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Atmospheric Instrumentation M. D. Eastin
Principles of Measurement and Instrumentation
Atmospheric Instrumentation M. D. Eastin
Outline
Principles of Measurement and Instrumentation
• Basics about Sensors and Instruments• Instrument Performance Characteristics• Measurement Standards• Interpretation / Reporting of Measurements• Preliminary Data Analysis
Atmospheric Instrumentation M. D. Eastin
Components of a Measurement System:
Instrument: A physical device or system, used to measure or monitor a measurand, that containsa sensor, an energy transfer device, an amplifier, a datadisplay, and a storage device
Parameter: Physical quantity to be measured (ex: pressure). Also called the measurand
Sensor: Device which responds directly to changes in the measurand and produces an output signal. Typical output signals include: (1) mechanical deflection,
(2) a rotation rate, (3) a voltage, (4) a resistance, or (5) a frequency
Primary input: Desired (ex: barometer measures pressure)Secondary input: Undesired / unavoidable (ex: barometer sensitive to wind)
Transducer: Device that converts an output signal to a more useful signal for convenient signal processing, display, transmission, and/or recording
Analog signal: Sensor output that fluctuates continuously with the measurandDigital signal: Sensor output that fluctuates in discrete steps for a given small
change in the measurand
Basics about Sensors and Instruments
Atmospheric Instrumentation M. D. Eastin
Components of a Measurement System:
Instrument: A physical device or system, used to measure or monitor a measurand, that containsa sensor, an energy transfer device, an amplifier, a datadisplay, and a storage device
Amplifier: Device that magnifies a small output signal into a larger signal, that extendsover a greater range, to better discern small changes in the measurand
[ These devices are common in most instruments due to a number ][ of electrical engineering considerations (see Chapter 3)
]
Meter: Displays the “final” (converted and amplified) output signal - digital or analogOften converts the electrical signal (ex: voltage) into physical units (ex: °C)
Recorder: Device employing a retrieval method whereby successive meter readings can be preserved (e.g., paper, film, computer hard drive)
Transmitter: Device that transfers any electronically recorded data to another site viawired or wireless communication systems (ex: phone lines or satellites)
Basics about Sensors and Instruments
Atmospheric Instrumentation M. D. Eastin
Instrument PerformanceCharacteristics
Atmospheric Instrumentation M. D. Eastin
Instrument Performance CharacteristicsTerms and Definitions:
Static Characteristics: Those that do not change in time. Examples include: (1) bias,(2) accuracy, (3) precision, (4) resolution, (5) sensitivity, and (6) dynamic range → all of these will be defined soon!
Specific values determined through careful calibration
Dynamic Characteristics: Those that involve changes with time. Examples include: (1) response time, and (2) drift
Specific values determined through careful calibration
Exposure Characteristics:Those resulting from unique local conditions at the site where an instrument is located. Examples include: (1) conduction (2) radiation effect, (3) wind effects, (4) wetting effects,
and (5) obstructions
Often dynamic in nature since they are related to changes inlocal weather (rain -- sunshine) and obstructions (trees grow)
Cannot be determined through careful calibration – Must be accounted for by instrument design and careful siting
Atmospheric Instrumentation M. D. Eastin
Instrument Performance CharacteristicsTerms and Definitions:
Calibration: A series of performance tests between a new instrument and a high-quality instrument used to determine performance and uncertainty characteristics
These tests can be conducted in a controlled laboratory setting or in the field through a series of inter-comparisons (or “buddy checks).
Laboratory Calibration Field Calibration
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Systematic Error: A consistent / repeated offset in a measurement as a result of a fixedor regular discrepancy in the instrument response
Also called a “bias” – can be accounted for or removed
Random Error: Variations in measurement due to statistical fluctuations in the quantitysensed, the internal operation of the instrument, or some combinationof the two – cannot be removed
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Accuracy: A measure of the overall uncertainty in the value of the measured parameter, when compared against an external standard – determined by the combination
of random and systematic errors
Precision: An instrument is precise if, in repeated trials, it is able to give the same output response for the same input parameter – determined by systematic errors
Note: A precise instrument can still be inaccurate
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Resolution: The smallest change in a parameter measureable by an instrument
Sensitivity: Ratio of the instrument response to a unit change in the parameter sensed
Linear response: Accuracy is the same regardless of parameter magnitudeResponse is characterized by a straight line
Non-Linear response: Accuracy varies with parameter magnitudeResponse is characterized by a polynomial function
Linear Non- Linear
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Dynamic Range: The full range of parameter values over which an instrument can detect a measureable response
Instruments unable to measure the full range of possible values will “saturate” at a maximum / minimum measureable value and report no further variation
An instrument with insufficient dynamic range
“Saturation” occurred atmaximum
measurablevalue
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Response Time (τ): How long an instrument takes to respond by a required amount to a sudden / step change in the parameter being sensed.
Requiredamount
Suddenchange
Full Response Time (3τ)
“Long”
“Short”
“Average”
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Response Time (τ): How long an instrument takes to respond by some fractional amount to a typical oscillation in the parameter being sensed.
“Long”“Small Fraction”
“Short”“Large Fraction”
“Average”
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Drift Error: Due to physical changes that occur in the sensor with time. These errors are difficult to detect during initial calibration, and are often compensated through
frequent calibrations
ASOS instruments are calibrated at least once per year to account for any drift
Days
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Examples: Davis Instrument Vantage Pro-2 surface weather station
See Pages 3-8 in Davis-Vantage-Pro2-Specfication.PDF on course website (Page 6 for Temperature Sensor performance shown below)
Resolution
DynamicRange
ResponseTime
Accuracy
??? +2 slides
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Examples: Davis Instrument Vantage Pro-2 surface weather station
Which of the following sensors exhibit → linear sensitivity?→ non-linear sensitivity?→ How do you know?
Pressure
Humidity
Temperature
Reported (Expected) AccuracyParameter
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Exposure Error: Due to imperfect coupling between the sensor and the desired parameter to be measured – site specific – cannot be accounted for in calibration
Example: A thermometer designed to measure air temperature willnever exactly measure the true air temperature due to anumber of errors introduced by thermometer mountingand exposure to other parameters that can influence the measured temperature
Stagnation heating(at low wind speeds)
(prevents a regular air flow)(that provides “offset cooling”)
(of any conduction heating)
Solar heating(in direct sunlight)
Evaporative Cooling(when wetted)
(by rain or dew)
Conduction heating(due to solar heating )(of mounting brackets)
Infrared heating(close to a “heated”)
(building)
Instrument Performance Characteristics
Atmospheric Instrumentation M. D. Eastin
Terms and Definitions:
Exposure Error: Example: Reduced by placing instruments away from large buildings and in well-designed fan-aspirated shelters
Rain shield
Sensor
ConcentricAir Intakes
Fan
Fan Aspiration(maintains regular)(flow of ambient air)
(prevents)(stagnation heating)
Rain Shield(prevents sensor wetting)(and evaporative cooling)
Bright WhiteShield
(Limits both )(solar heating)
(and) (conduction heating)
(by)(mounting hardware)
“Open” exposure(Placement away from)
(buildings)(limits)
(IR heating)
Atmospheric Instrumentation M. D. Eastin
Measurement Standards
Atmospheric Instrumentation M. D. Eastin
Measurement StandardsCalibration Standards:
• In the United States, the “calibration standards” (or high-quality instruments) are maintainedby the National Institute of Standards and technology (NIST)
• Each calibration laboratory must acquire their “standards” from NIST• Each country has its own “standards” institute
Performance Standards:
•A common set of methods used to determine instrument specifications, such as(1) sensitivity, (2) resolution, (3) dynamic range, (4) accuracy, and (5) time constant
•In the United States, the American Society of Testing and Materials (ASTM) establishand maintain these standards
Procedural Standards:
• A common set of methods and documentation, such as (1) averaging periods, (2) wireless communication frequencies, and (3) station description “metadata” that documents each station’s characteristics (location, land cover, exposure, instruments, calibration dates)
• World Meteorological Organization (WMO) sets averaging periods (wind speed → 10 min)• International agreements set communication frequencies (wx balloons → 400-405 MHz)• National Climate Data Center (NCDC) maintains station metadata files
Atmospheric Instrumentation M. D. Eastin
Measurement StandardsExposure Standards: Surface Stations
• World Meteorological Organization (WMO) sets exposure requirements for each parameter
Wind Instruments → Mounted at 10-m above ground level in open terrain, whereby the shortest distance between the anemometerand an obstruction (buildings, trees, etc.) must be at leastten times the height of the obstruction (e.g., at least 100-maway from a 10-m tall tree). No rooftop mounting!
PTH Instruments → Mounted inside a radiation / rainfall shield, with or withoutforced ventilation, at a height of 1.5 to 2.0-m above
ground level in open terrain, whereby the site is not located on a
steep slope or in a depression where thermal conditions might
not be representative of the larger scale. No rooftop mounting!
Precipitation → All gauges should be situated in open terrain with a gauge-mounted wind screen to minimize the effects of blowing
windon “gauge catch” – especially in areas expecting snowfall.Any frozen precipitation must be melted to obtain a liquid equivalent
Atmospheric Instrumentation M. D. Eastin
Measurement Standards
NOAAASOS
SurfaceStation
(operational)
Conformsto WMO
Standards
Most arelocated
at airports
Precipitation(heated)
(tipping bucket)(with)
(wind screen)
Wind Speed / Direction(cup anemometer)
(and wind vane)(at 10 m)
Temperatureand
Humidity(fan-aspirated)
(at 1.5 m)
Pressure(at 1.5 m)
Note the open and level terrain
away from obstructions
Atmospheric Instrumentation M. D. Eastin
Interpretation and Reportingof Measurements
Atmospheric Instrumentation M. D. Eastin
Interpretation and Reporting of MeasurementsWhat does an instrument REALLY measure and then RECORD?
•Most automated (electronic) instruments collect measurements (every 5-10 seconds)•Surface weather stations are required to report once per hour (usually at 5 min to the hour)
What happens to the 600+ individual measurements made each hour by the sensor?
Atmospheric Instrumentation M. D. Eastin
What does an instrument REALLY measure and then REPORT?
•Most automated (electronic) instruments collect measurements (every 5-10 seconds)•Surface weather stations are required to report once per hour (usually at 5 min to the hour)
What happens to the 600+ individual measurements made each hour by the sensor?
• These individual observations are NOT reported• Many are used to estimate a sample mean (X̅@ ) over some time interval
where: Xi = one individual observation
N = total observations collected in specified time interval
The instrument records this sample mean!
N
iiN X
NXXX
NX
121
1)...(
1
Interpretation and Reporting of Measurements
Atmospheric Instrumentation M. D. Eastin
How should YOU report the observation?
•Since we now know that an instrument’s “measurement” is really an average of manyindividual observations, it is important to include the expected variability in thereported value of that parameter
•If the instrument only reports a sample mean (X̅G ), how do we know the variability (σ) associated
with that sample mean?
X
where: μ = true meanX̅$ = sample meanσ = expected variability
Interpretation and Reporting of Measurements
Atmospheric Instrumentation M. D. Eastin
How should YOU report the observation?
•Since we now know that an instrument’s “measurement” is really an average of manyindividual observations, it is important to include the expected variability in thereported value of that parameter
•If the instrument only reports a sample mean (X̅G ), how do we know the variability (σ) associated
with that sample mean? → We don’t!
•However, we do know the expected variability associated with any sample mean observed by
that instrument → the reported accuracyobtained via calibration
X
where: μ = true meanX̅$ = sample meanσ = expected variability
Interpretation and Reporting of Measurements
Atmospheric Instrumentation M. D. Eastin
How should YOU report the observation?
•Suppose the temperature sensor on a Davis InstrumentsVantage Pro-2 reports the following:
T̅$ = 18.753°C
•We know from the specifications manual:
σ = ±0.5°C → sensor accuracy (see page 6)
•Thus, after accounting for significant figures,the reported observation should be:
18.8 ± 0.5 °C
Note: See Chapter 2 in the text for how to account for expected variability whenmultiple observed parameters are used to calculate another parameter
Example: Vantage Pro-2 stations calculate dewpoint temperature from theobserved temperature and relative humidity
X
where: μ = true meanX̅$ = sample meanσ = expected variability
Interpretation and Reporting of Measurements
Atmospheric Instrumentation M. D. Eastin
How should YOU report the observation?
•Such variability should also be reported in graphical displays using error bars
X
where: μ = true meanX̅$ = sample meanσ = expected variability
Incorrect(without error bars)
Temperature (°C) Temperature (°C)
Correct(with error bars)
Interpretation and Reporting of Measurements
Atmospheric Instrumentation M. D. Eastin
Preliminary DataAnalysis
Atmospheric Instrumentation M. D. Eastin
Preliminary Data AnalysisA very important step!
•All reported / recorded observations should be checked:
• Do the observations fall within the expected range• Are there any unphysical outliers that need to be removed.• Calibration procedures are not perfect!• Exposure errors can never be completely removed!
•This step is often called quality control and can be automated, but automation will only identify common errors, so a visual inspection is always wise
Step 1: Convert output signal to physical units
•This step is accomplished by applying transfer equations obtained during calibration(see the next slide)
•Most instruments and/or instrument display software come pre-programmed with the transfer equations obtained during the original calibration
•Additional transfer equations may need to be applied to account for any drift error or exposure error determined during subsequent field calibrations
Atmospheric Instrumentation M. D. Eastin
Preliminary Data AnalysisStep 1: Convert output signal to physical units
Atmospheric Instrumentation M. D. Eastin
Preliminary Data AnalysisStep 2: Check for outliers and
out-of-range values
•This step is accomplished by plotting ALL observations as a function of time using the provided display software or custom software, and then methodically removing the suspicious observations
Atmospheric Instrumentation M. D. Eastin
Summary
Principles of Measurement and Instrumentation
• Components of Instrument (terms and definitions)
• Instrument Performance Characteristics• Static / Dynamic / Exposure• Calibration• Bias / Accuracy / Precision• Resolution / Sensitivity / Dynamic Range• Response Time / Drift• Exposure errors (types and reduction methods
• Measurement Standards• Winds / PTH / Precipitation
• Interpretation / Reporting of Measurements• What does an instrument really record?
• Preliminary Data Analysis• Methods and Steps
Atmospheric Instrumentation M. D. Eastin
References
Bradley, J.T., K. Kraus, and T. Townsend, 1991: Federal siting criteria for automated weather observations, Preprints 7th Symposium on Meteorological observations and Instrumentation, New Orleans, LA, American Meteorological Society, pp 207-210.
Brock, F. V., and S. J. Richardson, 2001: Meteorological Measurement Systems, Oxford University Press, 290 pp.
Brock, F. V., K. C. Crawford, R. L. Elliot, G. W. Cuperus, S. J. Stadler, H. L. Johnston, M.D. Eilts, 1993: The Oklahoma Mesonet - A technical overview. Journal of Atmospheric and Oceanic Technology, 12, 5-19.
Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp.
Helmes, L., and R. Jaenicke, 1985: Hidden information within series of measurement – four examples from atmospheric science, Journal of Atmospheric Chemistry, 3, 171-185
Huffman, G. J., and J. N. Cooper, 1989: Design issues in nearly real-time meteorological systems and sites. Journal of Atmospheric and Oceanic Technology, 6, 353-358.
Parker, D.E., 1994: Effects of changing exposure of thermometers at land stations, International Journal of Climatology, 14, 1-31.
Stokes, G.M., and S. E. Schwartz, 1994: The Atmospheric Radiation Measurement (ARM) Program: Programmatic background and design of the cloud and radiation test bed. Bulletin of the American Meteorological Society, 75, 1201-1221.