Instruments for measuring physical quantities of the environment Standard EN ISO 7726 Accepted in 2001 Marianna Luoma

Scope The standard specifies the minimum characteristics of instruments and methods for measuring physical quantities characterizing an environment.

Physical quantities characterizing the environment Air temperature Mean radiant temperature Humidity of the air Air velocity Surface temperature

Characteristics of measuring instruments, examples Required accuracy given for different quantities to be measured: Air temperature: required ± 0.5 ºC, desirable ± 0.2 ºC Mean radiant temperature: required ± 2 ºC, desirable ± 0.2 ºC Air velocity: required ± (0.05 + 0.05 va) m/s, desirable ± (0.02 + 0.07 va) m/s

Recommended measuring heights Locations of the sensors Sitting person Standing Head level 1,1 m 1,7 m Abdomen level 0,6 m Ankle level 0,1 m 0,1

1. Measurement of air temperature Types of temperature sensors Expansion thermometer Liquid expansion thermometer Solid expansion thermometer Electrical thermometer Variable resistant thermometer Thermometer based on the generation of an electromotive force Thermomanometers Variation in the pressure of a liquid as a function of temperature

2. Mean radiant temperature Mean radiant temperature is measured by the black globe thermometer

3. Measurement of air humidity General characteristics of humid air At any given temperature, air cannot hold more than a certain amount of water vapour. Beyond that amount, the water vapour condenses. As the temperature of the air increases, so does the maximum amount of water vapour it can hold. Humidity can be expressed as absolute humidity and relative humidity Humidity is measured with hygrometers

Hygrometer types Dew-point hygrometers Condensation of the water vapour contained in the air on a mirror cooled to the dew-point of the mixture Electrical conductivity variation hygrometer Measuring the variation in electrical conductivity or electrical capacity of the sensor Absorption hygrometer (hair type) Psychrometer Cooling of a wet thermometer in a current of air by evaporation

This is page not in the standard! Vaisala HUMICAP® Sensor, a thin-film polymer sensor: Operating principle *The thin-film polymer either absorbs or releases water vapor as the relative humidity of the ambient air rises or drops. *The dielectric properties of the polymer film depend on the amount of water contained in it: As the relative humidity changes, the dielectric properties of the film change, and so the capacitance of the sensor changes. *The electronics of the instrument measure the capacitance of the sensor and convert it into a humidity reading.

Psychrometer: Principle of operation A psychrometer consists of two thermometers and a device to ensure ventilation of the thermometers at a minimum air velocity. The first thermometer is an ordinary thermometer indicating the air temperature ta. This will be referred as the ”dry” temperature of the air as opposed to the ”wet” temperature indicated by the second thermometer. This picture is not in the standard!

Psychrometer… The latter consists of a thermometer surrounded by a wet wick generally made from cotton. The end of the wick lies in a container of water. The water is raised by capillary attraction from the container to the thermometer and then evaporates at a rate dependent upon the humidity of the air. This results in a greater cooling of the thermometer the drier the air. The temperature indicated by the thermometer surrounded by the wet wick is referred to as the wet temperature (psychrometric) tw.

Psychrometer… The onserved dry temperature and the wet temperature are used in the determination of the absolute humidity of the air. Psychrometric chart or Mollier -diagram can be used to direct determination of the absolute or relative humidity of the air.

4. Measurement of air velocity Air velocity fluctuates randomly and most often changes its direction. Air velocity can be determined by an omnidirectional probe. Velocity is measured by anemometers, most often the anemometer has a hot-sphere type sensor.

5. Measurement of surface temperature Surface temperature is measured By a sensor in contact with the surface By an infrared sensor