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Instruments for measuring
physical quantities of the environment
Standard EN ISO 7726
Accepted in 2001
Marianna Luoma
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Scope
The standard specifies the minimum
characteristics of instruments and methods
for measuring physical quantities
characterizing an environment.
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Physical quantities characterizing the
environment
1.
2.
3.
4.
5.
Air temperature
Mean radiant temperature
Humidity of the air
Air velocity
Surface temperature
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Characteristics of measuring
instruments, examples
Required accuracy given for different
quantities to be measured:
–
Air temperature:
–
Mean radiant temperature:
–
required ± 0.5 ºC, desirable ± 0.2 ºC
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
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Recommended measuring heights
Locations of the
sensors
Sitting
person
Standing
person
Head level
1,1 m
1,7 m
Abdomen level
0,6 m
1,1 m
Ankle level
0,1 m
0,1
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1. Measurement of air temperature
Types of temperature sensors
–
Expansion thermometer
–
Electrical thermometer
–
Variable resistant thermometer
Thermometer based on the generation of an
electromotive force
Thermomanometers
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Liquid expansion thermometer
Solid expansion thermometer
Variation in the pressure of a liquid as a function of
temperature
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2. Mean radiant temperature
Mean radiant
temperature is
measured by the black
globe thermometer
tg =
t r + 2.35 t a v
1 + 2.35 v
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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
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Hygrometer types
Dew-point hygrometers
–
Electrical conductivity variation hygrometer
–
Condensation of the water vapour contained in the air on a
mirror cooled to the dew-point of the mixture
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
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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.
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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!
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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.
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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.
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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.
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5. Measurement of surface
temperature
Surface temperature is measured
–
–
By a sensor in contact with the surface
By an infrared sensor
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