The PMV and PPD of Fanger

Prof Ole Fanger proposed that the condition for thermal comfort is that the skin temperature and sweat secretion lies within narrow limits. Fanger obtained data from climate chamber experiments, in which sweat rate and skin temperature were measured on people who considered themselves comfortable at various metabolic rates. Fanger proposed that optimal conditions for thermal comfort were expressed by the regression line of skin temperature and sweat rate on metabolic rate in data from these experiments. In this way an expression for optimal thermal comfort can be deduced from the metabolic rate, clothing insulation and environmental conditions.

The final equation for optimal thermal comfort is fairly complex and need not concern us here. Fanger has solved the equations by computer and presented the results in the form of diagrams from which optimal comfort conditions can be read given a knowledge of metabolic rate and clothing insulation.

Predicted mean vote (PMV) and Predicted percentage dissatisfied (PPD).

Fanger extended the usefulness of his work by proposing a method by which the actual thermal sensation could be predicted. His assumption for this was that the sensation experienced by a person was a function of the physiological strain imposed on him by the environment. This he defined as "the difference between the internal heat production and the heat loss to the actual environment for a man kept at the comfort values for skin temperature and sweat production at the actual activity level" (Fanger1970). He calculated this extra load for people involved in climate chamber experiments and plotted their comfort vote against it. Thus he was able to predict what comfort vote would arise from a given set of environmental conditions for a given clothing insulation and metabolic rate. Tables of PMV are available for different environments for given clothing and metabolic rates. Such tables form the basis of ISO standard 7730 Note however that his method for PMV is inconsistent with the basic assumptions of his equation (Humphreys and Nicol 1995).

Fanger realised that the vote predicted was only the mean value to be expected from a group of people, and he extended the PMV to predict the proportion of any population who will be dissatisfied with the environment. A person's dissatisfaction was defined in terms of their comfort vote. Those who vote outside the central three scaling points on the ASHRAE scale were counted as dissatisfied. PPD is defined in terms of the PMV, and adds no information to that already available in PMV. The distribution of PPD is based on observations from climate chamber experiments and not from field measurements.

The model for Standard Effective Temperature also uses skin temperature as part of it's limiting conditions, but uses skin wettedness (w) rather than sweat rate for the other limiting condition. The values for Tsk and w are derived from the Pierce `two-node' model of human physiology (see Nevins & Gagge (1972)). SET relates the real conditions to the (effective) temperature in standard clothing and metabolic rate and 50% RH which would give the same physiological (??) response. Effective temperature can then be related to subjective response.