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Measuring degrees of combustibility using an OSU apparatus and oxygen-depletion principle
Fire Safety Journal 17 (1991) 291-299
Measuring Degrees of Combustibility Using an OSU Apparatus and Oxygen-Depletion Principlet Yoshio Tsuchiya & J. F. Mathieu Building M-59, Institute for Research in Construction, National Research Council of Canada, Montreal Road, Ottawa, Canada K1A 0R6 (Received 5 January 1990; revised version received 9 April 1990; accepted 4 May 1990)
ABSTRACT Responding to a demand from the code writing committees for a test capable of quantitatively measuring low degrees of combustibility instead of the pass~fail of the existing non-combustibility test, the Institute for Research in Construction has developed a degrees-ofcombustibility test. A version of the Ohio State University (OSU) heat release rate (HRR) apparatus, with Federal Aviation Administration modifications, has been further modified by adding an oxygenmeasuring system and by reducing the airflow rate. HRR values of four sample materials have been measured, the peak rates of which ranged from 8 to 300kW/m 2. For measuring small values of HRR, an enclosed system of OSU apparatus is shown to have advantages over the open system of the cone calorimeter. In the OSU apparatus, by reducing airflow to combustion, the oxygen depletion can be increased to increase the sensitivity and accuracy of HRR measurement.
INTRODUCTION CAN4-S114, ' D e t e r m i n a t i o n of Non-Combustibility in Building Materials', is the Canadian standard test for measuring non-combustibility of building materials 1 and is similar to I S O 1182, 2 A S T M E136-823 and several o t h e r national standard tests (e.g. the British and J a p a n e s e t Paper presented at the International Fires in Buildings Conference, 25-26 September 1989, Toronto, Canada. 291 © 1991 Government of Canada
292
Yoshio Tsuchiya, J. F. Mathieu
tests. 4,5 These tests have practical importance in defining noncombustible materials for building codes and regulatory purposes. In these tests, a specimen in the form of a small block is heated in a vertical tube furnace that is maintained at 750°C. The temperature rise of the exhaust gases caused by the combustion of the combustible contents in the specimen is measured. In the Canadian test, the temperature rise must not exceed 36 °C for a product to be considered non-combustible. There is, however, a technical problem in evaluating the temperature rise, because the temperature of the exhaust gases is affected by the heat-sink effect of the specimen and the airflow pattern surrounding the specimen. 6 In Canada, there has also been a demand from the code writing committees for a test capable of quantitatively measuring low degrees of combustibility (i.e. developing a scale of combustibility) instead of the pass/fail acceptance criteria of the existing test. The Institute for Research in Construction has developed this degrees-of-combustibility test in response to that request.
TEST A P P A R A T U S A N D TEST P R O C E D U R E S For testing the heat release rate (HRR) of aircraft interior materials, 7 the Federal Aviation Administration (FAA) in the US mandates a version of the Ohio State University (OSU) H R R apparatus, which is a modified ASTM E906 apparatus. 8 The changes include a modified sample injection mechanism and precisely defined calibration and H R R measuring procedures. These give the higher accuracy needed for the small values of H R R that are required for aircraft interior materials. Since materials to be tested in the present degrees-of-combustibility test may also have small values of H R R , the F A A modifications were adopted in the present test. Further modifications included the use of the oxygen-depletion method established by Huggett 9 and a reduced airflow rate to the apparatus. In the degrees-of-combustibility test, specimens are 150 x 150mm with a given thickness (maximum 55 mm). The specimen is tested in the vertical position with the finished side facing the burner and exposed to a specified level of radiant heat flux (25-50 kW/m2). Materials which are subjected to the test do not normally melt or drip. In the F A A H R R test, there are two pilot burners, one at the bottom of the specimen and the other at the top. The lower burner, with a single flame, is to ignite the specimen, and the 14-flame-port upper burner is for combustion of the unburned fraction of pyrolysis products.
Measuring degrees of combustibility
293
In the present degrees-of-combustibility test, the lower burner is the same as the F A A design. The upper pilot burner, however, has been removed because its flames are unstable and cause erratic results when flame-retardant treated materials are tested. Airflow to the apparatus is split into two parts, one to the combustion chamber and the other to the mantle over the pyramidal section of the chamber, as shown in Fig. 1. The purpose of mantle airflow is to reduce heat loss from the apparatus. The air splitting is attained by the flow resistance of the air distributing plates, as in the original ASTM E906 design, s The airflow rate in this degrees-of-combustibility test is 1.2 m3/min, half the rate used in the ASTM test. The reduced flow increases oxygen depletion, resulting in an increased sensitivity for the H R R measurement.
~Gas samplingprobe
1oo Chamber
T
Air
/
flow
AirdistdbuUngplates
Air flow
>
Fig. 1.
Cross section of the apparatus.
294
Yoshio Tsuchiya, J. F. Mathieu
For the measurement of oxygen concentration, a three-hole L-shaped sampling probe is positioned 50 mm below the upper edge of the combustion chamber walls. This is located below the convergence point of the chamber airflow and the mantle airflow in order to assure that no mantle air is taken into the probe (Fig. 1). An oxygen monitor using the paramagnetic principle (which has a faster response time than the chemical cell-type monitor) is used for measuring oxygen concentration in the exhaust gases from the combustion chamber. The monitor is calibrated by atmospheric air and a cylindered-oxygen-nitrogen mixture. The apparatus is calibrated by a square wave heat input from burning a prescribed flow of methane. The methane gas flow rate is 1 litre/min for 2 min; it is then increased to 4 litre/min and kept at that rate for 2 min. The sequence is repeated twice for one calibration. The original calibration burner specified in the ASTM E906 standard s is used for this purpose. The calibration constant, in kW/(oxygen depletion %), is calculated. The kW value in the calibration constant is calculated as a product of the methane flow rate, at standard conditions, and the net heat of combustion of methane.
Experimental Combustion experiments were conducted using the apparatus and method described above. In addition, concentrations of CO were measured to determine the extent of incomplete combustion which reduces the value of H R R . A schematic diagram of the complete gas analysis system is shown in Fig. 2.
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250 ml/min
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Measuring degrees of combustibility
295
TABLE 1 Description of Sample Materials
Samples Glass-fibre insulation Gypsum board Untreated plywood Flame-retarded plywood
Thickness (ram)
Density (kg/m 2)
Details
50 13 6 6
2-23 9.43 3.21 3.51
No vapour barrier Type X Fir, 3-ply
a Untreated plywood was immersed in a 20% aqueous solution of dibasic ammonium phosphate for 24 h; after blotting dry, it was left in the conditioning room for 7 days; the resulting conditioned sample had 9-3% by weight of add-on salt.
The H R R and the accumulated heat release of four different materials (Table 1) were measured. Specimens for the test were conditioned at 22 °C under 50% relative humidity for more than 24 h. The H R R and accumulated heat release (HR) curves for each specimen are shown in Figs 3-6. They are summarized in Table 2 together with peak CO concentrations. The highest concentration of CO observed was 0.2% in tests with flame-retardant treated plywood. A method for correcting the reduced H R R caused by CO generation has been presented previously. 1° The reduction of H R R caused by this amount of CO was calculated as 3%. This is considered insignificant and thus no correction of H R R based on CO concentration was performed. too
Glass-fibre insulation go 80 7o I1" n"1-
so
o n-
5O
"1-
+o
3o m 2
20 lo o
-1
Fig. 3.
0
1
2
3
q
Time,
rain
5
8
7
B
g
Heat release rate (HRR) and accumulated heat release (HR) of glass-fibre insulation.
296
Yoshio Tsuchiya, J. F. Mathieu 10o
Gypsum board go
DO 7o n" n" "l-
0O
!
rr
"t-
HRkw'm I -I
0
I
2
3
4
5
§
7
8
9
Time, mifl Fig. 4°
Heat release rate ( H R R ) and accumulated release ( H R ) of gypsum board.
Table 2 shows that the test method can be applied to a range of materials from low degrees of combustibility, such as glass-fibre insulation, to normal degrees of combustibility, such as generally accepted combustible material as plywood. The oxygen monitor used in this study had a range option of 20-21%, 16-21% and 11-21%. The second range was used for most materials. In testing the untreated plywood sample, however, the oxygen concentration dropped below 16% and the last range was used. 600
Flame-retarded plywood
rr
4O0
n" "1" n'"r-
30o1 '0o t ;
j o
...........
-I
, . . . . . . . . . . .
0
,kw,m2 i . . . . . . . . . . .
1
, . . . . . . . . . . .
2
, . . . . . . . . . . .
3
, . . . . . . . . . . .
4
=. . . . . . . . . . .
S
, . . . . . . . . . . .
5
, . . . . . . . . . . .
?
i . . . . . . . . . . .
8
Time, rain Fig, S.
Heat release rate (HRR) and accumulated heat release (HR) of flame-retarded
plywood.
Measuring degrees of combustibility
297
§t]0
Untreated plywood 5r~
40O
n" IT"
..r 21111
11111
o -1
0
1
2
3
4
S
8
?
8
Time, mln Fig,
6,
Heat release rate ( H R R ) and accumulated heat release (HR) of untreated plywood.
DISCUSSION A comparative study of the thermal method and oxygen-depletion method using the F A A test apparatus" showed that the latter had 40% better reproducibility. The oxygen-depletion method was also shown to be superior to the thermal method in its faster response, smaller baseline value and less dependency to flame emissivity. For those reasons the oxygen-depletion method has been adopted. TABLE 2 Heat Release Rate ( H R R ) and Accumulated Heat Release (HR) Results
Glass-fibre insulation Gypsum board Flame-retarded plywood Untreated plywood
Peak H RR a (k W /m 2)
Accumulated HR b (k W-min /m 2)
Peak CO concentration (%)
7.8 37-0 180.2 295-2
31.8 40.8 285.9 529-4
0.02 0-06 0.20 0-19
o Maximum rate of heat release during the 10-min test period. b Accumulated H R at the end of the first 5 rain. (This paper follows the practice used in the F A A test of expressing accumulated H R in kW-min) Incident heat flux density was 40 k W / m 2. Data are based on single measurement per material. Reproducibility was determined in the separate study on 11 materials; averaged relative standard deviations were 6.4% for the peak H R R and 5.3% for the accumulated HR.
298
Yoshio Tsuchiya, J. F. Mathieu TABLE 3
OSU Test versus Cone Calorimeter Airflow
Cone calorimeter OSU (this paper) OSU (alternative)
(m 3/min)
Specimen surface (m 2)
A /S (m/rain)
Uncertainty of measurement a (kW/m 2)
1-440 1-200 0-400
0-010 0 0.022 5 0.022 5
144 53 18
4.5 1.6 0.6
a When an oxygen analyser of 0-01% accuracyis used. Both the present method and the proposed ASTM cone calorimeter 12 test method use oxygen depletion as the basis for measuring HRR. In the cone calorimeter, a specimen burns in an open space and the air supply to the combustion is uncontrolled. However, in the OSU apparatus, in which the specimen burns in an enclosure, the supply of air can be easily controlled. For measuring small values of H R R , a controlled and reduced airflow is essential to attain significant oxygen depletion. When a material has a small value of H R R , the oxygen depletion is small; it also depends on an apparatus factor, the airflow/specimen surface area ratio (A/S ratio). The larger the A/S ratio, the smaller the oxygen depletion. When the depletion is small, the accuracy of oxygen measurement determines the accuracy of H R R measurement. The accuracy of an oxygen analyser is 1% of full scale, that is 0.01% oxygen concentration, if the analyser is provided with a 20-21% oxygen range option. The required H R R to result in 0.01% oxygen depletion can be considered to be the uncertainty of the measurement. The uncertainty is shown in Table 3 for the cone calorimeter, the present test method and an alternative airflow rate of 0-4m3/min in using the OSU apparatus. The alternative flow rate was originally planned to be used for testing 'non-combustible' materials. The present method and the alternative are shown in Table 3 to give lower levels of uncertainty than the cone calorimeter. CONCLUSION A degrees-of-combustibility test for building materials has been developed. The test is based on the OSU H R R test modified as per the F A A H R R test modifications. It also includes an oxygen-measuring system and a reduced combustion airflow. In testing four sample materials, the method was capable of measuring H R R in a range of
Measuring degrees of combustibility
299
materials from low degree of combustibility to normal degree of combustibility. The OSU method, using oxygen depletion and reduced combustion airflow, was shown to have advantages over the cone calorimeter method when low degrees of H R R were measured.
REFERENCES 1. National Standards of Canada, Standard method of test for determination of non-combustibility in building materials. CAN4-Sll4-M80, Underwriters' Laboratories of Canada, Toronto, 1980. 2. International Organization for Standardization, Fire tests---building materials---non-combustibility test. ISO 1182-1983, ISO, Geneva, Switzerland, 1983. 3. American Society for Testing and Materials, Behavior of materials in a vertical tube furnace at 750°C. 1988 Annual Book of ASTM Standards, ASTM E136-82, Vol. 04.07, Philadelphia, 1990, p. 312. 4. British Standards Institution, Method for assessing the heat emission from building materials. BS476 : Part 11 : 1982, London, 1982. 5. Japanese Standards Association, Testing method for incombustibility of internal finish materials and procedure of buildings. JIS A1321-1975, Tokyo, 1975. 6. Ahonen, A., Ojala, A., Weckman, H. & Yli-Penttila, M., Application of oxygen consumption calorimetry to non-combustibility testing. Report No. 291, Technical Research Centre of Finland, Espoo, 1984. 7. Federal Aviation Administration, 14 CFR Parts 25 and 121, Improved flammability standards for materials used in the interiors of transport category airplane cabins. Federal Register, 51, No. 139, 21 July, 1986; Federal Register, 52, No. 34, 20 Feb., 1987; Federal Register, 53, No. 165, 25 August, 1988. 8. American Society for Testing and Materials, Heat and visible smoke release rates for materials and products. Annual Book of ASTM Standards, ASTM E906-83, Vol. 04.07, Philadelphia, 1990, p. 707. 9. Huggett, C., Estimation of rate of heat release by means of oxygen consumption measurements. Fire and Materials, 4 (1980) 61. 10. Tsuchiya, Y., Methods of determining heat release rate: state-of-the-art. Fire Safety J., 5 (1982) 49. 11. Tsuchiya, Y., Heat release rate measurement for evaluating the flammability of aircraft materials, from Symposium on Aircraft Fire Safety, Advisory Group for Aerospace Research and Development, North Atlantic Treaty Organization, May 21-6, 1989, Lisbon, Proceedings No. 467, October 1989, p. 32. 12. American Society for Testing and Materials, Heat and visible smoke release rates for materials and products using an oxygen consumption calorimeter. Annual Book of ASTM Standards, ASTM E1354-90, Vol. 04.07, Philadelphia, 1990, p. 999.
Measuring Degrees of Combustibility Using an OSU Apparatus and Oxygen-Depletion Principlet Yoshio Tsuchiya & J. F. Mathieu Building M-59, Institute for Research in Construction, National Research Council of Canada, Montreal Road, Ottawa, Canada K1A 0R6 (Received 5 January 1990; revised version received 9 April 1990; accepted 4 May 1990)
ABSTRACT Responding to a demand from the code writing committees for a test capable of quantitatively measuring low degrees of combustibility instead of the pass~fail of the existing non-combustibility test, the Institute for Research in Construction has developed a degrees-ofcombustibility test. A version of the Ohio State University (OSU) heat release rate (HRR) apparatus, with Federal Aviation Administration modifications, has been further modified by adding an oxygenmeasuring system and by reducing the airflow rate. HRR values of four sample materials have been measured, the peak rates of which ranged from 8 to 300kW/m 2. For measuring small values of HRR, an enclosed system of OSU apparatus is shown to have advantages over the open system of the cone calorimeter. In the OSU apparatus, by reducing airflow to combustion, the oxygen depletion can be increased to increase the sensitivity and accuracy of HRR measurement.
INTRODUCTION CAN4-S114, ' D e t e r m i n a t i o n of Non-Combustibility in Building Materials', is the Canadian standard test for measuring non-combustibility of building materials 1 and is similar to I S O 1182, 2 A S T M E136-823 and several o t h e r national standard tests (e.g. the British and J a p a n e s e t Paper presented at the International Fires in Buildings Conference, 25-26 September 1989, Toronto, Canada. 291 © 1991 Government of Canada
292
Yoshio Tsuchiya, J. F. Mathieu
tests. 4,5 These tests have practical importance in defining noncombustible materials for building codes and regulatory purposes. In these tests, a specimen in the form of a small block is heated in a vertical tube furnace that is maintained at 750°C. The temperature rise of the exhaust gases caused by the combustion of the combustible contents in the specimen is measured. In the Canadian test, the temperature rise must not exceed 36 °C for a product to be considered non-combustible. There is, however, a technical problem in evaluating the temperature rise, because the temperature of the exhaust gases is affected by the heat-sink effect of the specimen and the airflow pattern surrounding the specimen. 6 In Canada, there has also been a demand from the code writing committees for a test capable of quantitatively measuring low degrees of combustibility (i.e. developing a scale of combustibility) instead of the pass/fail acceptance criteria of the existing test. The Institute for Research in Construction has developed this degrees-of-combustibility test in response to that request.
TEST A P P A R A T U S A N D TEST P R O C E D U R E S For testing the heat release rate (HRR) of aircraft interior materials, 7 the Federal Aviation Administration (FAA) in the US mandates a version of the Ohio State University (OSU) H R R apparatus, which is a modified ASTM E906 apparatus. 8 The changes include a modified sample injection mechanism and precisely defined calibration and H R R measuring procedures. These give the higher accuracy needed for the small values of H R R that are required for aircraft interior materials. Since materials to be tested in the present degrees-of-combustibility test may also have small values of H R R , the F A A modifications were adopted in the present test. Further modifications included the use of the oxygen-depletion method established by Huggett 9 and a reduced airflow rate to the apparatus. In the degrees-of-combustibility test, specimens are 150 x 150mm with a given thickness (maximum 55 mm). The specimen is tested in the vertical position with the finished side facing the burner and exposed to a specified level of radiant heat flux (25-50 kW/m2). Materials which are subjected to the test do not normally melt or drip. In the F A A H R R test, there are two pilot burners, one at the bottom of the specimen and the other at the top. The lower burner, with a single flame, is to ignite the specimen, and the 14-flame-port upper burner is for combustion of the unburned fraction of pyrolysis products.
Measuring degrees of combustibility
293
In the present degrees-of-combustibility test, the lower burner is the same as the F A A design. The upper pilot burner, however, has been removed because its flames are unstable and cause erratic results when flame-retardant treated materials are tested. Airflow to the apparatus is split into two parts, one to the combustion chamber and the other to the mantle over the pyramidal section of the chamber, as shown in Fig. 1. The purpose of mantle airflow is to reduce heat loss from the apparatus. The air splitting is attained by the flow resistance of the air distributing plates, as in the original ASTM E906 design, s The airflow rate in this degrees-of-combustibility test is 1.2 m3/min, half the rate used in the ASTM test. The reduced flow increases oxygen depletion, resulting in an increased sensitivity for the H R R measurement.
~Gas samplingprobe
1oo Chamber
T
Air
/
flow
AirdistdbuUngplates
Air flow
>
Fig. 1.
Cross section of the apparatus.
294
Yoshio Tsuchiya, J. F. Mathieu
For the measurement of oxygen concentration, a three-hole L-shaped sampling probe is positioned 50 mm below the upper edge of the combustion chamber walls. This is located below the convergence point of the chamber airflow and the mantle airflow in order to assure that no mantle air is taken into the probe (Fig. 1). An oxygen monitor using the paramagnetic principle (which has a faster response time than the chemical cell-type monitor) is used for measuring oxygen concentration in the exhaust gases from the combustion chamber. The monitor is calibrated by atmospheric air and a cylindered-oxygen-nitrogen mixture. The apparatus is calibrated by a square wave heat input from burning a prescribed flow of methane. The methane gas flow rate is 1 litre/min for 2 min; it is then increased to 4 litre/min and kept at that rate for 2 min. The sequence is repeated twice for one calibration. The original calibration burner specified in the ASTM E906 standard s is used for this purpose. The calibration constant, in kW/(oxygen depletion %), is calculated. The kW value in the calibration constant is calculated as a product of the methane flow rate, at standard conditions, and the net heat of combustion of methane.
Experimental Combustion experiments were conducted using the apparatus and method described above. In addition, concentrations of CO were measured to determine the extent of incomplete combustion which reduces the value of H R R . A schematic diagram of the complete gas analysis system is shown in Fig. 2.
~i I
GLASS WOOL-..~ TO REMOVE \ ~ PARTICULATES
r-. GAS SAMPLING
I
PROBE
I
Z
250 ml/min
4blfn
I COMPACT INLINE FILTER
BACKPRESSURE REGULATOR
PUMP
3-WAY VALVE
l~.Z. Gas analysis system.
® PURGE VALVE
>
Measuring degrees of combustibility
295
TABLE 1 Description of Sample Materials
Samples Glass-fibre insulation Gypsum board Untreated plywood Flame-retarded plywood
Thickness (ram)
Density (kg/m 2)
Details
50 13 6 6
2-23 9.43 3.21 3.51
No vapour barrier Type X Fir, 3-ply
a Untreated plywood was immersed in a 20% aqueous solution of dibasic ammonium phosphate for 24 h; after blotting dry, it was left in the conditioning room for 7 days; the resulting conditioned sample had 9-3% by weight of add-on salt.
The H R R and the accumulated heat release of four different materials (Table 1) were measured. Specimens for the test were conditioned at 22 °C under 50% relative humidity for more than 24 h. The H R R and accumulated heat release (HR) curves for each specimen are shown in Figs 3-6. They are summarized in Table 2 together with peak CO concentrations. The highest concentration of CO observed was 0.2% in tests with flame-retardant treated plywood. A method for correcting the reduced H R R caused by CO generation has been presented previously. 1° The reduction of H R R caused by this amount of CO was calculated as 3%. This is considered insignificant and thus no correction of H R R based on CO concentration was performed. too
Glass-fibre insulation go 80 7o I1" n"1-
so
o n-
5O
"1-
+o
3o m 2
20 lo o
-1
Fig. 3.
0
1
2
3
q
Time,
rain
5
8
7
B
g
Heat release rate (HRR) and accumulated heat release (HR) of glass-fibre insulation.
296
Yoshio Tsuchiya, J. F. Mathieu 10o
Gypsum board go
DO 7o n" n" "l-
0O
!
rr
"t-
HRkw'm I -I
0
I
2
3
4
5
§
7
8
9
Time, mifl Fig. 4°
Heat release rate ( H R R ) and accumulated release ( H R ) of gypsum board.
Table 2 shows that the test method can be applied to a range of materials from low degrees of combustibility, such as glass-fibre insulation, to normal degrees of combustibility, such as generally accepted combustible material as plywood. The oxygen monitor used in this study had a range option of 20-21%, 16-21% and 11-21%. The second range was used for most materials. In testing the untreated plywood sample, however, the oxygen concentration dropped below 16% and the last range was used. 600
Flame-retarded plywood
rr
4O0
n" "1" n'"r-
30o1 '0o t ;
j o
...........
-I
, . . . . . . . . . . .
0
,kw,m2 i . . . . . . . . . . .
1
, . . . . . . . . . . .
2
, . . . . . . . . . . .
3
, . . . . . . . . . . .
4
=. . . . . . . . . . .
S
, . . . . . . . . . . .
5
, . . . . . . . . . . .
?
i . . . . . . . . . . .
8
Time, rain Fig, S.
Heat release rate (HRR) and accumulated heat release (HR) of flame-retarded
plywood.
Measuring degrees of combustibility
297
§t]0
Untreated plywood 5r~
40O
n" IT"
..r 21111
11111
o -1
0
1
2
3
4
S
8
?
8
Time, mln Fig,
6,
Heat release rate ( H R R ) and accumulated heat release (HR) of untreated plywood.
DISCUSSION A comparative study of the thermal method and oxygen-depletion method using the F A A test apparatus" showed that the latter had 40% better reproducibility. The oxygen-depletion method was also shown to be superior to the thermal method in its faster response, smaller baseline value and less dependency to flame emissivity. For those reasons the oxygen-depletion method has been adopted. TABLE 2 Heat Release Rate ( H R R ) and Accumulated Heat Release (HR) Results
Glass-fibre insulation Gypsum board Flame-retarded plywood Untreated plywood
Peak H RR a (k W /m 2)
Accumulated HR b (k W-min /m 2)
Peak CO concentration (%)
7.8 37-0 180.2 295-2
31.8 40.8 285.9 529-4
0.02 0-06 0.20 0-19
o Maximum rate of heat release during the 10-min test period. b Accumulated H R at the end of the first 5 rain. (This paper follows the practice used in the F A A test of expressing accumulated H R in kW-min) Incident heat flux density was 40 k W / m 2. Data are based on single measurement per material. Reproducibility was determined in the separate study on 11 materials; averaged relative standard deviations were 6.4% for the peak H R R and 5.3% for the accumulated HR.
298
Yoshio Tsuchiya, J. F. Mathieu TABLE 3
OSU Test versus Cone Calorimeter Airflow
Cone calorimeter OSU (this paper) OSU (alternative)
(m 3/min)
Specimen surface (m 2)
A /S (m/rain)
Uncertainty of measurement a (kW/m 2)
1-440 1-200 0-400
0-010 0 0.022 5 0.022 5
144 53 18
4.5 1.6 0.6
a When an oxygen analyser of 0-01% accuracyis used. Both the present method and the proposed ASTM cone calorimeter 12 test method use oxygen depletion as the basis for measuring HRR. In the cone calorimeter, a specimen burns in an open space and the air supply to the combustion is uncontrolled. However, in the OSU apparatus, in which the specimen burns in an enclosure, the supply of air can be easily controlled. For measuring small values of H R R , a controlled and reduced airflow is essential to attain significant oxygen depletion. When a material has a small value of H R R , the oxygen depletion is small; it also depends on an apparatus factor, the airflow/specimen surface area ratio (A/S ratio). The larger the A/S ratio, the smaller the oxygen depletion. When the depletion is small, the accuracy of oxygen measurement determines the accuracy of H R R measurement. The accuracy of an oxygen analyser is 1% of full scale, that is 0.01% oxygen concentration, if the analyser is provided with a 20-21% oxygen range option. The required H R R to result in 0.01% oxygen depletion can be considered to be the uncertainty of the measurement. The uncertainty is shown in Table 3 for the cone calorimeter, the present test method and an alternative airflow rate of 0-4m3/min in using the OSU apparatus. The alternative flow rate was originally planned to be used for testing 'non-combustible' materials. The present method and the alternative are shown in Table 3 to give lower levels of uncertainty than the cone calorimeter. CONCLUSION A degrees-of-combustibility test for building materials has been developed. The test is based on the OSU H R R test modified as per the F A A H R R test modifications. It also includes an oxygen-measuring system and a reduced combustion airflow. In testing four sample materials, the method was capable of measuring H R R in a range of
Measuring degrees of combustibility
299
materials from low degree of combustibility to normal degree of combustibility. The OSU method, using oxygen depletion and reduced combustion airflow, was shown to have advantages over the cone calorimeter method when low degrees of H R R were measured.
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