Thermal Comfort
ISO Standard 7730 appropriate RH range:
30-70%
A lithium chloride cell
A lithium chloride cell absorbs water vapour from the air increasing its electrical conductivity which is then measured.
A nomogram to measure the MRT
A nomogram can then be used to calculate the MRT, which corrects the globe bulb reading for air velocity and air temperature.
Thermal Hydrograph
A thermo-hygrograph or hygrothermograph is a chart recorder that measures and records both temperature and humidity (or dew point). The Thermo Hydrograph measures air temperature and relative air humidity by showing a graphic recording of the two measurements which allows reliable recording on a double scale chart of humidity and temperature.
Thermal Comfort
According to the ANSI/ASHRAE Standard 55-2010, thermal comfort is defined as "that condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation."Also known as human comfort, thermal comfort is the occupants' satisfaction with the surrounding thermal conditions and is essential to consider when designing a structure that will be occupied by people.
Air velocity
Air speed. Air velocity influences the convective heat transfer, evaporation, and can cause draughts.
Heat transfer by convection
Air temperature can vary when determining heat transfer by convection through the person. The measurement of this quantity can in fact lead to errors if precautions are not taken (ISO 7726 p12). Prevent the thermometer from being exposed to radiation from other heat sources as the temperature measured would not be the actual temperature of the air but a temperature in between the air temperature, and the mean radiant temperature.
A consistent thermal environment
Air temperature, radiation, air velocity and humidity can be considered to be practically uniform around the worker. Air temperature is measured in degrees Centigrade
Non-contact Infrared Radiation (IR) thermometer
Direct infrared radiation from individual surfaces can be measured using a non-contact IR thermometer. The non-contact thermometer assumes all surfaces have an emissivity value of 0.95.
Calculation of Relative humidity using the wet and dry bulb thermometer
Dry bulb thermometer (Td) regular thermometer with a dry bulb. Wet bulb (Tw) - thermometer with bulb covered in clean grease free wick. Normally de-ionised water is used to wet the bulb.
Regnault's apparatus
Fill the container 1/3 with water and add crushed ice to the metal container slowly. The ice will cool the water and the metal container. This in turn cools the outside room air in contact with the container and eventually a fog is formed on the outside of the metal surface. The temperature at which the fog appears is known as 'the Temperature of appearance of fog' - this temperature is recorded. Cease addition of ice. After a short period, the fog will disappear this known as the 'temperature of disappearance of fog' and this temperature is recorded. More ice is added to the container to further reduce the temperature approx. by 0.5 - 1 degree. The 'Temperature of appearance and 'temperature of disappearance' is noted again. Repeat steps 3 - 5 above until the 2 temperatures (i.e. appearance and disappearance) are within ½ degree of each other. The average of those two temperatures = dew point temperature.
Hair hygrograph
Hydroscopic materials change dimension as the humidity changes i.e. human/horse hair.
Adaptive Comfort
If changes occur in the thermal environment to produce discomfort, then people will generally change their behaviour and act in a way that will restore their comfort. Such actions could include taking off clothing, reducing activity levels or even opening a window. In order to consider adaptive comfort, the space must have operable windows, no mechanical cooling system, and the occupants must be near sedentary and have a metabolic rate between 1.0 and 1.3 met. Also, occupants have the option of adding or removing clothing to adapt to the thermal conditions.
Relative humidity = saturated vapour pressure (SVP) at dew point divided by SVP at room temperature.
In the above example: Relative humidity = 14/20 = 0.7*100=70%
PR = Pw - A (Td -Tw)
PR = The pressure of the water vapour in the room, or S.V.P. @ dew point Pw = S.V.P. @ the wet bulb temperature (From graph) Td = Dry bulb temperature Tw = Wet bulb temperature A = constant depending on ventilation rate, A= 0.75 for still air, A= 0.5 for ventilated (sling)
Predicted Percentage of Dissatisfied (PPD)
Predicted Percentage of Dissatisfied (PPD) predicts the percentage of occupants that will be dissatisfied with the thermal conditions. It is a function of PMV, given that as PMV moves further from 0, or neutral, PPD increases. The maximum number of people dissatisfied with their comfort conditions is 100% and, as you can never please all of the people all of the time, the recommended acceptable PPD range for thermal comfort from ASHRAE 55 is less than 10% persons dissatisfied for an interior space.
Predicted Mean Vote (PMV)
Refers to a thermal scale that runs from Cold (-3) to Hot (+3), originally developed by Fanger and later adopted as an ISO standard. By subjecting a large number of people to different conditions within a climate chamber and having them select a position on the scale the best described their comfort sensation. A mathematical model of the relationship between all the environmental and physiological factors considered was then derived from the data. The result relates the size thermal comfort factors to each other through heat balance principles and produces a sensation scale.
Psychrometers (Sling -whirling hygrometer)
Rely on cooling produced by water evaporating from a fabric sleeve surrounding a thermometer bulb. The instrument consists of two thermometers and a device to ensure ventilation of the thermometers at a velocity greater than 4 m/s. The Relative humidity reading is read from an accompanying psychrometric chart or slide rule.
Mean Radiant Temperature (MRT)
The MRT is the temperature of a uniform imaginary enclosure in which radiant heat transfer from the human body is equal to the radiant heat transfer in the actual enclosure. The MRT is measured in degrees Centigrade.
Absolute humidity of air
The absolute humidity of the air characterises any quantity related to the actual amount of water vapour contained in the air as opposed to quantities such as the relative humidity or the saturation level, which gives the amount of water vapour in the air in relation to the maximum amount that it can contain at a given temperature and pressure. (ISO 7726, p3)
Clothing insulation (clo)
The amount of thermal insulation the person is wearing
Relative Humidity
The amount of water vapor in the air, expressed as a percentage of the maximum amount that the air could hold at the given temperature; the ratio of the actual water vapor pressure to the saturation vapor pressure.
Metabolic Rate (met)
The energy generated from the human body - activity level
The globe thermometer
The globe thermometer consists of a hollow black globe usually made of copper, in the center of which is placed a temperature sensor. Normally a 150 mm or 44 mm globe is used however other diameter sizes are available.
Kata Thermometer
The kata thermometer is suitable for the measurement of low air velocities (< 1m/s). It has a long response time, and thus averages air movement over sampling period. It consists of a stem, a shinny bulb containing alcohol with a red dye. The stem has an upper and lower graduation corresponding to a temperature drop of 3°C. The thermometer is heated in hot water to a temperature above the upper graduation, wiped dry, hung up in the area of interest and allowed to cool. The time taken for the temperature to cool from the upper to the lower graduation mark is recorded, this is known as the 'cooling time'. Air velocity is then calculated using the 'cooling time' the dry bulb temperature, a calibration factor (specific to the particular thermometer) using a nomogram.
ASHRAE 55
The recommended acceptable PMV range for thermal comfort from ASHRAE 55 is between -0.5 and +0.5 for an interior space.
ISO Standards 7726 and 7730.
The specific standards of measurement used for thermal comfort
Dew point
The temperature at which the air is saturated, by cooling the air until water vapor condenses on a surface.
Measurement of Heat Stress using the Metrosonics WBGT meter
The unit consists of a static wet bulb thermometer, a globe bulb thermometer (44 mm globe) and a dry bulb thermometer. The three readings are automatically linked to display the wet bulb globe temperature (WBGT).
Regulation 7 of the Safety, Health and Welfare at Work (General Application) Regulations 2007.
These regulations determine the parameters employers must maintain in order for workers to be productive in their workplace environment. Employers are required to maintain a temperature for workers which is appropriate for human beings, and to consider windows and ventilation.
Air Temperature
This is the temperature of the air, and can be measured using a mercury in glass thermometer, alcohol in glass thermometer, a bimetallic strip, a thermistor, a thermocouple, a platinum resistance thermometer, or a thermograph.
A bimetallic strip
Used to measure the air temperature. Bonding two metals with different thermal expansion coefficients can produce useful devices for detecting and measuring temperature changes.
Mercury in glass thermometer
Used to measure the air temperature. The volume of mercury changes slightly with temperature; the small change in volume drives the narrow mercury column a relatively long way up the glass tube.
0.7WB +0.3GT (indoors) or 0.7WB + 0.2 GT + 0.1DB (outdoors)
WB = wet bulb temperature (natural) °C GB = globe bulb temperature (44 mm diameter globe) °C DB = dry bulb temperature °C