solar radiation context/importance of solar radiation measurements –energy available to earth...

SOLAR RADIATION

• Context/Importance of solar radiation measurements– Energy available to Earth– Atmospheric circulation– Here: microscale energy balance: turbulence generation– Almost 50% of short-wave radiation absorbed by surface and changed into th-IR

• Types of radiation (irradiation W/m2)– Direct solar radiation– Diffuse solar radiation (scattering and recflections)– Global solar radiation

hemispherical (2 steradians)

– Spectral range from 300 to 3000 nm

SENSOR TYPES: AN OVERVIEW OF TECHNOLOGIES

• Direct solar radiation: Pyrheliometers– Tracking of solar position (azimuth and zenith angles)

• Net radiation level: Radiometers– Sum of direct and diffuse solar radiation minus reflected

• PPFD: Quantum sensors– Photosynthetic light spectrum or PPFD

• Total (global) radiation: Pyranometers– Thermopile arrays

• Electrical output prop. to absorbed radiation• Thermocouples in series or in parallel

– Photovoltaic detectors• Si – photodiodes (barrier-layer cell)• Potential at junction between two materials• Rapid response time

OUR CHOICE:THE DAVIS SOLAR RADIATION SENSOR 6450

• General information– Si photodiode with wide spectral response (sealed)– Convection cooling of body, run-off path for water– Cutoff ring for cosine response, level indicater, alignment fins– In-build amplifier

• Manufacturer’s specifications– Power requirements: 3 VDC (10%); 1 mA; 3 mW– Analog output [0,+3VDC]; 1.67 mV per W/m2

– Operating environmental conditions:• Temperature: -40° to +65°C• Temperature corrections: coeff = 0.12% per °C; ref temp = 25°C

– Sensor specifications• Spectral response: 400 (300) to 1100 nm• Output Range: 0 to 1800 W/m2

• Cosine Response 3% (0°-70° i. angle); 10% (70°-80°i.a.)• Accuracy: 5% of full scale (ref: Eppley PSP at 1000 W/m2)• Drift: up to 2% per year

OUR CHOICE:THE DAVIS SOLAR RADIATION SENSOR 6450

• Sensor components and dimensions– Diffuser with ‘excellent’ cosine response– Cable (industrial version: 5 m)– Dimensions: 51 mm x 70 mm x 57 mm– Weight: 226 g

• Mounting– Top of stand– Needs flat surface with holes for fixing– 3 screws and springs for levelling– Fins for optical alignment

CALIBRATION REQUIREMENTSIMPROVED ACCURACY LOW-COST SENSORS

• Studies by King et al. at Sandia National Laboratories– Achieve better accuracy (3%) using photodiodes instead of expensive

thermopile devices (2%)

• Main factors influencing accuracy– Solar spectral influence– Solar angle-of-incidence

• Sensor’s response to direct irradiance influenced by cosine of solar AOI & optical characteristics of front surface

• Significant measurement errors at high AOI

– Operating temperature

• Calibration methodology– Determine AMa and AOI functions (empirical curves)

– Determine calibration constant Cn (in mV) for standardized AMa solar spectra, at a reference temperature To

– Apply corrections for an improved estimate of the total (broadband) irradiance Et:

CALIBRATION REQUIREMENTSIMPROVED ACCURACY LOW-COST SENSORS

• Solar spectral influence– Changes of solar spectral distribution over the day– Compute spectral mismatch parameter M (ASTM)

– Option to relate M to the absolute optical airmass AMa

and obtain a continuous spectral mismatch parameter

• Solar angle-of-incidence– Influence depends on pyranometer’s optical

characteristics (diffuser)– AOI function

• Operating temperature

• Other factors:– Mechanical and optical asymmetries, linearity of

response, path across pyranometer, response time,

stability

CALIBRATION REQUIREMENTSCLASSIFICATION & INDICATIONS

• WMO classification of Davis 6450 pyranometer– Second class pyranometer

• WMO calibration methods (using sun or lab source)– Comparison with standard pyrheliometer– Comparison with standard pyranometer

(natural conditions)– Comparison on laboratory optical bench– Comparison in laboratory integrating chambers– Calibrate in the normal position of use

• Comparison with reference pyranometer– Simultaneous operation of both instruments– Long period for representative results in typical conditions– Calibration factor: k = R kr (R-response ratio test/reference)– Take temperature into account– Davis sensors individually calibrated against secondary standard – standard

calibrated against Eppley Precision Spectral Pyranometer in natural daylight

SITE SELECTION CONSIDERATIONS

• Site selection– Site should be free from any obstruction above plane of sensing element– Shadow possibilities at any time of year have to be taken into account– Should be away from light-coloured walls or reflective objects– Should not be exposed to artificial radiation sources– Keep at certain distance from buildings or obstructions– Here: these considerations will probably not be taken into account– Avoid vibrations due to high winds, use rigid platform

• Installation– Careful optical levelling of instrument, check plotting irradiance throughout a day– Maximum cable length and weatherproofed junctions

• Operation– Sampling rate– Integration of data

MAINTAINANCE REQUIREMENTS

• Inspection frequencies (ASTM standards)– Daily for cleaning– Semi-annual for inclination angle verification– Annual for recalibration and general deterioration inspection

• Care and maintainance– Wipe dry and clean diffuser using cloth and ethyl alcohol– Gently remove solid deposits: frozen snow, frost or rime

• Recalibration– Observed annual drift of 2%, for higher accuracy, calibrate once a year– Basic calibration procedures should be employed if preserved standards

available– Quality control with long-term plots of solar noon irradiance value during clear

sky conditions

Si Photodiode Visible to UV

• What is a photodiode?

-> A component semiconductor able to detect a radiation, which collects the visible signal and transforms it into electric signal. Family of the active photoreceivers, it requires to be polarized in reverse by an external food.

-> 3 distinct areas are located:

• charging space (ZCE) • Neutral region of type N• Neutral region of type P


• Dimensional outlines

-> Side View:

-> Top View:

Width: 5,5 mm

Width window: 3,1 mm

Height: 16,5 mm

Distance connects: 2,55mm


• General Characteristics :

- package= 18mm

- Active area size= 1,2 mm2

- Reverse voltage(VRMax)= 30 Volts

- Operating temperature(Topr)= -40 to +100

- Storage temp.(Tstg)= -55 to +125 celsius

-> Analogical dispositif with borosilicate glass


• Electrical and optical Characteristics :

-Spectral response range from 320 to 1100 nm

- Peak (sensitivity wave length) lambda= 960 nm « spectral response »

- PhotoSensitivity (S)= 0,6 (A/W) « photosensitivity »


Warning: Photodiodes (Ge) have a more significant photosensitivity but their dark current is notable ID = 10 uA. It is thus preferable to use photodiodes (Si) (ID near 10 pA) for the detection of weak illuminations.

-Dark current ID= 2 pA

- Temp. coefficient of ID= 1,12 times / degree


• Electrical and optical Characteristics :

- Rise time : 0,4 micro sec. «rise time/ load resistance»

- Terminal capacitance(Ct)= 140 pF

- Shunt resistance (Rsh)= 5 to100 GOhm « shunt

resistance »

This device goes to necessite the use of an amplifier of impedance. About alternative technologies we have: laser, and every works with wavelength.


• Precision and accuracy: - Near the microseconde

- High reliability

- High linearity(->relationship between the collected ray and the electrical current product is strictly linear)

• Maintenance requirements

- The cellule must be always clean to intercept radiations clearly with no wrong ways and any shade about objects have to affect our measurements.

InfraRed Thermometer (IRT)

• Sensor to get Skin Temperature

Canopy Surfaces(Ground, Wall, Roof, …)

• Anything above «Absolute Zero» radiates in the IR

+ : Everything can be measured

- : Risk of Perturbations

Compounds Infomations

• IR Thermopile Detector Target

Wave Length : 5 m -14 m

Atmospheric Window (H2O, CO2, CH4, …)

• Temperature Sensor Environment

IRT Specifications

• Operating Power Requirements :

• Output Format :

- Voltage Output Conversion (Analog)

(expected ranges : 0 - 10V, 4 - 20 mA)

- CPU (chip) converts Analog signal into Digital

Voltage 3V 4.5V

Current 4 A 6 A

• Range of environmental Conditions :

- Temperature : Measurement Range -33° 220°C Operating Range -10° 50°C

- Emissivity of Materials (Function of T°) : High Emissivity : Plastic, Glass, Ceramics, Water, Soil, … Low Emissivity : Gold, Aluminium, shiny Objects

- Field of View : Distance of Sensor Spot Size on Surface

IRT Specifications

• Accuracy :

Thermopile Depends on neighbourhood Temperature

IRT Specifications

Error on Measurment when ambiant T° = 25°

T°

eM

Relative Measurment Error min : ambiant T° = target T°

+/- 15°

+/- 0.5°

Alternative technologies

• Thermocouple :

2 dissimilar Metals are joined

When Junction is heated (or cooled), a Voltage is produced

• Resistance Temperature Detector (RTD) :Composed of a Material which has well known «Temp – Res» Relationship. Most accurate Sensor, Stability & Sepeatability

Both are contact Sensors, that means they have to be close to the Surface.

Calibrations & Maintenance

• Blackbody Cavity (commercial) :

Standard Method for Calibration (emissivity : ε =1) need to get ε-Target from Tables

• On Field :

Has to be done for specific Surface Properties

use Hand-held with auto ε-compensation system

• Test Calibration Frequency Requirement :

Drift ? Emissivity changes ?

Sites Selection

As many representative Samples as possible…

• Different Exposition Surfaces :

Ground, Wall and Roof

Sunny vs. Shaded Sites

• Different Surface Types (with high ε) :

Concrete, Brick, Asphalt, Glass, Gravels, …

Notices…

• Still no Information about how to fix IRT on Stands

• Many Certifications about the Products (e.g. ISO 9001)

• Not a lot of Specific Informations about the sensor TN0am Difficulty to be informed through the website !!

• References :www.zytemp.com (choosen Instrument)www.apogeeinstruments.comwww.hukseflux.comwww.omega.com

Measurement of Surface Wind

• Wind direction meteorological forecast

• Wind speed Naval war strategy

• Modern wind sensors

– Anemometers based on cooling effect of wind

– Anemometers based on air movement

– Anemometers based on wind force

Wind measurement

• Wind speed

Definition:

- Wind velocity

- Average over 10 min

• Wind direction

Definition:- Direction from which wind

is blowing - Geographical north- Calm

Units: degreesUnits: m/s or knots

Wind Sensors:Davis anemometer

• Direction vane

=> Measures wind direction

• Cup rotor

=> Measures wind speed

Wind speed sensor: Two sub-assemblies

Rotor:• Proportional to wind

speed• Problem with starting

threshold speed• Response to change in

wind speed = distance constant

Signal generator:• Reed switch in

anemometer head unit• Magnet in the wind cup

assembly

=> Circuit closure

N 2RRV (turn/sec) regimerotation N

radius R

elocityrotation v

Wind direction vane: Two sub-assemblies

Vane:• Well-balanced• One-single equilibrium

position

Signal generator:• Shaft angle transducer• 360º Potentiometer• Resistance: 0-20K Ohms• Input Voltage: 3V• Output voltage• Analog signal

Range and Accuracy:Range:• Wind speed 0.9-78 m/s• Wind direction 0-360° or 16 compass points

Accuracy:• Wind speed 5%• Wind direction 7°

Resolution:• Wind speed 1mph• Wind direction 1°

Environmental Conditions

• Freezing Conditions => Use of drip rings

• Hurricane and light breeze => Wind tunnel testing

Maintenance

• No maintenance required

Calibration requirement

• Calibration curve: nature of response to change in wind speed

• Rotation rate: function of wind speed• Usually linear calibration• Specific to each type and instrument

• Overspeeding: overestimation of wind speed

Distance constant: greater for deceleration than for acceleration => depends on the angle of attack

l0U-U

S

Considerations about site selection

• Cable length 42 m • International standard recommendation

– Height:– Open terrain

• Obstructions– Buildings– Trees

)]75.4(log656.0233.0[VV 1001h h

=> Ideal recommendation: wind sensors only on open, flat, rural terrain

Conclusion: Why choose a cup anemometer?

Advantages:• Robust assembling• Comfortable handling• Competitive price• Easy measurement

technique

Inconvenients:• Horizontal speed• Slow response to wind

variation• Constraining siting

=> Cup anemometers are simple and convenient instruments!

One device for measuring both relative humidity and temperature

MEASUREMENT OF TEMPERATURE AND RELATIVE HUMIDITY

MEASUREMENT OF TEMPERATURE AND RELATIVE HUMIDITY

• Sensor SHT75 (total weight = 168 mg)

Fig.: www.sensirion.com/humidity

SPECIFICATIONS OF SHT75

• Power requirements

- VOLTAGE supply is between : 2.4 and 5.5 V

- Maximum supply CURRENT depends on the conditions :

- when it is measuring : 550 A - in average : 28 A

(with one measurement of 12bit per second)


• For sending a command, we have to do the following steps :- initiate a transmission- enter the code for the command

- measurement sequence for RH and T



• How do we do practically for sending command ?




• Output format

- Digital output for both temperature and relative humidity (there is a 14bit analog to digital converter)

• Converting Output to physical values

- We need to use 2 formula with five coefficients to convert from digital readout to temperature and to relative humidity.


• Converting Output to physical values- for relative humidity :



• Converting Output to physical values- for temperature :



• Range of environmental conditions for proper operation:

In our case, there is no specific problem…

… except for the extreme values for both RH and T.



• Precision and accuracy



• Alternative technologie :- the most accurate humidity measurement instruments that exist on the market are the chilled mirror hygrometers.

- However, these instruments are very expensive.


CALIBRATION REQUIREMENTS

• Each of our sensors (SHT75) are already calibrated in a precision humidity chamber.


• Our operating conditions in any case on the campus, are fully included in the recommended conditions. Therefore there is no restriction for selecting the sites.

• (We must still be careful with the areas near the laboratories of air pollution because the chemical vapors may interfere with the polymers used for capacitive humidity sensors. High levels of pollutants could cause permanent damage to the sensing polymer.)

MAINTENANCE REQUIREMENTS

• There is an “end of battery” function that detects voltages below 2.47 V.

• No specific maintenance.

WIRED RAIN METERWS-7048U

LA CROSSETECHNOLOGY

Blablabla…

INTRODUCTION

• Floods have become a more and more frequent phenomenon. They are responsible for high human and financial losses.

• Good predictions of rain can allow to avoid such losses by building protection in key places for example.

• Predictions are based on previous rain measurements.

That’s why it is important to make good measurements in many different places. We can do them using our wired rain meter.

POWER REQUIREMENTS

• 2x AA, IEC LR6 1.5V batteries.

• Life time of a battery: about one year.

SPECIFIC FEATURES

• 3 displays of rainfall quantities.• Transmission of rainfall data through a connected wired

rain gauge.• Rainfall unit display in inch or in millimeters (1inch =

25.4mm).• Manual and auto reset of rainfall data.• Auto off function (after 12 hours).• Wall mounting or table standing.

DISPLAY OF RAINFALL DATA (1/ 2)

• 1 auto display : the rain quantity data will be displayed an added to the previous recorded data in the auto counter.

• If no rain is recorded during 2 hours, the auto counter will

display the last recorded data. The auto counter will

automatically reset to 0.0 mm when rain begins again.

• The rain data of auto counter cannot be reset manually.

DISPLAY OF RAINFALL DATA (2/ 2)

• 2 manual displays : the rain quantity data will be displayed in counter 1 and 2. New rain data will be added to the previous records and unless the counters are manually reset by pressing reset 1 or reset 2 key, the rain data will increase continuously to a maximum of 9999.9 mm.

ACCURACY

• The rain sensor features a self emptying tipping bucket that measures 0.01 inch per tip.

• So accuracy is 0.01 inch = 0.254 mm.

CALIBRATION REQUIREMENTS

• There is no need for calibration.• We can imagine improving the precision by comparing

the measurement with a graduated recipient during a rain period.


• We must place the rain gauge in a site where the rain cannot be intercepted before falling into the sensor.

• Wire length is 10 meters, the rain meter must be placed under shelter. So the rain gauge will be placed at a maximal distance of 10 meters from a sheltered location.

PROBLEMS THAT MAY OCCUR

• Something can fall into the rain gauge and disturb the measurements.

• If it is snowing the measurements won’t be correct.

• Even if snow is alternating with rain, the measurments will be affected.

Soil Moisture

The ECH2O Probe

Atmospheric Turbulent Flow in Urban Environments , V. Silva 2005

Plan

Introduction

Specifications

Calibration requirements

Site selection considerations

Maintenance requirements

Applications notes

Synthesis

Introduction

soil moisture

>> when to irrigate, how much water to apply,…

>> transport and storage of dissolved nutrients and pollutants

The Ech2o Probe

dielectric permittivity

>> sensitive measure of water content

ECH2O Probes in both 4” and 8” lengths

Specifications

Echo probe diagram

Specifications

Power

Requirements: 2.5VDC @ 2mA to 5VDC @ 7mA

Specifications

Output format

Analog : 10-40% of excitation voltage (250-1000mV) at 2500mV excitation.

Specifications

Accuracy

Typically ± (± 3%)With soil-specific calibration ± (± 1%)

Resolution

0.002m3/m3 (0.1%)

Allowable environmental conditions

Operating Environment

0 to 50º C

Range of Measurement

0 to saturated volumetric water content


ECHO probe : pre-calibrated for most soil types

• Soil type with high sand or salt content : standard calibration not accurate

>> specific calibrations


stability on sand soils


standard calibration (2500mV excitation)

(ECHO-20) :

(m3/m3) = 0.000695mv - 0.29

(ECHO-10) :

(m3/m3) = 0.000936mv - 0.376

volumetric water content

mv millivolt output


soil adjacent to the probe surface

>> strongest influence on the probe reading

air gaps or excessive soil compaction

>> profoundly influence the readings


large metal objects

>> attenuate the probe’s electromagnetic field


install the ECHO probe

>> pilot hole in the soil

the entire length of the probe

must

be covered


Orientation

>> flat side perpendicular to the surface


Removing the Probe

not pull it out of the soil by the cable !


durability of the Echo Probe wires at low temperatures :

breaking point : between -35° C and -45° C


problems with rodents

>> chewing on the ECHOprobe cables

Application notes

Application notes

An ECH2O Radio Logger in Southeastern Idaho potato field

>> multiple-depth

soil moisture readings

and precipitation

data

Synthesis

benefits• High resolution allows daily or hourly

tracking of water use.

• Voltage output proportional to water content.

• Low-cost dielectric water content sensor

• Very low power requirement.

Synthesis

inconvenients• Sensitivity to salt and temperature.

Model 264Very Low Differential Pressure

Transducer

Measuring a PRESSURE DIFFERENCES (unidirectional or

bidirectional)

Model 264Very Low Differential Pressure Transducer

What does it do?

It measures the pressure difference between two pressures (measured from two different medias at the same time), converting it to a proportional electrical output. Scale can be set for either unidirectional (i.e. 0 to maxp) or bidirectional (i.e. –maxp to maxp) pressure differences.

Models with different scale (maximum pressures) ranges are available Measuring either low or very low pressures.


Power requirement & Output format

Mode A – VOLTAGE

In: 9-30 VDC, current not specified

Out: 0 to 5 VDC (analog pressure indication)

Mode B – CURRENT

In: 9-30 VDC

Circle: 4-20mA (analog pressure indication), to any external load of 0-800 ohms

Uses 9-30 V, DC. ANALOG. Two models are available: Voltage and Current

Output: Signal (Voltage resp. Current) proportional to pressure

Internal Regulation permits use with unregulated power supplies


Conditions for proper operationPressure ranges:

- Measurement:

•Unidirectional: 0 – 0.00025 atm to 0 – 0.25 atm

•Bidirectional: 0 – 0.0012 atm to 0 – 0.12 atm

- Allowed overpressure: 0.7 atm in both directions

Temperature:

-18 to 79°C (error compensated for -18 to 65°C)

NO LIQUIDS OR CORROSIVE GASES

Sensor is quite robust to substances in the atmosphere; However, very aggressive compounds like chloric or sulfuric acid however could cause damage, and exposure to moisture or contaminants can lead to performance losses.


Precision & accuracy

Standard accuracy: ±1.0% full scale (i.e. 2.5E-6 to 2.5E-3 atm, depending on scale)

Higher accuracies available

Signal is temperature compensated to 0.033% FullScale/°C thermal error over the temperature range of -18 to 65°C


Alternative technologies

Virtually all modern pressure sensors, pressure is measured by monitoring a diaphragm which is being subjected to the test pressure on one side, and the specified reference pressure (varying or fixed through a hermetically sealed vacuum) on the other side.

In Setra Model 264, both sides of the diaphragms are accessible to external pressures Pressure DIFFERENCES are measured; the alternate method are instruments for absolute measurements.



No field calibration required (factory calibrated)

Mounting position effects < 0.0025 atm

•factory calibrated for vertical position

Manual zero & span adjustment possible

(open both ports to atmosphere -> adjust zero-screw, then apply full pressure and adjust FS-signal with span-screw)

However, setra recommends software correction



No special conditions (suitable for any air or nonconductive gases)

Exceeding moisture should be prevented from entering the instrument



No special maintenance required

No indication of durability


Tubing recommendations

Diameter of ideal tubes depend primarily on length:

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solar radiation context/importance of solar radiation measurements –energy available to earth...

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