ASTM F1868-23

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Designation: F1868 − 17 F1868 − 23
Standard Test Method for
Thermal and Evaporative Resistance Resistance,
Evaporative Resistance, and Total Heat Loss Measurements
of Clothing Materials Using a Sweating Hot Plate
This standard is issued under the fixed designation F1868; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Clothing is often made of materials that impede the flow of heat and moisture from the skin to the
environment. Consequently, people may suffer from heat stress or cold stress when wearing clothing
in different environmental conditions. Therefore, it is important to quantify the thermal resistance and
evaporative resistance resistance, evaporative resistance, and total heat loss of clothing materials and
to consider these properties when selecting materials for different clothing applications.
1. Scope
1.1 This test method covers the measurement of the thermal resistance and the evaporative resistance, under steady-state
conditions,resistance, evaporative resistance, and total heat loss under steady-state conditions of fabrics, films, coatings, foams, and
leathers, including multi-layer assemblies, for use in clothing systems.
1.2 The range of this measurement technique for intrinsic thermal resistance is from 0.002 to 0.5 K·m /W and for intrinsic
2 2
evaporative resistance is from 0.0 to 1.0 kPa·m /W. The total heat loss range is from 0.0 to 1300 W/m .
1.3 The values in SI units shall be regarded as standard. No other Other units of measurement are included in this provided in this
standard but are not regarded as standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to consult and establish appropriate safety and healthsafety, health, and environmental practices and
determine the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
This test method is under the jurisdiction of ASTM Committee F23 on Personal Protective Clothing and Equipment and is the direct responsibility of Subcommittee
F23.60 on Human Factors.
Current edition approved June 1, 2017June 1, 2023. Published June 2017June 2023. Originally approved in 1998. Last previous edition approved in 20142017 as
F1868 – 14.F1868 – 17. DOI: 10.1520/F1868-17.10.1520/F1868-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1868 − 23
C177 Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the
Guarded-Hot-Plate Apparatus
D1518 Test Method for Thermal Resistance of Batting Systems Using a Hot Plate (Withdrawn 2023)
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
F1291 Test Method for Measuring the Thermal Insulation of Clothing Using a Heated Manikin
F1494 Terminology Relating to Protective Clothing
F2370 Test Method for Measuring the Evaporative Resistance of Clothing Using a Sweating Manikin
F3426 Test Method for Measuring the Thermal Insulation of Clothing Items Using Heated Manikin Body Forms
2.2 Other Standards:
ISO 11092 Textiles – Physiological Effects – Measurement of Thermal and Water-Vapour Resistance Under Steady-State
Conditions (Sweating Guarded-Hotplate Test)
3. Terminology
3.1 Definitions:
3.1.1 clo, n—a unit of thermal resistance (insulation) equal to 0.155 K·m /W.
3.1.1.1 Discussion—
The value of the clo was selected as roughly the insulation value of typical indoor clothing, which should keep a resting manperson
(producing heat at the rate of 58 W ⁄m ) comfortable in an environment at 21 °C, air movement 0.1 m/s. When clo was developed,
typical indoor clothing consisted of a three-piece suit and light underclothes.
3.1.2 evaporative resistance, n—Thethe resistance to the flow of moisture vapor from a saturated surface (high vapor pressure)
to an environment with a lower vapor pressure.
3.1.2.1 Discussion—
The evaporative resistance in units of kPa·m /W can be calculated for several different cases.
A
R = apparent totalintrinsic evaporative resistance of the fabric test specimen only, when evaluated non-isothermally. The term
ef
apparent is used as a modifier for totalintrinsic evaporative resistance to reflect the fact that the properties of the specimen may
be altered in the testing condition and that condensation may occur within the specimen.
A
R = apparent total evaporative resistance of the fabric test specimen, liquid barrier, and surface air layer when evaluated
et
non-isothermally. The term apparent is used as a modifier for total evaporative resistance to reflect the fact that the properties of
the specimen may be altered in the testing condition and that condensation may occur within the specimen.
R = evaporative resistance of the air layer on the surface of the liquid barrier without a fabric test specimen (that is, bare plate).
ebp
This property reflects the instrument constant and the resistance of the liquid barrier, and in conjunction with R , is used in the
et
calculation of R .
ef
R = intrinsic evaporative resistance of the fabric test specimen only. In the calculation of this value, the assumption is made that
ef
the boundary layers of the bare plate and the boundary layers of the fabric are equal.
R = total evaporative resistance of the fabric test specimen, the liquid barrier, and the surface air layer.
et
3.1.3 permeability index (i ),n—the efficiency of evaporative heat transport in a clothing system.
m
3.1.3.1 Discussion—
An i of zero indicates that the clothing system allows no evaporative heat transfer. An i of one indicates that the clothing system
m m
achieves the theoretical maximum evaporative heat transfer allowed by its insulation. insulation; however, a value of one is not
approached in practice. The permeability index is calculated one of two ways.
i = permeability index calculated using the total thermal resistance and the total evaporative resistance of a material. The U.S.
m
military uses this value in their databases on fabrics and clothing systems.
i = permeability index calculated using the intrinsic thermal resistance and the intrinsic evaporative resistance of a material.
mf
ISO 11092 uses this value.
3.1.4 thermal resistance, n—the resistance to the flow of heat from a heated surface to a cooler environment.
3.1.4.1 Discussion—
Thermal resistance in units of K·m /W can be calculated for several different cases.
I = total insulation value of the test specimen and the air layer, expressed in clo units.
t
I = intrinsic thermal resistance of the fabric test specimen only, expressed in clo units.
f
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Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
F1868 − 23
R = thermal resistance of the air layer on the surface of the plate without a fabric test specimen (that is, bare plate). This property
cbp
reflects the instrument constant and is used to standardize the plate, and in conjunction with R , is used in the calculation of R .
ct cf
R = intrinsic thermal resistance of the fabric test specimen only. In the calculation of this value, the assumption is made that the
cf
boundary layers of the bare plate and the boundary layers of the fabric test specimen are equal.
R = total thermal resistance of the test specimen and the air layer.
ct
3.1.5 total heat loss, n—the amount of heat transferred through a material or a composite by the combined dry and evaporative
heat exchanges under specified conditions expressed in watts per square metre.meter.
3.1.5.1 Discussion—
This single criterion for comparing fabric assemblies was developed as a special case by the National Fire Protection
Assoc.Association. The specific conditions used by NFPA are a simulate skin at 35 °C fully sweating hot plate surface in a 25 °C,
65 % RH environment.environment, with a 2 m/s wind flowing parallel to the surface of the skin.
3.2 For definitions of other terms related to protective clothing used in this test method, refer to Terminology F1494.
4. Significance and Use
4.1 The thermal resistance and evaporative resistance resistance, evaporative resistance, and total heat loss provided by fabrics,
films, coatings, foams, and leathers, including multi-layer assemblies, is of considerable importance in determining their suitability
for use in fabricating protective clothing systems.
4.1.1 The thermal resistance, evaporative resistance, and total heat loss can be significantly affected by environmental conditions.
Extreme care must be taken when using results measured under standard testing conditions to determine a material’s suitability
for use in conditions outside the testing conditions.
4.2 The thermal interchange between people and their environment is, however, is an extremely complicated subject that involves
many factors in addition to the steady-state resistance values of fabrics, films, coatings, foams, and leathers, including multi-layer
assemblies. Therefore, thermal resistance values and evaporative resistance values values, evaporative resistance values, and total
heat loss measured on a hot plate may or may not indicate relative merit of a particular material or system for a given clothing
application. While a possible indicator of clothing performance, measurements produced by the testing of fabrics have no proven
correlation to the performance of clothing systems worn by people. Clothing weight, drape, tightness of fit, and so forth, can
minimize or even neutralize the apparent differences between fabrics or fabric assemblies measured by this test method.
4.3 The thermal resistance and evaporative resistance of clothing systems and items can be measured with a heated sweating
manikin in an environmental chamber in accordance with Test Methods F1291, F2370and , and F2370F3426.
5. Interferences
5.1 Departures from the instructions of this test method may lead to significantly different testtesting results. Technical knowledge
concerning the theory of heat flow, temperature measurement, and testing practices is needed to evaluate which departures from
the instructions are significant. Standardization of the method reduces,reduces but does not eliminate the need for such technical
knowledge. Report any departures from the instructions of Test Method F1868 with the results.
6. Apparatus
6.1 Hot Plate—The guarded flathot plate shall be composed of a test plate, guard section, and bottom plate, each electrically
maintained at a constant temperature in the range of human skin temperature (33 to 36 °C). The guard section shall be designed
to prevent lateral loss of heat from the test plate. The guard section shall be wide enough to minimize heat loss and moisture
transport through the edges of the test specimen under the conditions of the test. The bottom plate shall prevent downward loss
of heat from the test plate and guard section. A system for feeding water to the surface of the test plate and guard section is also
needed for testing Parts B and C. See Test Methods D1518, C177, and ISO 11092 for additional information on hot plates.
6.2 Temperature Control—Separate, independent temperature control is required for the three sections of the hot plate (test plate,
guard section, and bottom plate). Temperature control may be is achieved by independent adjustments to the voltage or current,
or both, supplied to the heaters using solid-state power supplies, solid-state relays (proportional time on), adjustable transformers,
variable impedances, or intermittent heating cycles. The test plate, guard, and bottom plate sections shall be controlled to measure
the same temperature to within 60.1 °C of each other.
F1868 − 23
6.3 Power-Measuring Instruments—Power to the hot plate test section shall be measured to provide an accurate average over the
period of the test. If time proportioning or phase proportioning is used for the power control, then devices that are capable of
averaging over the control cycle are required. Integrating devices (watt-hour transducers) are preferred over instantaneous devices
(watt metres).meters). Overall accuracy of the power-monitoring equipment must be within 62 % of the reading for the average
power for the test period.
6.4 Temperature Sensors—Temperature sensors shall be thermistors, thermocouples, resistance temperature devices (RTDs), or
equivalent sensors. The test plate, guard section, and bottom plate shall each contain one or more temperature sensors that are
mounted flush with the hot plate surface and in such a manner that they measure the surface temperature within plate surface. Each
temperature sensor shall have an overall accuracy of 60.1 °C.
6.5 Controlled Atmosphere Chamber—The hot plate shall be housed in an environmental chamber that can be maintained at
selected temperatures at a minimum between 20 and 35 °C. The test chamber wall temperature shall be 60.5 °C of the air in the
chamber. The relative humidity shall be maintained as specified in the individual procedure section.
6.6 Measuring Environmental Parameters—The air temperature, relative humidity, and air velocity shall be measured as follows:
6.6.1 Relative Humidity Measuring Equipment—Either a wet-and-dry A wet and dry bulb psychrometer, a dew point hygrometer,
or other electronic humidity-measuring device shall be used to measure the relative humidity and calculate the dew point
temperature inside the chamber. The relative humidity-sensing devices shall have an overall accuracy of at least 64 %.
6.6.2 Air Temperature Sensors—Shielded air temperature sensors shall be used. Any sensor with an overall accuracy of 60.1 °C
is acceptable. The sensor shall have a time constant not exceeding 1 min. The sensor(s) is suspended with the measuring point
exposed to air inside the chamber at a point in the air stream such that the air temperature sensor is not influenced by the plate
temperature.
6.6.3 Air Velocity Indicator—Air velocity shall be measured with an accuracy of 60.1 m/s using a hot wire anemometer. Air
velocity is measured at a point 15 mm (nominal) from the plate surface or from the top of the test specimen surface to the bottom
of the anemometer sensing element. The air velocity shall be measured at one position perpendicular to the airflow, at the center
of the plate.
6.6.3.1 The air velocity is to be measured 15 mm above the plate surface for bare plate measurements. The air velocity is to be
measured 15 mm above the test specimen surface when testing fabric or systems. The 15-mm 15 mm distance is to be the distance
from the plate or test specimen to the anemometer sensing element (wire)—not to the bottom of the sensing element housing.
6.6.3.2 At a minimum, annually verify that air velocity spatial variation does not exceed 610 % of the mean value. Measurements
of air velocity shall be measured at three positions located along a horizontal line perpendicular to the airflow, including a point
at the center of the plate and at points at the centers of the guard section on both sides of the plate.
6.6.3.3 The additional two anemometers needed for spatial variation must meet the same requirements as defined in section 6.6.3
and shall be permitted to be external anemometers or integral anemometers to the system.
6.6.4 Air Temperature Variations—Air temperature variations during testing shall not exceed 60.1 °C.
6.6.5 Relative Humidity Variations—Relative humidity variations during testing shall not exceed 64 %.
6.6.6 Air Velocity Variations—Air velocity variations shall not exceed 610 % of the mean value for data averaged over 5 min.
7. Materials
7.1 Water—For the evaporative resistance and total heat loss measurements in Parts B and C, distilled, de-ionized, or reverse
osmosis-treated osmosis treated water shall be used to wet the test plate surface.
7.2 Liquid Barrier—For the evaporative resistance measurements in Parts B and C, a liquid barrier shall be used to cover the test
plate so that water does not contact the test specimen. The permeability index of the liquid barrier shall be greater than 0.7, ≥0.7,
where i = 0.060 (R /R ). Examples include untreated cellophane film and microporous polytetraflouroethylene film.
m cbp ebp
F1868 − 23
5 2
7.3 Verification Fabrics —A verification fabric is required for the verification in Part C. The verification fabric is 7.5 oz ⁄yd , plain
weave, yellow color with a fiber blend of 93 % meta-aramid, 5 % para-aramid, and 2 % anti-static with a durable water-repellent
finish. Sources for the verification fabric are given in Footnote 4.a footnote.
8. Sampling and Preparation of Test Specimens
8.1 Sampling—Test three specimens from each laboratory sampling unit.
8.2 Specimen Preparation—Use test specimens large enough to cover the surface of the hot plate test section and the guard section
completely. Remove any undesirable wrinkles from the test specimens. Possible techniques for removing wrinkles include
smoothing, free-hanging, pressing, steaming, ironing, and so forth.
8.3 Conditioning—Allow the test specimens to come into equilibrium with the atmosphere of the testing chamber by conditioning
them in the chamber for a least 4 h.
9. Procedure Part A – Thermal Resistance (R and R ; I and I )
ct ctcf t f
9.1 Test Conditions:
9.1.1 Temperature of the Test Plate, Guard Section, and Bottom Plate—Maintain the temperature of these sections at 35 6 0.5 °C
and without fluctuating more than 60.1 °C during a test.
9.1.2 Air Temperature—Maintain the air temperature of the air flowing over the plate between 4 and 25 °C without fluctuating
more than 60.1 °C during a test.
9.1.2.1 Select an air temperature that will generate a power level in the middle range of the instrument consistent and measurable
amount of power while maintaining the plate temperature at 35 °C. Thicker materials will need to be tested at lower temperatures.
9.1.3 Relative Humidity—Maintain the relative humidity of the air flowing over the plate between 20 and 80 % without fluctuating
more than 64 % during a test.
9.1.3.1 The relative humidity has little or no effect on fabric insulation under steady-state conditions. Under transient conditions,
the absorption of moisture from the air will generate heat in the fabric, and the desorption of moisture will produce a cooling effect.
9.1.4 Air Velocity—Maintain the air velocity between 0.5 and at 1.0 m/s without fluctuating more than 60.1 m/s over the duration
of the test measurement.
9.1.4.1 The method described in Test Method D1518 does not specify air velocity over the hot plate, and ISO 11092 specifies an
air temperature of 20 °C, a relative humidity of 65 %, and an air velocity of 1.0 m/s.
9.2 Procedures:
9.2.1 Measure the bare plate thermal resistance,resistance (R ),) in the same manner as that for R except that the test plate shall
cbp ct
not be covered with a test specimen.
9.2.2 Measure the total thermal resistance,resistance (R ),) by placing a fabric or fabric system on the test plate. Place the test
ct
specimen on the test plate with the side normally facing the human body towards the test plate. In the case of multiple layers,
arrange the specimens on the plate as on the human body. Eliminate bubbles and wrinkles within the test specimen and air gaps
bubbles, wrinkles, and air gaps within and between the specimen layers and the plate or between specimen layers by smoothing
the specimen without compressing. This smoothing of bubbles and wrinkles is one reason that the results from this test may does
not represent the performance of actual clothing worn by people. Still people, as still air trapped between clothing layers can
contribute to the insulation of the fabric system when worn on the body.
Verification fabrics are available from TestFabrics at Testfabrics.com.
F1868 − 23
NOTE 1—Fabrics and fabric systems thicker than 0.5 cm should be tested on plates with a large guard section (for example, 12.7 cm) to prevent lateral
heat loss through the edges of the fabric. If a large guard is not used, a lower insulation value will be measured.
9.2.3 After the fabric or fabric system reaches steady-state conditions, record measurements for power input and the conditions
given in Measurement 9.1 (with the exception of air velocity) every 1 min for a minimum test period of 30 min to determine the
total thermal resistance of the fabric plus the air layer, (of thermal resistance shall be complete when equilibrium is reached.R ).
ct
9.2.3.1 Data used to calculate the thermal resistance shall be collected at least once every minute.
9.2.3.2 Equilibrium shall be a rate of change of less than 3 % per hour of the calculated thermal resistance over a period not less
than 30 min.
9.2.3.3 The coefficient of variation of calculated thermal resistance shall be less than 5 %.
9.3 Calculations—Calculate the total resistance to dry heat transfer, (R ), for a fabric system, including the surface air layer
ct
resistance using Eq 1.
R 5 ~T 2 T ! A/H (1)
ct s a c
where:
R = total resistance to dry heat transfer provided by the fabric system and air layer (K·m /W),
ct
A = area of the plate test section (m ),
T = surface temperature of the plate (°C),
s
T = air temperature (°C), and
a
H = power input (W).
c
9.3.1 Average the data from three specimens for the dry thermal resistance tests to determine the average R for the laboratory
ct
sampling unit.
9.3.2 Determine the intrinsic thermal resistance provided by the fabric alone, R , by subtracting the thermal resistance value
cf
measured for the air layer, R (that is, bare plate test) from the average total thermal resistance value measured for the fabric
cpb
system and air layer, R .
ct
9.3.3 To convert the insulation values measured in SI units to clo units, multiply by 6.45.divide by 0.155. R is often designated
ct
as I and R is designated as I when insulation is expressed in clo units.
t f f
10. Procedure Part B – Evaporative Resistance (R and R ) and Permeability Index (i and i )
et ef m mf
10.1 Test Conditions:
10.1.1 Temperature of the Test Plate, Guard Section, and Bottom Plate—Maintain the temperature of these sections at 35 6 0.5 °C
without fluctuating more than 60.1 °C during a test.
10.1.2 Isothermal Conditions—The air temperature is the same as the plate temperature, so no dry heat exchange is occurring
between the plate and the environment. This is the preferred condition for measuring evaporative resistance.
10.1.2.1 Air Temperature—Maintain the air temperature of the air flowing over the plate at 35 6 0.5 °C and without fluctuating
more than 60.1 °C during a test.
10.1.2.2 Air Velocity—Maintain the air velocity between 0.5 and 1 at 1.0 m/s without fluctuating more than 60.1 m/s over the
duration of the test measurement. The air velocity shouldshall be the same for the dry thermal resistance test and the evaporative
resistance test if both are being conducted on a fabric system.
10.1.2.3 Relative Humidity—The relative humidity shall be 40 6 4 % during a test.
10.1.3 Non-Isothermal Conditions—The materials are tested under environmental conditions that simulate actual conditions of
use. The same environmental conditions are used for the insulation test (Part A) and the non-isothermal sweating hot plate test.
The air temperature is lower than the plate’s temperature, so dry heat loss is occurring simultaneously with evaporative heat loss,
F1868 − 23
and condensation maywill develop between the plate and the fabric, or between fabric layers, or both. The evaporative resistance
determined under non-isothermal conditions shall be referred to as the apparent evaporative resistance value. The apparent
evaporative resistance values for materials shall only be compared to those of other materials measured under the same
environmental conditions.
10.1.3.1 State the air temperature, air velocity, and relative humidity used in the non-isothermal tests. (See Part C for the
non-isothermal protocol used to evaluate materials used in NFPA protective clothing.)
NOTE 2—ISO 11092 Test Conditions—The environmental conditions specified in the ISO standard are an air temperature of 35 °C (isothermal), a relative
humidity of 40 %, and an air velocity of 1.0 m/s.
10.2 Procedures:
10.2.1 Feed water to the surface of the test plate and guard section.
10.2.2 Cover the test plate and guard section with a liquid barrier that prevents wetting of the fabric specimens by liquid water.
Adhere the liq
...