Fabric Thermal Resistance & Water Vapor Resistance and Sweating Guarded Hotplate Testing Method

Everyone wears clothes, and often buys clothes, we may have many criteria when choosing clothes, but most of them will mention the word “comfort”. The comfortable performance also has a variety of forms, the clothes may be soft, maybe light, and snug, and there is a comfort called “thermal and moisture comfort”, what does it mean?

The “thermal and moisture comfort” is measured by thermal resistance and water vapor resistance. Simply, thermal resistance is the ability to block heat transfer. Clothing was born to insulate and keep warm, to prevent excessive heat from escaping, and to ensure that the body is in a comfortable temperature environment role. When the temperature outside the body is lower than the body temperature, will inevitably bring heat transfer from the inside to the outside, the fabric in the middle to absorb heat, establish a ladder, and gradually stabilize the temperature difference between the two sides, which forms a steady state process requires energy is thermal resistance. The greater the value of thermal resistance, the better the warmth of the garment. Similarly, the water vapor resistance is the energy required to stabilize the fabric on both sides of the water vapor pressure differences. The greater the water vapor resistance value, the worse the perspiration of the garment, wear it on the body will feel sticky wet uncomfortable.

Why do they study the Thermal Resistance & Water Vapor Resistance of fabrics?

When the external temperature is high or the human body is engaged in strenuous activities, the clothing wrapped in the surface of the human body will be moistened by a large amount of sweat. In recent years, with the deepening of the research on the development of functional textiles, the research on the thermal and wet comfort of fabrics in this state has attracted more and more attention from researchers. The balance of heat production and heat dissipation in the human body is a necessary condition for the continuation of life. Heat production depends on the physiological change process in the body, while heat dissipation is carried out in four ways: convection, conduction, radiation, and evaporation. Heat dissipation is related to the surrounding environmental conditions and the performance and state of the clothing fabric, especially in the clothing after wetting, clothing thermal resistance changes and the balance of human heat production and heat dissipation is broken, so the evaluation of the warmth of the fabric after wetting, you can know the amount of heat dissipation through the fabric when sweating, it is guidance on what to wear when the body is in a sweating state.

Commonly used Thermal Resistance & Water Vapor Resistance Test Method

Constant Temperature Method

The fabric will be placed on one side of the thermostatic hot plate, the other sides of the thermostatic hot plate are adiabatic protection, determine the heat required to maintain the constant temperature of the hot plate when not putting on the specimen and put it on the specimen, thus calculating the insulation rate of the fabric to explain the thermal insulation performance of the fabric. Firstly test the  power required to maintain the test board’s constant temperature  by not putting the specimen, then test  the power required by putting the specimen, through the formula (1) to calculate:

Wr = (1-b/a ) × 100% (1)

Wr is the insulation rate, %; a is the thermal power consumed by the test plate when no specimen is put on, W; b is the thermal power consumed by the test plate when a specimen is put on, W.

Currently used to test the evaluation of flat material insulation performance of the single plate method is used in this test method.

However, there is no protective cover above the single plate method, so the airflow on the surface of the specimen will cause a certain amount of convective heat dissipation, and the test result becomes the combined value of convective and conductive heat dissipation. Due to the unstable airflow in the constant temperature and humidity chamber, which leads to the low insulation rate measured when the airflow rate in the upper part of the specimen is high, the insulation material is usually thicker, and the heat dissipation at the edge of the cut specimen is obvious, etc., which makes the existing single-plate instrument’s test stability not good. In addition, GB 11048-89 “textile insulation performance test method” and FZ/T 01029-93 “textile steady-state conditions in the determination of thermal and water vapor resistance in the test of thermal resistance”, both belong to the constant temperature method.

Cooling Rate Method

The cooling Rate Method is to stop the power supply after heating the hot body to a certain temperature, cover the fabric to one side of the hot body or wrap the hot body with fabric in the case of adiabatic on all other sides, and then let it cool naturally, measure the time required for the hot body to cool to a certain temperature, or measure the temperature reduction value of the hot body in a certain time, and express the heat insulation performance of the fabric with the cooling rate. Some also set up a pressurization device on device to determine the thermal insulation performance of the fabric under a certain pressure. This method is faster than the Constant Temperature Method to determine, but only can roughly compare the insulation performance of clothing materials, but can not quantitatively determine the thermal resistance of the fabric.

Evaporation Heat Plate Method

The evaporation Heat  Plate Method also known as the Guarded Hotplate Method, which is called the “skin model”, can simulate the heat and moisture transfer process that occurs close to the human skin, and is the most accurate device to measure the fabric thermal resistance and water vapor resistance. The Guarded Hotplate includes a temperature and water vapor control and measurement device and a heat guard ring and temperature control device (environmental chamber).

Static Plate Method

The fabric is sandwiched between two thermostatic hot and cold plates with different temperatures, and the heat flow through the fabric is measured by a thin flat plate heat flow sensor, i.e. the hot plate method. Calculating the thermal resistance and the thermal conductivity of the fabric to judge the performance of heat transfer. However, when using this method to test the thermal insulation performance of the fabric, there is an obvious edge effect at the boundary of the specimen. In order to eliminate the edge effect and reduce the test error, a material with a known thermal conductivity needs to be measured for calibration to obtain the calibration factor. This method is not suitable for testing materials with a thermal conductivity lower than 0.15W/(m×k), because the calibration error will be amplified and the accuracy of the instrument will be reduced. Corresponding standards are ASTMD1518-2014 “Standard Test Method for Thermal Resistance of Batting Systems Using a Hot Plate”, GB/T11048-2008 “Textiles Physiological Comfort Determination of Thermal Resistance and Water-Vapor Resistance under Steady State Conditions” and JISL1096-2010 Test Methods for Fabrics and Knitted Fabrics”.

“Sweating Torso”

“Sweating Torso” is the skin simulator. The device is made of an electrically heated metal cylinder, similar in shape to the human torso, but cannot simulate the behavioral actions of the human body. This instrument can be simulated with high accuracy and measure the parameters related to warmth and moisture permeability.

Moisture Permeable Cup Method

The Moisture Permeability Cup Method measures the water-vapor resistance of the garment fabric from the definition of water-vapor resistance. In the measurement, firstly, the relative humidity of the environment is measured, then the measured value and the difference between the humidity of the water surface is found, and the actual moisture permeability is divided by the difference to get the total water vapor resistance (total water vapor resistance = cup in the air layer, fabric surface air layer moisture resistance + fabric itself moisture resistance). Finally, the total water vapor resistance minus the blank experiment in the cup of air layer and the fabric surface air layer water vapor resistance is the water-vapor resistance of the fabric value. In the process of measurement, it is necessary to consider the influence of the area factor.

Sweating and warming Torso Simulation Device

The Sweating and Warming Torso Simulation Device includes the torso solid part and the measurement and control part, which can better simulate the dynamic heat and moisture transfer. Under the equilibrium stable condition, the device can measure the temperature and humidity values and change rate of the body surface, environment, and torso body, and get the fabric water-vapor resistance in the stable equilibrium state and constant heat and humidity flux, non-stable moisture dispersion case through certain equilibrium equation.

The Test Method of Sweating Guarded Hotplate

 Thermal Resistance Test: The specimen is covered on the electric heat test plate, and the test plate and its surrounding and bottom heat guard ring can maintain the same constant temperature. The whole test device is placed in a climate chamber with constant temperature, humidity, and wind speed, and the temperature on the electric heat test plate forms a temperature gradient with the temperature of the climate chamber.

An electric heat test plate dissipates heat through the specimen to the surrounding area, The power consumed in this heat dissipation process is brought into the corresponding equation to obtain the thermal resistance value.

Water Vapor Resistance Test: For the determination of water vapor resistance, it is necessary to cover the porous electric heating test plate with a breathable but impermeable film, and the water entering the electric heating plate evaporates after passing through the film in the form of water vapor (without liquid water contacting the specimen). After the specimen is placed on the film, determine the heat flow required to maintain a constant test temperature under a certain water evaporation rate, and the water vapor pressure through the specimen together with the formula to calculate the specimen water vapor resistance value.

The Sweating Guarded Hotplate is composed of a human skin simulation test board, automatic water supply system, wind speed stabilization system, test host, etc. It also needs to be equipped with an environmental chamber, because the test board is placed in the environmental chamber for the whole test in order to meet the temperature and humidity conditions required for the test.

Equipped with an automatic water supply system to ensure a stable water supply without making any adjustments and with no leakage. Equipped with a wind speed stabilization system to ensure that the wind speed can be continuously adjustable from 0 to 2m/s, and uniform and stable to meet various testing standards. Independent integrated design, the test host, and environmental experimental chamber completely separate design, test sample thickness automatic sensing, through the sample thickness, calculate the most suitable PID heating parameters, which can quickly reach the test conditions.

The instrument is used as follows.

Before turning on the machine, check the water level of the constant temperature and humidity water tank to indicate whether there is enough water or no water, please add water first. Otherwise, even if the machine is turned on, the Guarded Hotplate will not work. Add water method: open the front door, unscrew the left stainless steel cover, take the accessory funnel and fill it with distilled water, which is used to provide microclimate humidity adjustment. Just fill the water between the water level indicator line.

After turning on the power, display the setting panel for setting the temperature and humidity of the climate chamber. For the thermal resistance test, the temperature of the climate chamber is 20℃ and the humidity is 65%, and for the water vapor test, the temperature of the climate chamber is 35℃ and the humidity is 40%.

 

Thermal Resistance Test

(1)The sequence of testing is: cold machine preheating – thermal resistance parameter setting – thermal resistance empty plate test – thermal resistance test

(2)Cold machine preheating: After turning on the power, the whole machine needs to preheat for 45min, add a piece of medium-thickness fabric on the test plate, wait for the test plate to 35 degrees Celsius, and take out the fabric. Then observe the heating plate temperature and the bottom plate temperature to about 35.2  degrees Celsius, complete the cold machine preheating, you can put the test specimen (or standard sample) into the test bench.

(3)Thermal resistance parameter setting: set according to the standard requirements.

(4)Thermal resistance empty plate test: Measure the thermal resistance under the temperature gradient without putting the test specimen. Empty plate tests need not be done every time, because the repetition error of the empty plate test is quite small, it is recommended to test once in 3-6 months.

(5) Thermal Resistance Test: put the tested sample on the upper surface of the test plate, adjust the lift button at the front of the test bench inside the test chamber, and cover the four sides of the metal crimp, when the metal crimp is the exact horizontal position, then put down the Plexiglas cover, cover the instrument door, press the start button, and the instrument will run automatically. When it is stable, press the stop button, and the instrument will keep displaying the thermal resistance value as the test result.

Replace the specimen and repeat the previous steps for the test. Finally, take the average value for evaluation. The results are kept in 3 valid digits.

 

Water Vapor Resistance Test

(1) The sequence of the test is: cold machine preheating – water vapor resistance parameter setting – humidification and water replenishment and test film placement – water vapor resistance empty plate test – -water vapor resistance test

(2) Cold machine preheating: after turning on the power supply, the whole machine needs to preheat for about 60min, add a piece of medium-thickness fabric on the test plate, wait for the test plate to 35 degrees Celsius, and remove the fabric. Then observe the heating plate temperature and the bottom plate temperature to about 35.2 degrees Celsius, complete the cold machine preheating, you can test the specimen (or standard sample) on the test bench.

(3) In addition to humidification and water replenishment and film placement, the test of water vapor resistance and thermal resistance is similar, here do not repeat. Humidification and water replenishment: press the wet resistance key on the control panel, and adjust the rod waterproof joint loosening a little, the water inside the water replenishment box will automatically flow into the test chamber, observe the water level indicator on the right side of the test bench and the test body porous plate surface,

When touching the surface of the porous plate by hand and water comes out, you can stop the water level adjustment lever to pull up and tighten the waterproof joint.

(4) Take two films, flexible side for testing, spread out on the surface of the porous plate, between the two layers of film can be properly wetted with distilled water. Take the attached cotton block to smooth out the film and remove the air bubbles between the film and the porous plate. The rest of the steps show the thermal resistance test, and finally, record the test results.

Note: This type of  Guarded Hotplate requires a customized computer. With the corresponding test software such as SGHP software, you can see the test results directly on the computer.

Also, prepare the specimen in advance according to different standards, usually cutting the sample into 50 cm x 50 cm, and placing the processed sample on the surface of the test area. If the thickness of the sample is more than 1 mm, then adjust the height of the wind speed sensor so that the height between the sample and the sensor is about 15 mm.

Another example is that according to ISO139:2005 “Standard Atmosphere for Textile Humidity Regulation and Testing” standard, there are some requirements:

-Requirement 20 amp single-phase power supply

-Three times distilled water (not tap water direct water supply)

-Maximum sample size: 300mm x 300mm  sample thickness <5mm

214 mm x 214 mm sample thickness 5-70 mm

-Metal plate: 3 mm thick porous metal plate (phosphor bronze alloy) 200 mm x 200 mm

-Metal plate temperature control: 35℃ ±0.1℃

-Thermal protection temperature control: 35℃ ±0.1℃

– Air flowing parallel through the metal plate and thermal protection ring after moisture conditioning

– Airflow waveguide height: 55mm ±3mm above the table

– Airflow temperature range: 18℃-40℃ ±0.1℃

– Relative humidity: 30-70% ±3%

– Accuracy of airflow temperature in the test: ±0.5℃ thermal and moisture resistance < 0.5m2.K/W and 100m2.Pa/W

– Air velocity: 1 ± 0.05 m/s, measured at 15 mm on the bench at 20°C temperature, etc.

Some of the characteristics of high quality Guarded Hotplate

(1) High practicality

A separate design system is easy to control; the height of the thermal plate can be automatically adjusted according to the thickness of the test sample to ensure that the wind speed sensor is higher than the thermal plate by 15mm, making the test environment more realistic.

(2) Remote monitoring

Windows-based software is provided, and the host computer in the test chamber is connected to the software via Wi-Fi, so the operator can control and monitor the test situation in the office.

(3) More reference value of test results

The complex specially designed environment ensures the strictest control and monitoring of air temperature, air velocity, and relative humidity, enabling a more objective test of the comfort of the test sample.

(4) Stable and reliable data processor

An optional computer for displaying all real-time data, remote control and creation of test reports, automatic calculation of thermal resistance, water vapor resistance (evaporation resistance), water vapor permeability index, insulation value, heat loss, display of measurement results, and dynamic curves.

(4) Applicable to common international standards

Built-in programmable test procedures to meet the requirements of relevant ASTM, GB/T, ISO, and other standards.

 

Guarded Hotplate Specification& Parameters (using a particular one as a reference)

(RCT) thermal resistance range 0.002-2 .0 m² k/w

Repeatability ≤ ± 2%

Resolution 0.0001 m²k/watt

(Ret) Wet resistance range 0-1000 m² Pa/W

Resolution 0.001 m²- Pa/watt

Test temperature range 0 – 35°C

Temperature control accuracy ± 0.2°C

Wind speed 0 ~ 1 m/s

Wind Speed Accuracy ± 1%

Sample thickness 0 ~ 50 mm

Test plate area 254 mm x 254 mm

Protective ring size 512 mm x 512 mm

Protective ring width 127 mm

Standards ISO 11092, ASTM F1868, GB/T 11048

Voltage 110 V / 220 V 50 Hz

Weight 62 Kg

Dimensions 770 x 670 x 430 mm (L x W x H)

These Guarded Hotplates are widely used in commercial testing laboratories, military, research institutions, sports, leisure and home apparel, and other textile or textile-like testing.

Clothing (fabric) Heat and Moisture Transfer Characteristics of Physical Indicators

Characterize the heat transfer characteristics of clothing materials using thermal conductivity, thermal resistance value, CLO value, warmth, and other indicators; characterize the water vapor transfer characteristics of clothing materials using moisture permeability, moisture resistance, moisture permeability index, and other indicators. The significance of these indicators is briefly described as follows.

(1) Thermal Conductivity K.

When the thickness of textile Δ x=1m, surface area A=1m^2, and temperature difference Δ T=1℃, the heat Q transferred from one side of the textile to the other side by heat conduction in unit time, the unit of thermal conductivity K is W/m ℃.

 

(2) Thermal Resistance Rd

Thermal resistance Rd is an index to evaluate the thermal insulation performance of textiles. It is the ratio of the temperature difference Δ T between the two sides of the textile test and the heat flow rate q per unit area passing through the specimen vertically, and the unit of thermal resistance R is ℃ – m2/W.

(3) CLO Value

The CLO value is the index to evaluate the cold insulation performance of the garment. The average skin temperature Ts=33℃, the temperature Ta=21℃, the relative humidity≤50%, the wind speed≤10cm/s, the person sitting quietly or doing light mental work, the metabolic heat production Hc=209.34J/m2-h, the comfortable feeling, the heat insulation value of the worn clothes I=1CLO.

α =1.55

(4) Thermal Insulation Rate β

The Thermal Insulation Rate β is the same as the CLO value, which is also an index to evaluate the cold insulation performance of the garment. It is the ratio of the difference between the heat conducted before covering fabric Q0 and the heat conducted after covering fabric Q, and the heat conducted before covering fabric Q0.

(5) Moisture Permeability

The amount of moisture permeability is to evaluate the performance of the fabric wet transfer index, sometimes also use the amount of moisture permeability B to express the fabric moisture permeability performance, which is defined as the percentage of water G through the fabric specimen and the same container not covered with the specimen evaporated in the same conditions and time G0.

(6) moisture permeability WVT

Moisture permeability WVT is the amount of moisture transmission through a unit area A in unit time t, and the unit of moisture permeability WVT is g/m2-d.

(7) Water Vapor  Resistance  Re

Re for the two sides of the specimen of the water vapor concentration difference Δ C and unit time perpendicular through the specimen of wet flow M’ or wet heat flow ratio, wet resistance R units for Pa – m² / W.

(8) Moisture Permeability Index Ia

Considering that heat loss is not only related to a temperature gradient but also related to the diffusion heat effect caused by water vapor concentration gradient, Woodcock proposed the moisture permeability index Ia in 1962 as the evaluation index of wearing comfort under hot climate conditions. The moisture permeability index Ia means Ia = 1, that the water vapor on the fabric has complete permeability; Ia = 0, that the water vapor on the fabric has no permeability. In order to adapt to large environmental changes, Ia is expected to be as large as possible.

Evaluation Methods of Fabric Heat and Moisture Transfer Characteristics

The current evaluation system is mainly divided into two categories, subjective evaluation method and objective evaluation method. Among them, the objective evaluation method can be divided into two categories: the physical evaluation method and the physiological evaluation method; the subjective evaluation method is also known as the psychological evaluation method.

The objective evaluation method in the physical evaluation method is through the use of different instrumentation devices, such as flat moisturizer, moisture permeability cup, fabric precision thermal materiality tester, sweating warm body dummy, etc., to form the description and characterization method of thermal and wet comfort characteristics. The physiological evaluation method in the objective evaluation method refers to an objective method of judging the comfort of clothing by the changes in physiological parameters of the human body when wearing different types of clothing under specific activity levels and environments. It serves as an objective method to study the thermal and wet comfort of clothing that is complementary to the physical method mentioned above. The main measurement indicators are (1) skin temperature, body temperature, and heart rate. If the average skin temperature of the subject changes less in the three stages, the temperature rise and fall process are smooth, and there is no significant change in the subject’s body core temperature and heart rate in the three stages, it means that the garment has excellent performance and good comfort. (2) sweating volume, and evaporation rate. The amount of sweating reflects the warmth of the clothing, if the amount of sweating, then the corresponding clothing warmth is good; the size of the evaporation rate reflects the strength of the clothing moisture permeability, the better the clothing moisture permeability, the better the wet comfort performance of the clothing.

The subjective evaluation method, also known as the psychological evaluation method, is a pre-designed questionnaire form, which can include questions and answers on stickiness, coldness, heat, stuffiness, wetness, etc. The subjects fill in the questionnaire according to their different sensations during the test and rate the comfort of the clothing worn. The subjective evaluation indexes include (1) wearability, (2) heat and humidity, and (3) PMV and PPD values. Among them, PMV and PPD values are calculated in accordance with the ISO 7730 iso thermal environment – PMV and PPD indices and the description of thermal comfort conditions: applicable human metabolism, external work done by the human body, the thermal resistance of clothing, the surface area covered by clothing, air temperature, average radiation temperature, relative air velocity, partial pressure of water and gas, convection conversion coefficient and clothing surface temperature, to calculate the pretest average vote PMV and PPD. PPD, and then compared with the 7-point comfort scale to evaluate the thermal comfort of the human body.

In summary, although scholars at home and abroad have made a lot of research results on the evaluation system of garment (fabric) comfort and identified a variety of methods to measure the evaluation of thermal and wet comfort of garments, such as physical index evaluation method, microclimate parameter evaluation method, warm body dummy method, physiological evaluation method, psychological evaluation method, comprehensive evaluation method, etc., various methods have certain meaningful shortcomings, such as single Physical indicators can not characterize the clothing wearing state, movement and other factors are important: microclimate parameters evaluation method, are only the respective microclimate instrument parameters developed to evaluate; psychological evaluation method and can not objectively and fairly evaluate the thermal and wet comfort of the fabric, etc. And the indicators used to characterize the thermal and wet comfort performance of garments (fabrics) obtained by testing with different instruments do not have a standard. Therefore, more research on the testing and evaluation methods of heat and moisture transfer performance of garments and their fabrics is needed to improve the theory of heat and moisture transfer and to provide correct guidance to people’s dressing styles.