin, the investigations have been performed independently.

Section snippets

Round robin design

The fire laboratories participating in the round robin were BRE/FRS, FM Global, BFRL, SINTEF, and SP. The heat flux gauge calibration facilities utilized by the participating laboratories differ substantially from one another. An approach was adopted in which each fire laboratory carried out calibrations using their existing method following standard internal procedures.

The pattern (nomenclature utilized here is taken from the Guide for Interlaboratory Comparisons [5]) was the “basic circular”

BRE/FRS

The calibration of heat flux gauges for use as working standards is carried out by comparing their response at various levels of irradiance with the response of a secondary standard gauge at the same levels of irradiance. The measurements are made at multiple heat flux levels by varying the distance between the radiant source and the gauges.

The radiant source is a 0.3 m×0.3 m porous refractory burner operating on pre-mixed natural gas with air. It is mounted vertically and is operated in the

First round robin

Each fire laboratory reported the results for the calibrations of the two heat flux gauges in terms of the coefficients determined from linear least squares curve fits of the data when plotted as kW/m2 versus the response of the gauge in mV. In some cases the y-intercept was forced to pass through the plot origin, while in others it was allowed to “float,” and a y-intercept value from the fit is reported. Normal laboratory protocols were followed.

There are many different approaches that might

Discussion

Given the significant differences between the calibration methods used in the various fire laboratories, the agreement for the calibrations, as reflected by the variations from the average values for the nominal full-scale readings of the four gauges, is viewed as good. However, there are systematic variations in calibration results. SINTEF reported the lowest sensitivity for the two gauges tested during the first round robin, while BRE/FRS reported the highest. When the results from these two

Summary

The results of the two round robins of heat flux gauge calibrations have demonstrated that the agreement between the five fire laboratories is satisfactory for most fire research purposes and standard fire testing. Absolute values and variations in results between these laboratories are comparable to those reported by measurement laboratories. Systematic variations observed in the results suggest it should be possible to further improve agreement between calibration results from the fire

Recommendations

The role of convective heat transfer in the response of gauges sensitive to both radiation and convection is not very well understood and needs to be elucidated, both for calibration purposes and for their use in fire tests and other applications. The absorptivity, which is a property of the surface coating, also requires additional study because it affects the relative amounts of radiative and convective heat transfer to the gauge surface.

When the calibration is performed as a transfer

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