18-20 June, 2003, Budapest, Hungary OSSKI Center (Törley Palace)

with Exhibition and Pre-Session on Thermal Energy in Hungarian
"THERMO-BRIDGE"
between East and West for technology transfer and information exchange

To design a new range of body support systems (BSS) (e.g., chair cushions and backs) for thermal regulation at their user interface, multi-sensor temperature measurements (up to 64 channels), and high-resolution infrared thermal imaging techniques, were used. Furthermore, thermal properties of constituent materials were independently measured beforehand, so that composite multi-layer conductivity or insulation values could then be calculated and compared to a range of thermal power outputs of different users performing variable tasking functions, i.e., at variable metabolic rates.

Comfort provided to a user by a body support system, such as a seat cushion, depends on a variety of factors. One such factor is the ability of the body support system to provide for thermal regulation with the user and the surrounding environment through the variously embedded heat exchange processes/mechanisms within the system. Instead of the user endeavoring the self-regulatory processes to maintain thermal equilibrium with the surrounding environment, regulatory processes could now be vested to the system rather than the user.
Despite the increasing demand, in the past decade, for more open-plan offices, and recognizing the significant impact of thermal comfort on worker’s productivity, this aspect of comfort, between the user and the surrounding environment through the chair, seems to not have been sufficiently studied. However, various researchers had studied thermal comfort or models of heat exchange between humans and environments, through various clothing ensembles ^[1-10].

In this research program and within the design development stage, real-time digital data from data acquisition system and from infrared thermal images (maps) were concurrently used to study user’s thermal signatures (including emissivities) occurring at chair backs and seat-rest cushion surfaces. Such measurements occurred while the user was seated, and upon departing different types of chairs or design prototypes. Consequently, angle factors between seated human bodies (typical of mesh chairs) and rectangular planes became measurable. This approach allowed studying and comparing design variations for calibrated thermal interaction between typical users and various cushion types, as well as with surrounding environment, as the user endeavored changing tasking functions (resulting in variable metabolic rates), and different user-chair orientations within the workspace. It also allowed introducing numerous measurable design parameters that may be sought by a BSS designer. Some of these parameters are briefly presented in the figures of this publication. With this research program, and its measurement capabilities, new state-of-the-art office seat cushions were designed to achieve calibrated and validated thermal performances. This new upholstery solution was then evaluated against conventionally padded (polyurethane) upholsteries and against transpiring open-weave mesh, which both represent customary, however thermally extreme, upholstery solutions.

Conventional contemporary office seats are usually made of either upholstered padding or synthetic mesh in a frame assembly. Each of these types of upholstery has its own characteristic thermal properties. For example, padded upholstery (e.g., polyurethane) seating provides limited heat exchange with the user, which however, mainly occurs through conduction and sweat evaporation processes. As a result, upholstered padding is better suited for lower workspace temperatures on the order of 16-25°C (61-77°F) and shorter sitting times before heat starts building up and un-evaporated sweat starts developing at the user-body support system interface, especially with users engaged in higher, more concentrated, tasking function (metabolic rate). On the other hand, mesh seating provides excessive heat exchange between the surrounding environment and the user, mainly through radiation and convective heat transfer. Unfortunately, these two heat exchange processes do not entirely depend on the intrinsic (i.e., invariant) properties of the body support system, but also on extrinsic factors such as surrounding temperature and air speed, and on factors such as workspace configuration due to various orientations and temperatures of surrounding thermally reflective surfaces. As a result, mesh chairs may be better suited for higher workspace temperatures of 25-35°C (77-95°F) and longer sitting times. Clearly, neither of the conventional seating designs provides for thermal regulation in a wide variety of office temperatures, user/task types, or workspace configurations.

To provide for thermal regulation in a body support system, body-generated heat should not be excessively lost (dissipated) to the nearby environment. In this case discomfort may be caused by engaging the user in undue self-regulatory thermal processes, and could as well aggravate low back pain as a result of repeated/prolonged exposure. On the other hand, the body support system should not retain excessive heat at the user interface so as to cause discomfort by prompting sweating and inhibiting its evaporation. Clearly, thermal regulation, coupled with proper vapor (e.g., sweat) permeation, at the interface, prevents the human body from endeavoring self-regulatory thermal processes (further sweating, developing goose pumps, endeavoring postural adjustments, in chair movement, shivering, or excessive heat generation), which are all direct measures (indications) of discomfort.

Thermal properties are major ergonomic features that should be considered in the design of an office chair. The human body always works to retain its core temperature near 37°C (98.6°F). It does so by means such as postural adjustments, varying skin temperatures by perspiration, regulation of cardiovascular, or by pulmonary activity such as changing pulse and breath rates to affect blood flow and vessel sizes, especially in skin areas close to a heat-exchanging interface, i.e., with a body support system. Obviously, a body support system that prompts sweating after a relatively short period of interfacing time and which requires the human body to engage in such thermal self-regulatory processes to attain or restore thermal balance, will be uncomfortable. Such response may affect work efficiency/productivity as it prompts the user to lose concentration on the task or to ultimately disengage from it. With conventional upholstered padding, heat can quickly build up at the user/seat interface causing the user to limit metabolic rates such as muscular activity to reduce the rate of heat generation. In this case, the user may also begin sweating to initially expedite the thermal transfer across the user skin, and to attempt to invoke the sweat evaporation cooling process.

When the user/seat interface inhibits sweat evaporation due to low cushion vapor permeability under even small pressures, stagnant heat that is not dissipated at the interface (or transmitted through it), would prompt further sweating, therefore leading to even greater user’s discomfort. On the other hand, open-weave mesh upholstery has high vapor permeability and heat dissipation. As a result, they do not allow for any heat build up at the seat user interface, therefore preventing user’s body from retaining any of its generated heat, unduly placing it as a heat source for the whole environment. With colder workspace environments, and closer, oppositely oriented, highly reflective, and cold office space surfaces (floors/walls, etc.), and with high (transient or steady) airspeeds, the user is set to generate heat that seeks thermal equilibrium with the whole environment; a condition that prompts discomfort. This might allow for excessive user heat loss and, therefore, could lead to user’s disengagement from the task. Consequently, it is postulated that, for design purposes, a limited heat build up at the interface would be favorable to smoothen, or even reverse (break), the thermal gradient across the interface. This gradient usually exists between the user’s skin surface (as a heat source) and the environment, as a heat sink.

Figure (1): A thermal-data acquisition system (left) showing instrumentation (wiring) for a typical test across various interfacial layers. A single layer (i.e., cushion surface) instrumentation is shown (right).

In this research program, a new office chair cushion was conceptualized and designed to provide for proper thermal regulation qualities. The cushion, and its design prototypes, was thermally evaluated against conventionally padded (polyurethane) cushions and against transpiring open-weave mesh seat upholstery without padding. The new seat cushion was conceptualized to embed several thermal exchange mechanisms to be integrally built into its cellular and layered construction, which would then match the heat dissipation requirements for a wide range of user types performing various tasking functions. Thermal matching would then be established within upper and lower bounds of comfort.
By embedment of various heat exchange mechanisms in the seat, expedited steady state thermal exchange conditions (balance) are achieved between the user and the environment within the narrow bounds of thermal comfort, obtained from psychometric measurements/evaluations. In the thermal design of body support systems, the designer would aim at achieving thermal balance (steady state conditions) at the interface, with uniform temperatures that are consistently less than the skin temperature by 2-5 degrees Fahrenheit, or by 5-12 degrees Fahrenheit, from body core temperature (i.e., 98.6°F). Substantially based on its inherent thermal qualities, the body support system should establish these design goals at the interface regardless of the external thermal qualities of the surrounding environment, the configuration of the workspace, and/or the type of user (being heat acclimated or not), or the nature of the tasking function that prompts the user to generate heat at different “metabolic” rates. As a result, thermal comfort becomes an inherent (invariable/ characteristic) property of the interface itself, not the surrounding environment or user’s type.

In addition to the various heat exchange mechanisms embedded into the new upholstery design, an additional thermal regulation process is provided by the placement of a thick gelatinous layer possessing high heat “retention” capacity as a middle layer, whereby excess heat generated by the user could be conducted through the overlay media (of known composite-layer conductivity), to that layer, which substantially acts as a “heat sink”. The overlay material and the fabric cover may as well be treated with phase changing materials (PCM) to provide for a similar heat exchange (i.e., storage/release) process. In this process, as the ambient workplace environment become colder, or user’s skin temperature warrants less heat be drawn from it, the gel layer (having latent/stored heat) will decrease the drawn heat rate from the surface or even reverse conduction of heat back towards the user. This thermal regulating process will maintain the overlay material (including the cushion top) within the pre-established envelope of thermal comfort. Thermal regulation of cushions would save the user’s body from endeavoring self-regulatory thermal processes that usually signifies discomfort.

To allow sweat evaporation as an effective thermal cooling process, and to allow for vapor permeation and convective heat transfer, the cushion would be covered with a microporous fabric that rests on porous (open cell) polyurethane media, with a composite moisture vapor transmission rate exceeding 250 (g/m²/24 hrs.) at 73°F and 50% RH, according to ASTM E-96, and air permeability exceeding 10 liters/m².s according to ASTM D-737. The cover fabric should have an indoors insulation of less than 0.15 K⁰.m²/W (=1.0 clo), but more than 0.120 K⁰.m²/W (=0.80 clo), at +20 °C, according to ASTM D-1518. Across the user-cushion interface and the cushion top surface, the cover fabric connects air outside the cushion to that residing inside a networked system of connected air within the cushion. Furthermore, the cushion construction allows air pumping, and circulation, within the channels, due to in-chair movements (ICMs). This action occurs as a result of pressurizing/depressurizing an air plenum inside the cushion, that pumps air laterally outwards or inwards through the cushion top surface and the upwardly tapered side channels. This action was studied and reported in a different publication.

Thermal measurements were made for twenty healthy acclimated and non-acclimated, variably sized, volunteers aged 20 to 55, performing various tasking functions. Volunteers consisted of 10 males and 10 females, who are already “white-collar” users of office chairs. Figure (1) shows a thermal data acquisition system (up to 64 channels), which was used to monitor and record temperature data, sampled in 1 to 30 seconds intervals, across the multiple layers of the seat and back interfaces (i.e., user’s skin surface, up to 3 clothing layers, the cushion surface, as well as under cushion fabric cover). Figure (2) depicts three selected unmarked (raw) thermal images (frames) of a seat cushion. These measurements allowed retrieval of several potential design parameters.

Using Flir Systems ^® ThermaCam Researcher 2002 ^® to process thermal images and retrieve their temperature and emissivity data, areas on the cushion were used to calculate, average and maximum temperatures, as well as standard deviation of all temperature measurements within that area. Hundreds of data points make up the resolution of the image, and therefore, the data population for such measurements. Figure (3) shows a thermal image with an outline bounding a “study area”. This one image, out of many captured and processed, in real-time, corresponds to the frame occurring immediately after a user had separated from the chair. Point temperature, or temperature along a line, may also be profiled as shown in the Figure. It also shows various directly retrievable statistical data and temperature profiles that may be captured and monitored in real-time.

Figure (2): Unmarked Thermal Image of the Body Support System (new seat cushion), immediately after a user had separated from it (left), 30 seconds after (middle), and 45 seconds after (right).
As conformance of the user to the body support system increases, the interfacial (bounded), i.e., study area, increases, and therefore, the more the population of the temperature points. The number of temperature points, should not, however, significantly affect the quality of the calculated averages, maximums, or standard deviation of data reported in Figure (4).

Figure (3): A thermal image captured by Flir’s^® ThermaCam Researcher 2002 ^® with an outline, bounding a “study area.”

Thermal signatures of users on the top surface of cushions were monitored in real time using a 60 Hz high-resolution infrared thermal camera by Flir^® Systems. Measurements were sampled at a rate ranging from 1 measurement every 5 seconds (0.2 Hz) to 5 measurement every 1 second (5 Hz). All measurements were made under uniform testing conditions in one continuous session. To inspect for consistent repeatability, measurements were also repeated for different users. All measurements were taken when, and after, the user was sitting in the upright position with a seat pan height of 19 inches from the ground. Figure (3) shows the outline bounding a “study area” on the cushion, and corresponding to the frame immediately after a user had separated from the chair. It also shows various statistical data and profiles that may be monitored in real-time.

Figure (4) shows an averaged multi-sensor temperature profile (left), with data sampled in one layer (on a conventionally padded seat cushion surface). The measurements were taken as the cushion interfaced the user endeavoring a preset task-sequence protocol. The protocol consists of an uninterrupted sedentary task-intensive VDT work period (A-B-C) that lasts for 60 minutes, followed by a 6-minute interval of user separation (C-D), and a continuous re-sit (D-E-F) for a period of 15 minutes. The protocol ends in a series of intermittent sit-stand tasks for additional 15 minutes (F-G). The Figure shows several potential design parameters that could be directly measured from the profile. Figure (4) (right) shows an averaged temperature profile for multiple interfacial layers in the 60 min. interval (A-C). Data obtained from the various layers may be processed and used in conjunction with thermal balance models such as those used in Refs. [1-4]. With variously estimated or calculated power output by users, and temperature data recorded across the various layer thickness, thermal conductivity of individual, and composite-layer seat cushions, may be estimated, and designed accordingly.

Figure (4): Averaged temperature data obtained from the thermal-data acquisition system at the seat cushion surface of various upholstery alternatives (left). Exponential curve-fitted temperature decay curves, corresponding to various upholstery solutions, after user had separated from chair, as measured by real-time IR thermal imaging (right).

Figure (5) shows averaged temperature data obtained from the thermal-data acquisition system with various upholstery alternatives, under repeatable and reproduce-able test conditions. Results clearly demonstrate the difference in thermal performance between the two thermally extreme upholstery solutions (i.e., conventionally padded and open-weave mesh), and the intermediate response presented by thermally regulating body support systems. Figure (5) (right) also shows that the various exponential (curve-fitted) interfacial temperature-amplitude decay curves, corresponding to various upholstery solutions, have heat loss (dissipation) rates ranging between 0.03 Nepers (N_P) for open-weave mesh (lowest curve), and 0.04 Nepers (N_P), for conventionally padded cushions. The results shown with these measurements represent a result of all thermal exchange processes working together to produce the measurable quantities of temperature, heat loss rate, and other statistical parameters. Other statistical parameters may be used to characterize the thermal dissipation qualities.

Figure (5): Averaged temperature data obtained from the thermal-data acquisition system at the seat cushion surface of various upholstery alternatives (left). Exponential curve-fitted temperature decay curves, corresponding to various upholstery solutions, after user had separated from chair, as measured by real-time IR thermal imaging (right).

Figure (6): Average/Maximum temperature values for different upholstery solutions (left), and standard deviation of temperature measurements (right), after user had separated from chair, as measured by real-time IR thermal imaging.

Because infrared images only capture externally irradiating (emitting) surfaces, its measurements might not be directly used to measure interfacial temperatures when the user is seated. However, the thermal trace (signatures) of users on various cushions types, and the dissipation pattern of such signatures, may be observed. Within the context of energy/power-based models, such measurements carry significant information about the thermal qualities of the body support system. When examining the distribution of temperature points around their mean (average), after the user (as a heat source) had separated from chair, with various cushions, and cooling process takes place.
From Figure (6), conventionally padded cushions were observed to exhibit least directional gradients in temperatures, expressed in terms of higher average/maximum temperature values (left). At fist instance, this might suggest least dispersion of data, as might be indicated by the standard deviation of all temperature measurements on the cushion surface. The more the standard deviation the more the temperature values are dispersed away from their mean. Interestingly, and on contrary, as depicted in Figure (6) (left), one could observe that open-weave mesh upholstery has lowest such dispersion, signifying uniform temperature gradients, in the various orientations. This demonstrates that open-weave mesh, not retaining any significant heat, would be insensitive to temperature variations of the user’s skin temperature. This also signifies the high influence of the environment on making most temperature points follow their mean. As a result, one could infer that open-weave mesh upholstery would not be able to redistribute or to regulate the thermal exchange process at the interface.

Results demonstrate that advanced thermal measurement technologies could be used advantageously in the design of thermal comfort for body support systems. This research indicates that upholstery solutions could be designed to neither correspond to the thermal extremes of open-weave mesh, or conventionally padded support systems, in a wide range of workplace environments, and configurations (exchanging surface orientations). Such techniques were used to help designers develop a new range of upholstery solutions that would thermally regulate the thermal exchange process at the interface between the user and the body support system.

Author wishes to extend thanks to the technical staff at Allsteel^®, and at the HON INDUSTRIES Stanley M. Howe Technical Center. Particularly, Author wishes to acknowledge help of Mr. Woody Witherow and Mr. Travis Flenker in carrying out some of the experimental procedures.

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