Measurement and Quantification of Thermal Environments; ISO 7726 -Ergonomics of the Thermal Environment - Instruments and methods for measuring and monitoring physical quantities

Keywords: Microclimatic monitoring, Measurements, Physical quantities, Thermal Comfort.

Introduction

Measuring and quantifying the thermal environment is important to understand and predict how people will respond to them. The measurement of the thermal environment has been ongoing for centuries. In the modern context, evaluating the thermal environment with particular focus on how people respond, then Bedford [1] presents early guidance on measuring the thermal environment based on investigations into heat stress on battleships during World War 2. Ellis, Smith and Walters [2] updated Bedford’s work and Olesen and Madsen [3] provided guidance for the assessment of work spaces.

When evaluating the impact of the thermal environment on humans it is important to quantify the condition that the person is exposed to. For this we need to be able to accurately measure the thermal environment; air temperature, mean radiant temperature, relative humidity and air velocity, the four basic environmental parameters, with metabolic activity and clothing insultation being the two personal parameters. To gather this information suitable instruments are needed to measure these physical quantities. It is also necessary that there is consensus on the definition of the measurements and the required accuracy of any instruments measuring them. This can be done by standardization.

The International Standards Organization (ISO) brings together consensus on topics to provide Standards that can be used worldwide. ISO was set up in 1947 and has over 170 member countries. Its principles of a single representative organization from each country and a democratic system of voting support the notion of a democratic process of globalization and a fairly operating world market in a world economy. Within Technical Committees (TC) working groups (WG) develop and produce the Standards. The WG’s are made up of experts nominated by their national Standards bodies to represent their country. This international panel of experts work to establish consensus on the topic and aim to establish the best practice based upon the most up to date evidence and literature.

Standards concerning with the Thermal Environment are the remit of ISO TC159 SC5 WG1. This covers Standards for measurement and evaluation of cold (ISO 11079 Determination and interpretation of cold stress when using required clothing insulation (IREQ) and local cooling effects) [4], thermal comfort (ISO 7730 Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria) [5] and heat stress (ISO 7933 Analytical determination and interpretation of heat stress using calculation of the predicted heat strain index [6] and ISO 7243 Assessment of heat stress using the WBGT (wet bulb globe temperature) [7]). In addition to these evaluation Standards there are a series of supporting Standards which give detail about how to measure metabolic rate (ISO 8996) [8], Insulation of clothing (ISO 9920) [9] and measurement of the physical environment (ISO 7726) [10].

ISO 7726 -Ergonomics of the thermal environment - Instruments and methods for measuring and monitoring physical quantities specifies the minimum characteristics of instruments for measuring physical quantities characterizing an environment as well as the methods for measuring the physical quantities of this environment.

The basic physical quantities considered in the revision proposal of ISO 7726 are:

·         Air temperature, ta (°C)

·         Surface temperature, ts (°C)

·         Globe temperature, tg (°C)

·         Radiation, W (m-2)

·         Plane radiant temperature, tpr (°C)

·         Dew point temperature, td (°C)

·         Relative humidity, RH (%)

·         Wet bulb temperature, tw (°C)

·         Natural wet bulb temperature, tnwb (°C)

·         Air velocity, va (m·s-1)

In addition, specification for derived physical quantities is also given:

·         Mean radiant temperature, tr (°C)

·         Radiant temperature asymmetry, (°C)

·         Operative temperature, to (°C)

·         Partial vapour pressure, ∆tp (Pa)

·         Humidity ratio, g·(kg-1)

This International Standard gives definitions for the terms to be used in the methods of measurement, testing and interpretation, taking into account Standards already in existence. It details specifications relating to the methods for measuring the physical quantities which characterise thermal environments. It also provides guidance for factors that could impact the measurement of a given physical quantity

Accuracy

When selecting measurement instruments for the evaluation of the thermal environment, particularly those where people will be working or living it is important that they have an appropriate level of accuracy. Accuracy defines how close the reported measured values are to the true value. ISO 7726 provides required accuracy tolerances and desirable tolerances. The required accuracy can be considered as the minimum level of accuracy whilst the desirable accuracy can be considered as the optimal accuracy for that physical quantity. Table 1 details the basic parameters for the quantification of the thermal environment and the measurement tolerances required for each.

By specifying the accuracy required of a measurement for a physical quantity rather than the method for measurement of that physical quantity the Standard does not restrict any specific existing or future measurement technology. Air temperature can therefore be measured by a mercury-in-glass thermometer, thermocouple, resistance thermometer, thermistor or alternative providing it can measure to a required accuracy of ±0.3°C or desirable accuracy of ±0.1°C.

The Standard also considers the specification of accuracy depended upon application, so has two measurement classes, C and S. Class C (Comfort) is defined for moderate thermal environments (buildings, vehicles etc) and Class S (Stress – cold or heat) for more extreme thermal environments (cold stores, foundries etc). Considering air temperature then the range of operation for Class C measurement would be 10°C – 35°C and for Class S -60°C – 150°C.

Response time

The time taken for a measurement instrument to reach a stable output within the environment also important. Any instrument needs to respond within a suitable timeframe. ISO 7726 provides standardised conditions and procedure for the determination of time constants for instruments. From a practical perspective, it is worth noting that if there is a large difference between the environment that an instrument was in and that which it is moved to for measurement, i.e., air temperature 20°C to -10°C then the response time can differ.

Measurement considerations

ISO 7726 also considers the practicalities of measuring the physical quantities in real world situations. An environment may be considered to be homogeneous if, air temperature, mean radiant temperature, mean air velocity and partial vapour pressure are practically uniform around the person. Often the case in many built environments. It can then be assumed that a limited number of measurements will be required to quantify the physical environment. In many spaces, particularly industrial areas, the environment is more likely to be heterogeneous. Then the physical quantities shall be measured at several locations at or around the person or in a position representative of the occupation and account taken of the partial results obtained in order to determine the mean value of the quantities to be considered in assessing the comfort or the thermal stress.

Spatial variation

Deviation in air temperature, mean radiant temperature, or air velocity, within the space should be considered, both from a horizontal and vertical perspective. Considering the person, there can be variation from the feet to the head, then three measurements should be taken; at 1.1 m (head), 0.6 m (abdomen) 0.1 m (ankle) for seated occupants and 1.7 m (head), 1.1 m (abdomen) 0.1 m (ankle) for those standing.

Temporal variation

The monitoring of indoor environments is an important for the evaluation of the thermal comfort in moderate environments and the prevention of stress in hot and cold severe workplaces. Thermal environment measurements should be carried out in the building (or in a given working position) at a representative sample of locations where the occupants are known to, or are expected to spend their time. In addition, the choice of the measurement points should be addressed to highlight the presence of possible heterogeneities (e.g. potentially occupied areas near windows, diffuser outlets, heat sources, seasonal aspects). When parameters in the space surrounding people are not homogeneous, measurements are made at the position where the parameters are estimated to be the highest. Finally, the measurement periods shall represent a sample of the total occupied hours in the period selected for the evaluation, i.e., 8.00am, 11.00am. 14.00pm, 17.00pm, to capture the timeframe where people are in the space.

Other Standards ISO 7730, ISO 7933 or ISO 11079 provide information about the application of measurements obtained in relation to those people working or living in such spaces.

Practical implications for the assessment of thermal comfort

The marketplace offers an impressive amount of inexpensive measurement devices that often fall short of expectations [11]. As such, the metrological characteristics summarised in Table 1 are crucial when selecting the most accurate instruments and techniques for the objective evaluation of microclimatic conditions. This applies both in moderate environments, where high IEQ levels should be warranted, and in severe environments (cold and hot), where health protection is the main goal [11,13]. In fact, the measurement uncertainty affects the evaluation of thermal comfort [12,14] and stress indices [13].

Table 1. Characteristics of measuring instruments.

More particularly, the global thermal comfort requirements for mechanically conditioned buildings considered by ISO 7730 and EN 16798-1 [15] for each environmental category in Table 2 refer to the specific ranges of PMV values in Table 3.

Table 2. IEQ categories description according to EN 16798-1.

Table 3. Categories of overall thermal comfort according to ISO 7730 and EN 16798-1 as a function of the PMV value. PD=percentage of dissatisfied.

Well, what usually professionals and even researchers overlook at assessment stage is the effect of metrological features of instruments on the final PMV value. Figure 1 depicts the PMV sensitivity to measurement errors of the physical quantities (air temperature, mean radiant temperature, air velocity, and relative humidity) under thermal neutrality conditions (e.g., PMV=0). As clearly shown, while the measurement of the air temperature and the relative humidity scarcely affects PMV, the measurement of the air velocity and the mean radiant temperature with instruments characterised by the lowest accuracy (required accuracy) bring PMV uncertainties of a pair of decimals. That means an unacceptable environmental classification because the final uncertainty of the PMV is of the same magnitude as the category I width [14].

Figure 1. PMV uncertainty related to the physical quantities under neutral conditions (PMV=0 in winter and summer conditions). Basic clothing insulation value: 0.60 clo (1.0 clo) in summer (winter); air velocity value: 0.1 m s-1; metabolic rate value: 1.2 met; relative humidity value: 60% (40%) in summer (winter).

It is also important to note that qualified and certified staff should calibrate instruments regularly. In addition, the assessment or measurement of the subjective quantities (such as clothing insulation and metabolic rate) adds another degree of uncertainty to the PMV calculation.

Overview

To fully understand the impact of thermal environment on people’s comfort and health it is important to precisely quantify the physical parameters (air temperature, mean radiant temperature, air velocity and relative humidity). This can only be achieved accurately with appropriately specified measurement equipment. ISO 7726 -Ergonomics of the thermal environment - Instruments and methods for measuring and monitoring physical quantities provides a framework for defining the physical parameters and the measurement accuracy required for each parameter. It also gives a guidance on the practical assessment of working environments for people. It is a fundamental Standard for those wanting to assess thermal environments for people, whether it be in the workplace, homes, or outdoor spaces.

References

[1]     Bedford T (1946) Environmental warmth and its measurement, Med. Res. Coun. War Memo., No. 17. H.M.S.O., London.

[2]     Ellis, F.P., Smith, F.E. and Walters, J.D. (1972) ‘Measurement of environmental warmth in SI units’, 29(4), pp. 361–377.

[3]     Olesen, B.W. and Madsen, T.L. (1995) ‘Measurements of the physical parameters of the thermal environment’, Ergonomics, 38(1), pp. 138–153.

[4]     ISO 11079 (2007) Ergonomics of the thermal environment - Determination and interpretation of cold stress when using required clothing insulation (IREQ) and local cooling effects, International Organization for Standardization, Geneva.

[5]     ISO 7730 (2005) Ergonomics of the thermal environment – Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria, International Organization for Standardization, Geneva. Currently under revision.

[6]     ISO 7933 (2023) Ergonomics of the thermal environment – Analytical determination and interpretation of heat stress using calculation of the predicted heat strain, International Organization for Standardization, Geneva.

[7]     ISO 7243 (2017) - Ergonomics of the thermal environment - Assessment of heat stress using the WBGT (wet bulb globe temperature) index, International Organization for Standardization, Geneva.

[8 ISO 8996 (2021) Ergonomics of the thermal environment - Evaluation of metabolic rate, International Organization for Standardization, Geneva.

[9]     ISO 9920 (2007) Ergonomics of the thermal environment - Estimation of thermal insulation and water vapour resistance of a clothing ensemble, International Organization for Standardization, Geneva.

[10]   ISO 7726 (1998) Ergonomics of the thermal environment - Instruments and methods for measuring and monitoring physical quantities, International Organization for Standardization, Geneva. Currently under revision.

[11]   d’Ambrosio Alfano, F.R., Dell’Isola, M., Ficco, G., Palella, B.I., Riccio G. (2022). ‘Small globes and pocket heat stress meters for WBGT and PHS evaluations. A critical analysis under controlled conditions’, Building and Environment 226, 109781.

[12]   Dell'Isola, M., Frattolillo, A., Palella, B.I., Riccio, G. (2012). ‘Influence of Measurement Uncertainties on the Thermal Environment Assessment’, International Journal of Thermophysics (33), pp. 1616-32.

[13]   d’Ambrosio Alfano, F.R., Palella B.I., Riccio, G. (2007). ‘The role of Measurement Accuracy on the Heat Stress Assessment according to ISO 7333: 2004’, WIT Transactions on Biomedicine and Health (11) pp. 115-24.

[14]   d’Ambrosio Alfano, F.R., Palella, B.I., Riccio, G. (2011). ‘The Role of Measurement Accuracy on the Thermal Environment Assessment by means of PMV Index’, Building and Environment 46(7), pp. 1361-69.

[15]   EN 16798 - Part 1 Energy performance of buildings. Ventilation for buildings. Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics. Module M1-6.