Transcript CE/ARE 397 Indoor Air Quality: Field Measurements

Schedule of classes in PRC
I need your availability on
- Wednesday
- Thursday
- Friday
afternoon
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Objectives
• Compare lab and field work
• Examples
• Introduce measurement terminology
• Discus quality control
Lab Measurement
• Strictly design experiments
• Focus on maintaining one group of parameter in a controlled
environment to measure other
Field Measurements
• Often the only way to document the real world
• Often conducted in conjunction with laboratory measurements
• Many phenomena can not be meaningfully modeled or
reproduced in the laboratory
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Lab vs. Field measurements
Field results
Lab results
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Las class Example Lab work
Convection Correlation Development
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Example of Field Work:
Energy Implications of Filters
• Does using a better filter increase energy use?
• Conventional wisdom: Yes
• For smaller buildings: Maybe not
• Flow, fan energy, system energy, SHR, AC capacity
• All DECREASE
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Experimental Design
• Why can’t this study be done in a laboratory?
• Monthly measurements in 17 buildings over
the course of a year with different filters
installed
• Additional measurements in test house
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Instrumentation
• Power draw
• Fan and compressor
• Pressure drop
• Filter and coil
• Temp. and RH
• Capacity
• Fan flow
• Duct leakage
• Major issue?
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Summary
• Fieldwork is very messy
• Confounding variables and outliers
• Need large sample sizes
• Expensive and time consuming
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Terminology
• What is the difference between accuracy and
precision?
• Note that these terms are often confused and conflated with
other terms
• Accuracy – “Capability of an instrument to indicate
the true value of a measured quantity.”
• Precision – “Repeatability of measurements of the
same quantity under the same conditions; not a
measure of absolute accuracy”
• Precision not often reported
Reference ASHRAE Guideline 2
Terminology
• Example of accuracy and precision:
High accuracy,
low precision
Low accuracy,
High precision
Good measurement result is both: accurate and precise
Some Comments about
Instrument Accuracy
• Manufacturers are almost always optimistic
• Make the difference between accuracy defined for
full scale and reading
Instrument 1:
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Accuracy: ±1.5% of full scale
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Repeatability: ±0.5% of full scale
Instrument 2:
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Accuracy: ±1.5 % of reading
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Repeatability: ±0.5%
(limited in certain range)
• What is Repeatability?
Some Comments about
Instrument Accuracy
• Accuracy is rarely constant over Range
• Assume frequent calibration
• Requires standard
• Calibrate over range of interest
• Don’t use complicated calibration curves
• Anything other than linear requires justification
• Consider arrangement with multiple sensors
Example of built-in calibration
system
• Automatic Tracer Gas Monitor
Measured
variable
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0
signal
Other things that you should care
about
Sensitivity
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Sensitivity of the
sensor is defined as
the slope of the
output characteristic
curve
Thermistors
Resistor
(1/Voltage)
Temperature range
Which one is more sensitive?
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Other things that you should care about
• Response time
Examples
Standard
definition
Can be defined for other % values
Other things that you should care about
• Response time
• Hobo U12 internal temperature sensor
• Response time in airflow of 1m/s (2.2mph)
• 6 minutes, typical to 90%
• Telaire 7001 CO2 sensor
• <60 seconds to 90% of step change
• How do you use these values?
Other things that you should care
about
Hysteresis
• Sensor should follow the
changes of the mesured
parameter regardless of
which direction the
change is made; hysteresis
is the measure of this
property
How this affects the instrument accuracy?
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Other things that you should care about
Resolution
• the smallest detectable incremental change of input parameter that can be
detected in the output signal
• Hobo U12 internal relative humidity sensor
• 0.03% RH
• Telaire 7001 CO2 sensor
• ±1 ppm
• How do you use these values?
• Note that resolution can be limited by data logger
Other things that you should care about
• Range and detection limit
• How do you use these values?
• Note that you are often trading off range and
resolution and/or accuracy
• Example:
• Measuring CO2 with
Telaire 7001 CO2 sensor
Other things that you should care
about
Example:
In our test house we use CO2 as tracer gas
We use Telaire 7001 CO2 sensor for
concentration measurement
What is the range accuracy
and detection limit?
http://www.microdaq.com/telaire/index.php
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(Some) Real World Concerns
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First and operating cost
Ease of use
Safety
Durability
Flexibility
Reliability
Power requirements
Environmental requirements/conditions
Quality Assurance (QA)
Quality Control (QC)
How to incorporated QA/QC into your experimental study?
Experiment Design Phase:
• Define objective
- What question are you trying to answer?
- How will you know you are finished?
• Choose
- Factors of interest
- Parameters to measure
- Experiments control method(s)
- The data analysis techniques
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How to incorporated QA/QC into
your experimental study?
Experiment Design Phase:
For measured parameters
consider:
- Range
- Number of points
- Number of repetitions
Create an experimental matrix
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How to incorporated QA/QC into
your experimental study?
Experimental matrix
Be real:
- Consider available time and funding
- Predict potential for failure
- predict more experiments than minimum
- predict extra time for repetition
- Preliminary experiments help
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How to incorporated QA/QC into
your experimental study?
Measuring Phase:
- Use measuring techniques that will meet the needs of
your experiment
- Collect sufficient data (including repetition) to
adequately characterize the measured parameter
- Record all available conditions/parameters (even those
that are not in your matrix)
- Use experiment control methods
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How to incorporated QA/QC into
your experimental study?
Data Analysis Phase:
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Graphs & descriptive statistics first
Hypothesis testing
Regression next
Interpret the results
Draw conclusions
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How to incorporated QA/QC into
your experimental study?
Be ready to modify and/or go back and
forth between phases
Example…..
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