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Calculator: Comparison of sensor response time (τ 63.2%) versus
temperature error.
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Copyright©
2019 BARANI DESIGN Technologies s.r.o.
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Author: Jan Barani
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τ
pronounced "tau" is usually given in seconds and is defined as the
amount of time it takes for a meteorological sensor to respond to a rapid
change in a measurand. It is very important in measurement of temperature,
dew point temperature, humidity and air pressure. Radiosondes are especially
affected due to their rapid increase in altitude.
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The difference between sensor response time and sensor time
constant τ (tau) 63.2% article.
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CALCULATOR INPUTS
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Author
is the designer of the helical solar radiation shield for precision
atmospheric air temperature measurement. Lessons learned during its
development include the effects of sensor and shield response time on
measurement error as demonstrated in this calculator.
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1st Sensor + Solar Shield time constant τ 63.2% (seconds)=
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Sensor
response rate is usually given as τ (tau) 63.2 %. It is a physical property
of each sensor.
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2nd Sensor + Solar Shield time constant τ 63.2% (seconds) =
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Fluctuation in
Air temperature (°C) =
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You
can change the rate of air temperature change with these parameters to see
the effect air temperature change has on sensor error and radiation shield
error.
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Speed of Air Temperature fluctuation (sec) =
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Additional Fluctuation in Air temperature (°C) =
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Speed of additional True Air Temperature change (sec) =
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Time period =
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Time scale of plot
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10 minute average air temperature measurement error due to slow
sensor temperature response and radiation shield airflow restriction
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MEAN temperature measured over time period
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MAXIMUM
sensor temperature over time period
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MINIMUM
sensor temperature over 10 minutes
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RANGE
of recorded temperatures over time period
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ERROR in MEAN temperature reading
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ERROR
in recorded MAXIMUM temperature
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ERROR
in recorded MINIMUM temperature
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Apart from fan-ventilated shields, the helical MeteoShield Professional has the
lowest time constant (shortest response time) of any professional solar
radiation shield to date.
The advantage of naturally ventilated solar radiation shields over their
fan-ventilated counterparts is the fact that they don’t suck in airborne
water droplets and dirt onto the internal temperature and humidity
sensors. Fan-aspirated shields in
humid, wet and rainy conditions tend to saturate sensors with water to
produce unrealistically low air temperature readings during times of morning
dew, fog, drizzle, rain and snow. This is due to cooling effects from
evaporation. In the short-term, the opposite effect of humid air condensing
on metal temperature probes can beneficially temporarily reduce their
response time due to heating from water condensation onto the metal surface,
but as soon as the probe temperature nears equilibrium with air temperature,
the evaporative cooling will overshadow this effect to produce large
temperature measurement errors.
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Elapsed
Time (seconds)
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"True" Air Temperature
(°C)
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1st Sensor + Solar Shield
temperature (°C)
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2nd Sensor + Solar Shield
temperature Reading (°C)
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1st Sensor + Solar Shield
temperature error (°C)
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2nd Sensor +
Solar Shield temperature error (°C)
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Difference in instantaneous
temperature between sensors (°C)
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10min AVERAGE (Mean) =
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10min Maximum Value =
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10min Minimum Value =
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0
sec
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0.00 °C
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0.00 °C
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