3024 Albedometer
Contents
1. change in ambient temperature 4 Wm Non stability percentage change responsivity per year 1 Non linearity percentage deviation from the responsivity at 500 Wm due to the change in irradiance within 100 Wm to 1000 Wm Directional response for beam radiation The range of errors caused by assuming that the normal incidence responsivity is valid for all directions when measuring from any direction a beam radiation whose normal incidence irradiation is 100 Wm Spectral selectivity percentage deviation of the product of spectral absorptance and spectral transmittance from the corresponding mean within 0 35 pm and 1 5 pm Temperature response percentage deviation due to change in ambient temperature from 10 to 40 C relative to 20 C Tilt response percentage deviation from the responsivity at 0 tilt horizontal due to change in tilt from 0 to 90 at 1000 Wim irradiance Viewing angle Irradiance Spectral range Sensitivity Impedance 8 3024 User s Manual 1 2 20 Wm 2 2 1 2p sr 0 1400 W m max 2000 W m 305 2800 nm 50 points 335 2200 nm 95 points Between 9 and 15 pV Wm Depends on connection Albedometer Construction Receiver paint Glass domes Desiccant Spirit level Materials Weight Cable length Dimensions Carbon black Schott K5 optical glass 2 mm thick 30 mm and 50 mm outer diameter Silica2. reflected radiation by the global radiation arrester is noble gas filled has infinite impedance and recovers after breakdown The breakdown volt age is 90 V and the peak pulse current is 10 kA HIGH SENSING ELEMENT SURGE ARRESTER SENSING ELEMENT SURGE Figure 5 ARRESTER Model 3024 Electrical Connection 3024 User s Manual 5 Albedometer MAINTENANCE Once installed the albedometer requires little main tenance The outer domes should be inspected at regular intervals and cleaned regularly preferably every morning On clear windless nights the outer dome tem perature of horizontally placed albedometers will de crease even to the dew point temperature of the air due to IR radiation exchange with the cold sky The effective sky temperature can be 30 C lower than the ground temperature which results in an infrared emission of 150 W m When this hap pens dew glazed frost or hoar frost may form on the top of the outer domes and may remain there for several hours during the morning An ice cap on the domes is a strong diffuser and increases the albedometer s signal drastically up to 50 in the first hours after sunrise 6 3024 User s Manual In some networks the exposed domes of the al bedometer are ventilated continuously by a blower to keep the domes above the dew point tempera ture This ventilation also decreases the sensitivity to thermal radiation
3. The albedometer should be far from light colored walls or other objects likely to reflect sunlight onto it Mounting In principle no special orientation of the instrument is required due to the rotational symmetric sensor The World Meteorological Organization recom mends that the emerging leads are pointed to the nearest pole to minimize heating of the electrical connections Accurate measurement requires proper leveling of the thermopile surface To level the sensor ad just the sensor arm at the mounting fixture Electri cal contact with ground must be maintained to lead off current induced by lightning 4 3024 User s Manual According to the WMO the sensor should be installed 1 2 meters above a surface covered by short cut grass or whatever terrain is typical of the area under study The mounting device should not in terfere with the field of view of the instrument A setup such as that shown in Figure 4 is suitable The upper screen prevents excessive heating of the albe dometer body by solar radiation and shields the lower sensor from precipitation The lower screen prevents direct illumination of the domes by the sun at sun rise and sunset The shadow of the Model 3024 is minimized by its compact construction Also inci dent and reflected radiation are measured at nearly the same point The mast shown in Figure 4 intercepts a frac tion D 2pS of the radiation coming from the ground In the most unfavorable situatio
4. Model 3024 quali fies as First Class according to World Meteoro logical Organization classifications see Table 1 Secondary First Second Sage Regie Resolution smallest detectable change in W m Stability percentage of full scale change year Cosine response percentage deviation rom ideal at 10 solar elevation on a clear day Azimuth response Percentage deviation rom the mean at 10 solar elevation on a clear day Temperature response percentage maximum error due to change of ambient temperature within the operating range Non linearity percentage of full scale Spectral sensitivity percentage deviation from mean absorptance 0 3 to Sum Response time 99 response Table 1 WMO pyranometer classifications The non linearity error the sensitivity variation with irradiance is the same for any given 3024 see Figure 1 Albedometer 103 Yon inearity and Hit dependence Spectral selectivity is the product of spectral absorptance of the black coating and spectral trans mittance of the glass domes see Figure 3 100 Arbitrary units 0 97 Figure 1 Non linearity of Model 3024 albedometer dl Figure 3 1 Relative spectral transmittance of two pyranometer domes Four surface reflections and index The temperature dependence of the sensitivity change with wavelength are taken into account is an individual function For a given 3024 the curve 2 Spectral
5. zero offset I by an approxi mate factor of two When the blue silica gel in the drying cartridge turns completely pink normally after several months it must be replaced with active material Pink silica gel can be dried out and reactivated by placing it in an oven at 130 C for several hours Albedometer CALIBRATION The pyranometers used in the Model 3024 Albe dometer should ideally have a constant ratio of volt age output irradiance level outside the instrument in the plane of the sensing element This ratio is called sensitivity or responsivity The irradiance value can be simply computed by dividing the output signal of each pyranometer by its sensitivity figure or by multiplying the signal value by the reciprocal of the sensitivity figure of ten called calibration factor The sensitivity figure of a particular pyranom eter is an individual one determined in the manufacturer s laboratory by comparison against a standard pyranometer Pyranometer sensitivity changes with time and with exposure to radiation often due to deteriora tion of the black paint Periodic calibration at least every two years is advised 3024 User s Manual 7 Albedometer SPECIFICATIONS Specifications apply to each of the two pyranometers used in the Model 3024 Performance Response time for 95 response lt 30s Zero offset a response to 200 Wim net thermal radiation ventilated 15 Wm b response to 5 K h
6. Model 3024 Albedometer allweatherinc Albedometer TABLE OF CONTENTS INTRODUCTI N RE 1 TREDRAY OF OPERATION iii 2 General Descriptio risse lei 2 EE rilanci 2 INSTALLATION cedesi iano 4 ee e 4 e edu rca 4 ST le ee e alri 5 MAINTENANCE sli 6 GALIBRATI N D 7 Ree 8 PARTS RE 10 3024 User s Manual ii Albedometer INTRODUCTION The Model 3024 Albedometer is designed for measuring net global radiation and or albedo over surfaces of different natures The Model 3024 Albedometer is constructed of two pyranometers that com ply with the specifications for First Class which is second to Secondary Stan dard as published in the Guide to Meteorological Instruments and Methods of Obser vation Fifth Edition 1983 of the World Meteorological Organization WMO Geneva Switzerland The WMO classification list is adopted improved and ex tended by the International Standard Organization ISO and published as ISO 9060 3024 User s Manual 1 Albedometer THEORY OF OPERATION General Description The Model 3024 consists of two matched First Class Pyranometers mounted on a 500 mm long rod The top sensor faces up and measures incoming global radiation while the lower sensor faces down and measures reflected solar radiation If the sensors are connected in anti series the net global radiation is measured When the outputs are measured sepa rately the albedo can be calculated by dividing re flec
7. distribution of solar radiation at sea level Sun at is somewhere in the shaded region of Figure 2 Zenith Airmass 1 Sensitivity variation with temperature 102 50 a Figure 2 The curve of relative sensitivity variation with instrument temperature of a Model 3024 albedometer is in the shaded region A typical curve is shown 3024 User s Manual 3 Albedometer INSTALLATION Location Ideally the site for the albedometer should be free from any obstructions above the plane of the sens ing element and at the same time the albedometer should be readily accessible to allow cleaning of the domes and inspection of the desiccator If this is not possible the site should be chosen in such a way that any obstruction over the azimuth range between earliest sunrise and latest sunset should have an elevation not exceeding 5 the ap parent sun diameter is 0 5 This is important for accurate measurement of direct solar radiation Dif fuse solar radiation is less influenced by obstruc tions near the horizon For instance an obstruction with an elevation of 5 over the whole azimuth range of 360 decreases the downward diffuse solar radia tion by only 0 8 The albedometer should be located in such a way that a shadow will not be cast on it at any time for example by masts or exhaust pipes Mind that hot over 200 C exhausted gas streams will produce radiation in the spectral range of the 3024 albedometer
8. gel Sensitivity 0 5 degree bubble half out of the ring coincide with base of the instrument Detector surface and base are coplanar within 0 1 Anodized aluminum case stainless steel screws in stainless steel bushes white plas tic screen of ASA drying cartridge PMMA 1 9 kg 10m See Figure 6 Figure 6 Model 3024 Albedometer 3024 User s Manual 9 Albedometer PARTS LIST Model 3024 Replaceable Parts and Options Description Part Number Desiccator cartridge 30161 White sun shield 30162 Rubber ring for outer glass dome 30163 Outer glass dome with metal ring 30164 10 3024 User s Manual O allweatherinc All Weather Inc 1165 National Drive Sacramento CA 95818 Fax 916 928 1165 Phone 916 928 1000 Toll Free 800 824 5873 www allweatherinc com 3024 001 ECN 4476 July 1995
9. n sun at zenith the albedometer shadow decreases the sig nal with a part R H upper screen white tower screen white mast black o equator Figure 4 Typical Sensor Installation Albedometer E lectrical Connection The shield is isolated from the case with a surge arrestor so no shield current can exist Shield and The Model 3024 is provided with a 10 meter cable white leads may be connected to the same ground with shield and five leads The color code is at the readout equipment red upper sensor The albedometer can be connected to a com blue upper sensor puter or data acquisition system as well A low volt green lower sensor age analog input module with A D converter must yellow lower sensor be available The span and resolution of the A D white case converter in the module must allow a system sensi tivity of about 1 bit per W m More resolution is There are two modes of operating the Model not necessary during outdoor solar radiation mea 3024 Albedometer surements because albedometers exhibit offsets up e With the sensors connected in anti series to 2 W m due to a lack of thermal equilibrium the net global radiation is measured A surge arrester is incorporated into the sensor e When the outputs are recorded separately to lead off induced lightning current to the case For the albedo can be calculated by dividing the this reason the case should be grounded The surge
10. ted radiation by global radiation The Model 3024 s sensing elements are thermal detectors This type of sensing element responds to the total power absorbed and is theoretically non selective as to the spectral distribution of the radia tion This implies that the naked thermal detector is also sensitive to long wave infrared radiation ther mal radiation gt 3000 nm from the environment The radiant energy is absorbed by a black painted disk The heat generated flows through a thermal resistance to the heat sink the albedometer body The temperature difference across the thermal resis tance of the disk is then converted into a voltage The temperature rise is easily affected by wind rain and thermal radiation losses to the environment cold sky Therefore the detector is shielded by two glass domes Glass domes allow equal transmis sion of the direct solar component for every posi tion of the sun on the celestial sphere The spectral range of the albedometer is limited by the transmis sion of the glass A desiccator in the body prevents dew on the inner side of the domes which can cool down considerably on clear windless nights 2 3024 User s Manual Accuracy The albedometer s sensitivity is cross correlated to a number of parameters such as temperature level of irradiance vector of incidence etc The upper lim iting values of the resulting sensitivity variations are listed in the specifications The
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