There are numerous occupational and leisure tasks for which the participants are at risk of heat stress related illness. Heat stress illness effects can be either acute, such as heat stroke, heat exhaustion, heat cramps, fainting, and decline of performance; or chronic, such as loss of ability to tolerate heat, hypertension, heart muscle damage, reduced libido and impotence.
Existing practices for protection against heat stress are limited to awareness education of antagonistic conditions. Persistent monitoring of individuals is not an existing practice due to a variety of factors, including drawbacks to the sensor devices and lack of quantitative definition of heat stress limits.This project presents a Bondgraph model to illustrate the body-ambient heat exchange and how it would be measured using a heat flux transducer. Individual contributions to body heat are shown as r-elements. A measurement device of a heat flux transducer is shown as a transformer element. The equation layer of the model can be tailored for various operating conditions, using either derived or empirical formulas to describe heat transfer.
The model shows the same various components of body ambient heat exchange as are found in most occupation educational literature. Bondgraph figures further demonstrate causality and direction of power flow. Not fully quantified in the literature uncovered in research are how the body thermoregulatory functions begin to break downThis data suggest that given a sound understanding of the heat transfer mechanisms operating with the human body during high risk tasks and environments, a heat flux transducer should provide a leading indicator of heat stress illness for any variety of tasks.
Introduction
For the understanding of heat transfer between a human body and the ambient, a bondgraph model can provides a complete system view of the contributors to heat stress, and the equation layer of the model can be tailored for various operating conditions, using either derived or empirical formulas to describe heat transfer.
In this paper, we provide a query and answer on a theoretical situation to demonstrate how a heat flux transducer in conjunction with the body ambient bondgraph model would be used to monitor against the onset of heat stress illness.
Query
If a runner were to be instrumented with a heat flux transducer prior to running a marathon, how would the measured data be used to estimate the onset of heat stress illness, and what percentage of error needs to be figured into the measured data?
Answer
Model
A Bondgraph model is a graphical technique to describe energy flow in and amongst physical systems. Presented below is a Bondgraph model to illustrate the body-ambient heat exchange and how it would be measured using a heat flux transducer
Figure 1: Bond Graph Model of Heat Flow and Temperature of the Human Body
Where,
HF = Heat Flow
T = Temperature
Eva = Evaporation
Rad = Radiation
Con = Convection
Amb = Ambient
Sk = Skin
Bc = Body Core
Assumptions
The runner is 6’ tall (183cm), 150lbs (68.2kg) and running at a steady pace to complete the marathon in a time of 3.9 hours (3.0m/s).
The ambient temperature is 30ºC with a relative humidity of 60%; resulting in an apparent temperature of 35ºC.
Still air environment, resulting in an apparent wind speed equal to running speed.
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Source:
OpenStax, Body ambient bondgraph model using heat flux transducer. OpenStax CNX. May 15, 2008 Download for free at http://cnx.org/content/col10530/1.1
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