- Email: [email protected]
A small, sensitive anemometer system for agricultural meteorology
Agricultural Meteorology -
Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
A SMALL, SENSITIVE A N E M O M E T E R SYSTEM F O R A G R I C U L T U R A L METEOROLOGY E. F. BRADLEY 1
Division of Plant Industry, Cormnonwealth Scientific and Industrial Research Organization, Canberra, A.C.T. (Australia) (Received August 9, 1968)
SUMMARY
The paper describes a cup anemometer system suitable for use close to the ground and within crop canopies. The anemometers are small, sensitive, weatherproof, rugged and sufficiently inexpensive to be constructed in array proportions. The anemometer is about 6 cm high and the rotor arms about 6 cm long. The counting mechanism employs an automobile lamp whose voltage is stabilized by a Zener diode, a light-chopper unit, a CdS photocell and a highspeed electromagnetic counter. Details of the design, construction, circuitry and field use of the system are discussed. Starting speeds of the anemometers are between I0 and 15 cm/sec. The design of the anemometers is such that uniformity between different batches is high. Twelve prototype instruments had almost identical calibration curves, within + 1%o of the mean over a range 2-8 m/sec. INTRODUCTION
Micrometeorologists come from a wide variety of backgrounds. Agronomists, botanists, ecologists, and entomologists, for example, have for some time recognized the influence of the microclimate in their work, and the need to make accurate measurements of meteorological variables. Often, however, suitable sensors are not available, and the experimenter must either devise and build his own, or make use of instruments which were intended for another purpose. It is a regrettable fact that much micrometeorological data are obtained with sensors which, because of their size, response characteristics or construction are basically unsuited to the task in hand. This paper describes a cup anemometer system which was designed for use near the ground surface, and which is also well suited for measurement of wind
I At present, visiting Air Force Cambridge Laboratories, Bedford, Mass., U.S.A. Agr. Meteorol.,
6 (1969) 185-193
186
E.F. BRADLEY
velocities within crop canopies. Care has been taken with the detailed design to make it a reasonably-priced commercial proposition 1. PROBLEMS OF WIND-SPEED MEASUREMENT The shortcomings of the rotating-cup anemometer have been discussed by DEACON (195l), and more recently by MACCREADY (1966) and BERNSTEIN (1967). They show that very large errors in measurement of the absolute windspeed are possible, a consequence of the fluctuating nature of the wind. There are several reasons, however, why alternatives such as sonic and thermal anemometers are not practicable in much micrometeorological research. In many investigations, for example, an array of several anemometers is needed for the measurement of velocity gradients, divergences and anomalies. With these latter instruments, provision of an array is a costly and complex exercise, involving, amongst other things, the amplification and recording of a large number of simultaneous analogue signals. The cup anemometer, linked to an electro-mechanical counter, remains a digital recording instrument of unusual simplicity and reliability. Attention to certain points of design can reduce the measurement error to within acceptable limits. It can also be argued that the micrometeorologist is more commonly concerned with gradients of wind-speed; and since the errors may be expected to affect all sensors of an array, the error in gradient measurement becomes of second-order. It is surprising, in the light of this experience, that attempts to extend commercially available cup anemometers to array proportions create so many complications. ]'he problems are not only economic; there are also such practical problems as voltage drop along multiple signal cables, stability of power supplies and portability. The anemometer system described here has been designed as a portable array. The instruments are, in addition, small enough to permit exposure in the confined spaces of a crop canopy and sufficiently sensitive to respond to the low wind-speeds characteristic of this environment. Their construction requires a minimum of hand-work, and, despite their size and sensitivity, expensive instrument-making skills and machine tools are unnecessary. The temptation to achieve the ultimate in miniaturization has been resisted so that inexpensive, readilyavailable components may be used. The literature contains descriptions of many cup anemometers, built or modified for various purposes. One of the most recent (FRITSCHEN, 1967) includes a useful summary of the factors affecting cup design. No novelty is claimed for the operating principles of the present anemometer. Its value lies in a design 1 The anemometer system described here is now available from Rimco Instruments, Melbourne, Australia. Agr. Meteorol., 6 (1969) 185-193
ANEMOMETER FOR AGRICULTURAL METEOROLOGY
187
philosophy which takes account of the practical and economic problems of the scientist and of the manufacturer. THE ANEMOMETER
A photograph of the anemometer is given in Fig.l, and of its component parts in Fig.2. The scale may be gauged by the rotor tube which is 5.7 cm long. The cups are formed from halves of table tennis balls, remoulded under slight heat to the usual conical shape. The rotor arms are of nickel hypodermic tubing, and extend across the cup diameter to improve rigidity. They are cemented to the cups and aluminum centre boss. The latter overhangs the rotor tube to prevent direct ingress of water, and is secured to the shaft with a small set-screw. Shafts are made from 1 mm diameter stainless steel wire. The bearings are industrial jewels, the lower one consisting of an " o + end" pair in a threaded brass mount. The upper bearing is a single " o " jewel. It is mounted in the brass plug which, fitted with two rubber " o " rings, is a press-fit in the top of the rotor tube. The light-chopper is a slotted plastic rod, drilled along the axis to be a firm push-fit on the shaft. This method of construction is quite reliable with components of this size and weight, and avoids the use of watch-size set screws. A washer is similarly mounted near the top of the shaft to restrict vertical movement.
Fig.1. Small, sensitive anemometer.
Agr. Meteorol., 6 (1969) 185-193
188
E.F. BRADLEY
Fig.2. Component parts of anemometer.
Overestimation of the fluctuating wind, the effect which MACCREADY (1966) calls "u-error", is reduced by decreasing the moment of inertia of the rotor, the most significant contribution to which is the weight of the cups. Each cup weighs about 500 mg, and the complete rotor assembly weighs only 31/2 g. This also reduces friction on the end bearing, and helps to lower the stalling speed of the anemometer. The assembly may be balanced with a small amount of melted paraffin wax in the appropriate cup. The rotor tube is made from 3/8 inch diameter stainless steel rod, drilled axially at 5/16 inch diameter to leave a closed end which is subsequently threaded for the lower bearing mount. The two diametrically opposite holes are of different sizes, chosen to transmit maximum light without undue leakage around the chopper. After machining, the tube is heated to a dark color. This is adequate to Agr. Meteorol., 6 (1969) 185-193
189
ANEMOMETER FOR AGRICULTURAL METEOROLOGY
reduce internal reflections, and avoids the use of black paint which has been found to flake and foul the bearings. The anemometer body is an aluminum cylinder l l/s inch in diameter and length, drilled from above for the rotor unit, and machined from below to accommodate the lamp and photocell. These components are mounted on the base of the anemometer, together with a miniature 3-pin connector, so that they are easily removed as a unit. The CdS photocell is one commonly used in domestic and industrial appliances, and the lamp is an automobile type. Neither component is the smallest manufactured, but both are inexpensive and easily obtained. Whenever possible, and particularly in the rotor unit where mechanical precision is important, the design is based on cylindrical symmetry, to take advantage of the intrinsic accuracy of turning and concentric drilling operations. A symmetrical body is, in any case, desirable as discussed by RIDER (1961). THE COUNTING CIRCUIT
The light chopper unit is designed so that most of the area of this large photocell is illuminated, and its power capacity exploited. Individual lamp-photocell combinations vary in their operating characteristic, but most cells change their resistance from about 1 Mr2 in the dark to less than 2,000 ~ when illuminated in the anemometer. This provides good discrimination, and sufficient current to
TO OTHER ANEMOMETERS
COUNT SWITCH ÷12-24V
....
ii
I"
III
--------~2 V
III r IN 1603 •2V 10W
~
N
156"e'3568
.... IO
R
LAMP PHOTOCELL 12V B873105 1.5W (PHILIPS) ANEMOMETERS
,A/V~
JUNCTION
BOX
r
CABLE
COUNTING
CIRCUITS
Fig.3. Circuit of anemometer system.
Agr. Meteorol., 6 (1969) 185 193
190
E. F. BRADLEY
operate (without prior amplification or trigger circuits) the transistor switch illustrated in Fig.3. The limited frequency response of this type of photocell (150 c/sec) is entirely adequate in this application. Many suitable makes of counter are available, and since their operating characteristics depend on mechanical as well as electrical parameters it is not meaningful to discuss the operation of this simple circuit in any detail. It is the author's experience that high-speed (say 40 impulse/sec) counters have a more precise mechanism than many slower types, and that they will run reliably at reduced count rates on considerably lower voltage. Operated in this way, the impulse on the mechanism is less violent, resulting in less wear and longer life. The circuit shown is for a Hengstler F 403, 24 V, 40 impulse/sec counter. They are seldom called upon to run at more than 20 impulse/sec, which they will do reliably on 12 V. As indicated in the circuit, however, the counter supply voltage can be independent of the lamp supply. The transistor has a relatively high breakdown voltage (60 V), and needs no protection against the back e.m.f, produced by this counter coil. T H E ANEMOMETER A R R A Y
In crop studies, particularly, where singularities of growth are common and replication desirable, a fixed anemometer installation is a considerable restriction. These small and light instruments insure a portable system which may be set up in a very short time. Fig.4 shows the complete equipment needed to operate 24 anemometers. The box contains 24 counters and circuits, although only eight instruments are shown. In this installation a 12 V battery is the only power supply. With alternative counters a second 6 or 12 V battery may be necessary as discussed in the circuit description. The stand is the most convenient support over the ice surface, but on land the 1-inch diameter mast is supported only by guy-ropes. Cables are a perpetual headache to the field worker, and often compromise the flexibility of an installation. They must usually be fairly long, so that, apart from the irritations of handling and tangling, cable resistance is often significant. When several photo-chopper circuits share a common cable and power supply the combined lamp currents may cause a substantial voltage drop along the cable. More seriously, a variation in the number of lamps burning, due to failure or disconnection, alters the voltage drop and affects the operation of all other instruments. This effect, which may also depend on the state of charge of the battery, is overcome by stabilizing the lamp voltage at the instrument mast with 8.2-V Zener diodes. This voltage ensures long life of the 12 V lamps. Each Zener diode is housed in a small aluminum box (two are visible in Fig.4) which serves as its heat-sink, and also as a junction-box between the signal cable and four anemometers. An installation of twelve anemometers with four Zener boxes gives great Agr. Meteorol., 6 (1969) 185-193
ANEMOMETER FOR AGRICULTURAL METEOROLOGY
191
Fig.4. Complete anemometer system. flexibility, allowing for combinations of 3, 4, 6 or 12 instruments at each location. Fig.3 shows the circuit of this arrangement. The series resistance R is calculated so that the Zener passes about 1 0 ~ of the combined lamp currents at the "end-point" of the accumulator (11.5 V), and the power rating of R and Z allow for disconnection of all four lamps with the accumulator fully charged to 14 V. The installation shown in Fig.4 used 200 ft. of fairly light cable, the resistance of each wire being about 31/2 fL Each lamp draws 90 mA at 8.2 V, so that R calculates as about 11/4 fL The 10 W Zener has such a large safety margin that no series resistance was in fact, added. The lower anemometers illustrated in Fig.4 were equispaced at 12 cm, to observe mutual interference in this situation. The 36 cm anemometer was displaced Agr. Meteorol., 6 (1969) 185-193
192
E.F. BRADLEY
50 cm across wind for alternate 10 min profile measurements in a moderate (4-5 m/sec) steady wind. No systematic displacement in its reading was apparent, and 12 cm is regarded as a safe minimum vertical spacing. For profile measurement over surfaces, LETTAU (1967) has suggested criteria for the choice of anemometer spacing, having regard to surface structure, and the proper determination of profile parameters. PERFOR MANCE
The starting speeds of a set of twelve prototype anemometers were in the range 10-15 cm/sec. A typical calibration is given in Fig.5. It was notable that the twelve instruments had almost identical calibration curves, within _+ 1 ~ of the mean over the range 2 8 m/sec. No special care was taken in this case to ensure identical rotor geometry, but had rotors been set up by jig it is probable that uniformity would have been even better. This is a useful feature, since it enables the experimenter to obtain a rapid indication of profile shape by graphing directly from the counter readings. 1400
/f
1200
I000
+
+
z_
800
,o0//+/i/+/+/+/
600
"+~
1
2
3
4 WlNO
_1
t
I
I
I
5
6
7
8
9
SPEED(M/SEC)
Fig.5. C a l i b r a t i o n curve of a n e m o m e t e r .
The twelve prototype anemometers have been in use for about 2 years. They have performed reliably in the height of an Australian summer, with temperatures around 90°F, and more recently in the winter at Madison (Wisc.) in a temperature of - 12°F. After a period of 12 months, despite exposure to a dusty environment, only one of the twelve calibrations had changed perceptibly. Agr. Meteorol., 6 (1969) 185-193
ANEMOMETER FOR AGRICULTURAL METEOROLOGY
193
The anemometers are by no means as fragile and easily damaged as might be imagined. The white cups and reflective bodies make them clearly visible amongst foliage, and help to prevent accidental damage. The rotor arms and shaft are sufficiently flexible to withstand considerable clumsiness of handling without suffering permanent deformation. ACKNOWLEDGEMENTS
The author is happy to acknowledge the work of Mr. O. A. Simakoff and Mr. A. J. Bryan for their skill and patience in constructing a succession of prototype anemometers and circuits. Mr. C. Hazelton contributed substantially to the design of the final system. REFERENCES
BERNSTEIN, A. B., 1967. A note on the use of cup anemometers in wind profile experiments. J. Appl. Meteorol., 6: 280-286. DEACON, E. L., 1951. The overestimation error of cup anemometers in fluctuating winds. J. Sci. Instr., 28: 231-234. FRITSCHEN, L. J., 1967. A sensitive cup-type anemometer. J. Appl. Meteorol., 6: 695-698. LETTAU, H. H., 1967. Problems of Micrometeorological Measurements. In: E. F. BRADLEY and O. T. DENMEAD(Editors), Tile Collection and Processing of Field Data. Wiley, New York, N.Y., pp.3-40. MACCREADY, P. B., 1966. Mean wind speeds in turbulence. J. Appl. Meteorol., 5: 219-225. RIDER, N. E., 1960. On the performance of sensitive cup anemometers. Meteorol. Mag., 89: 209-215.
Agr. Meteorol., 6 (1969) 185 193
Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
A SMALL, SENSITIVE A N E M O M E T E R SYSTEM F O R A G R I C U L T U R A L METEOROLOGY E. F. BRADLEY 1
Division of Plant Industry, Cormnonwealth Scientific and Industrial Research Organization, Canberra, A.C.T. (Australia) (Received August 9, 1968)
SUMMARY
The paper describes a cup anemometer system suitable for use close to the ground and within crop canopies. The anemometers are small, sensitive, weatherproof, rugged and sufficiently inexpensive to be constructed in array proportions. The anemometer is about 6 cm high and the rotor arms about 6 cm long. The counting mechanism employs an automobile lamp whose voltage is stabilized by a Zener diode, a light-chopper unit, a CdS photocell and a highspeed electromagnetic counter. Details of the design, construction, circuitry and field use of the system are discussed. Starting speeds of the anemometers are between I0 and 15 cm/sec. The design of the anemometers is such that uniformity between different batches is high. Twelve prototype instruments had almost identical calibration curves, within + 1%o of the mean over a range 2-8 m/sec. INTRODUCTION
Micrometeorologists come from a wide variety of backgrounds. Agronomists, botanists, ecologists, and entomologists, for example, have for some time recognized the influence of the microclimate in their work, and the need to make accurate measurements of meteorological variables. Often, however, suitable sensors are not available, and the experimenter must either devise and build his own, or make use of instruments which were intended for another purpose. It is a regrettable fact that much micrometeorological data are obtained with sensors which, because of their size, response characteristics or construction are basically unsuited to the task in hand. This paper describes a cup anemometer system which was designed for use near the ground surface, and which is also well suited for measurement of wind
I At present, visiting Air Force Cambridge Laboratories, Bedford, Mass., U.S.A. Agr. Meteorol.,
6 (1969) 185-193
186
E.F. BRADLEY
velocities within crop canopies. Care has been taken with the detailed design to make it a reasonably-priced commercial proposition 1. PROBLEMS OF WIND-SPEED MEASUREMENT The shortcomings of the rotating-cup anemometer have been discussed by DEACON (195l), and more recently by MACCREADY (1966) and BERNSTEIN (1967). They show that very large errors in measurement of the absolute windspeed are possible, a consequence of the fluctuating nature of the wind. There are several reasons, however, why alternatives such as sonic and thermal anemometers are not practicable in much micrometeorological research. In many investigations, for example, an array of several anemometers is needed for the measurement of velocity gradients, divergences and anomalies. With these latter instruments, provision of an array is a costly and complex exercise, involving, amongst other things, the amplification and recording of a large number of simultaneous analogue signals. The cup anemometer, linked to an electro-mechanical counter, remains a digital recording instrument of unusual simplicity and reliability. Attention to certain points of design can reduce the measurement error to within acceptable limits. It can also be argued that the micrometeorologist is more commonly concerned with gradients of wind-speed; and since the errors may be expected to affect all sensors of an array, the error in gradient measurement becomes of second-order. It is surprising, in the light of this experience, that attempts to extend commercially available cup anemometers to array proportions create so many complications. ]'he problems are not only economic; there are also such practical problems as voltage drop along multiple signal cables, stability of power supplies and portability. The anemometer system described here has been designed as a portable array. The instruments are, in addition, small enough to permit exposure in the confined spaces of a crop canopy and sufficiently sensitive to respond to the low wind-speeds characteristic of this environment. Their construction requires a minimum of hand-work, and, despite their size and sensitivity, expensive instrument-making skills and machine tools are unnecessary. The temptation to achieve the ultimate in miniaturization has been resisted so that inexpensive, readilyavailable components may be used. The literature contains descriptions of many cup anemometers, built or modified for various purposes. One of the most recent (FRITSCHEN, 1967) includes a useful summary of the factors affecting cup design. No novelty is claimed for the operating principles of the present anemometer. Its value lies in a design 1 The anemometer system described here is now available from Rimco Instruments, Melbourne, Australia. Agr. Meteorol., 6 (1969) 185-193
ANEMOMETER FOR AGRICULTURAL METEOROLOGY
187
philosophy which takes account of the practical and economic problems of the scientist and of the manufacturer. THE ANEMOMETER
A photograph of the anemometer is given in Fig.l, and of its component parts in Fig.2. The scale may be gauged by the rotor tube which is 5.7 cm long. The cups are formed from halves of table tennis balls, remoulded under slight heat to the usual conical shape. The rotor arms are of nickel hypodermic tubing, and extend across the cup diameter to improve rigidity. They are cemented to the cups and aluminum centre boss. The latter overhangs the rotor tube to prevent direct ingress of water, and is secured to the shaft with a small set-screw. Shafts are made from 1 mm diameter stainless steel wire. The bearings are industrial jewels, the lower one consisting of an " o + end" pair in a threaded brass mount. The upper bearing is a single " o " jewel. It is mounted in the brass plug which, fitted with two rubber " o " rings, is a press-fit in the top of the rotor tube. The light-chopper is a slotted plastic rod, drilled along the axis to be a firm push-fit on the shaft. This method of construction is quite reliable with components of this size and weight, and avoids the use of watch-size set screws. A washer is similarly mounted near the top of the shaft to restrict vertical movement.
Fig.1. Small, sensitive anemometer.
Agr. Meteorol., 6 (1969) 185-193
188
E.F. BRADLEY
Fig.2. Component parts of anemometer.
Overestimation of the fluctuating wind, the effect which MACCREADY (1966) calls "u-error", is reduced by decreasing the moment of inertia of the rotor, the most significant contribution to which is the weight of the cups. Each cup weighs about 500 mg, and the complete rotor assembly weighs only 31/2 g. This also reduces friction on the end bearing, and helps to lower the stalling speed of the anemometer. The assembly may be balanced with a small amount of melted paraffin wax in the appropriate cup. The rotor tube is made from 3/8 inch diameter stainless steel rod, drilled axially at 5/16 inch diameter to leave a closed end which is subsequently threaded for the lower bearing mount. The two diametrically opposite holes are of different sizes, chosen to transmit maximum light without undue leakage around the chopper. After machining, the tube is heated to a dark color. This is adequate to Agr. Meteorol., 6 (1969) 185-193
189
ANEMOMETER FOR AGRICULTURAL METEOROLOGY
reduce internal reflections, and avoids the use of black paint which has been found to flake and foul the bearings. The anemometer body is an aluminum cylinder l l/s inch in diameter and length, drilled from above for the rotor unit, and machined from below to accommodate the lamp and photocell. These components are mounted on the base of the anemometer, together with a miniature 3-pin connector, so that they are easily removed as a unit. The CdS photocell is one commonly used in domestic and industrial appliances, and the lamp is an automobile type. Neither component is the smallest manufactured, but both are inexpensive and easily obtained. Whenever possible, and particularly in the rotor unit where mechanical precision is important, the design is based on cylindrical symmetry, to take advantage of the intrinsic accuracy of turning and concentric drilling operations. A symmetrical body is, in any case, desirable as discussed by RIDER (1961). THE COUNTING CIRCUIT
The light chopper unit is designed so that most of the area of this large photocell is illuminated, and its power capacity exploited. Individual lamp-photocell combinations vary in their operating characteristic, but most cells change their resistance from about 1 Mr2 in the dark to less than 2,000 ~ when illuminated in the anemometer. This provides good discrimination, and sufficient current to
TO OTHER ANEMOMETERS
COUNT SWITCH ÷12-24V
....
ii
I"
III
--------~2 V
III r IN 1603 •2V 10W
~
N
156"e'3568
.... IO
R
LAMP PHOTOCELL 12V B873105 1.5W (PHILIPS) ANEMOMETERS
,A/V~
JUNCTION
BOX
r
CABLE
COUNTING
CIRCUITS
Fig.3. Circuit of anemometer system.
Agr. Meteorol., 6 (1969) 185 193
190
E. F. BRADLEY
operate (without prior amplification or trigger circuits) the transistor switch illustrated in Fig.3. The limited frequency response of this type of photocell (150 c/sec) is entirely adequate in this application. Many suitable makes of counter are available, and since their operating characteristics depend on mechanical as well as electrical parameters it is not meaningful to discuss the operation of this simple circuit in any detail. It is the author's experience that high-speed (say 40 impulse/sec) counters have a more precise mechanism than many slower types, and that they will run reliably at reduced count rates on considerably lower voltage. Operated in this way, the impulse on the mechanism is less violent, resulting in less wear and longer life. The circuit shown is for a Hengstler F 403, 24 V, 40 impulse/sec counter. They are seldom called upon to run at more than 20 impulse/sec, which they will do reliably on 12 V. As indicated in the circuit, however, the counter supply voltage can be independent of the lamp supply. The transistor has a relatively high breakdown voltage (60 V), and needs no protection against the back e.m.f, produced by this counter coil. T H E ANEMOMETER A R R A Y
In crop studies, particularly, where singularities of growth are common and replication desirable, a fixed anemometer installation is a considerable restriction. These small and light instruments insure a portable system which may be set up in a very short time. Fig.4 shows the complete equipment needed to operate 24 anemometers. The box contains 24 counters and circuits, although only eight instruments are shown. In this installation a 12 V battery is the only power supply. With alternative counters a second 6 or 12 V battery may be necessary as discussed in the circuit description. The stand is the most convenient support over the ice surface, but on land the 1-inch diameter mast is supported only by guy-ropes. Cables are a perpetual headache to the field worker, and often compromise the flexibility of an installation. They must usually be fairly long, so that, apart from the irritations of handling and tangling, cable resistance is often significant. When several photo-chopper circuits share a common cable and power supply the combined lamp currents may cause a substantial voltage drop along the cable. More seriously, a variation in the number of lamps burning, due to failure or disconnection, alters the voltage drop and affects the operation of all other instruments. This effect, which may also depend on the state of charge of the battery, is overcome by stabilizing the lamp voltage at the instrument mast with 8.2-V Zener diodes. This voltage ensures long life of the 12 V lamps. Each Zener diode is housed in a small aluminum box (two are visible in Fig.4) which serves as its heat-sink, and also as a junction-box between the signal cable and four anemometers. An installation of twelve anemometers with four Zener boxes gives great Agr. Meteorol., 6 (1969) 185-193
ANEMOMETER FOR AGRICULTURAL METEOROLOGY
191
Fig.4. Complete anemometer system. flexibility, allowing for combinations of 3, 4, 6 or 12 instruments at each location. Fig.3 shows the circuit of this arrangement. The series resistance R is calculated so that the Zener passes about 1 0 ~ of the combined lamp currents at the "end-point" of the accumulator (11.5 V), and the power rating of R and Z allow for disconnection of all four lamps with the accumulator fully charged to 14 V. The installation shown in Fig.4 used 200 ft. of fairly light cable, the resistance of each wire being about 31/2 fL Each lamp draws 90 mA at 8.2 V, so that R calculates as about 11/4 fL The 10 W Zener has such a large safety margin that no series resistance was in fact, added. The lower anemometers illustrated in Fig.4 were equispaced at 12 cm, to observe mutual interference in this situation. The 36 cm anemometer was displaced Agr. Meteorol., 6 (1969) 185-193
192
E.F. BRADLEY
50 cm across wind for alternate 10 min profile measurements in a moderate (4-5 m/sec) steady wind. No systematic displacement in its reading was apparent, and 12 cm is regarded as a safe minimum vertical spacing. For profile measurement over surfaces, LETTAU (1967) has suggested criteria for the choice of anemometer spacing, having regard to surface structure, and the proper determination of profile parameters. PERFOR MANCE
The starting speeds of a set of twelve prototype anemometers were in the range 10-15 cm/sec. A typical calibration is given in Fig.5. It was notable that the twelve instruments had almost identical calibration curves, within _+ 1 ~ of the mean over the range 2 8 m/sec. No special care was taken in this case to ensure identical rotor geometry, but had rotors been set up by jig it is probable that uniformity would have been even better. This is a useful feature, since it enables the experimenter to obtain a rapid indication of profile shape by graphing directly from the counter readings. 1400
/f
1200
I000
+
+
z_
800
,o0//+/i/+/+/+/
600
"+~
1
2
3
4 WlNO
_1
t
I
I
I
5
6
7
8
9
SPEED(M/SEC)
Fig.5. C a l i b r a t i o n curve of a n e m o m e t e r .
The twelve prototype anemometers have been in use for about 2 years. They have performed reliably in the height of an Australian summer, with temperatures around 90°F, and more recently in the winter at Madison (Wisc.) in a temperature of - 12°F. After a period of 12 months, despite exposure to a dusty environment, only one of the twelve calibrations had changed perceptibly. Agr. Meteorol., 6 (1969) 185-193
ANEMOMETER FOR AGRICULTURAL METEOROLOGY
193
The anemometers are by no means as fragile and easily damaged as might be imagined. The white cups and reflective bodies make them clearly visible amongst foliage, and help to prevent accidental damage. The rotor arms and shaft are sufficiently flexible to withstand considerable clumsiness of handling without suffering permanent deformation. ACKNOWLEDGEMENTS
The author is happy to acknowledge the work of Mr. O. A. Simakoff and Mr. A. J. Bryan for their skill and patience in constructing a succession of prototype anemometers and circuits. Mr. C. Hazelton contributed substantially to the design of the final system. REFERENCES
BERNSTEIN, A. B., 1967. A note on the use of cup anemometers in wind profile experiments. J. Appl. Meteorol., 6: 280-286. DEACON, E. L., 1951. The overestimation error of cup anemometers in fluctuating winds. J. Sci. Instr., 28: 231-234. FRITSCHEN, L. J., 1967. A sensitive cup-type anemometer. J. Appl. Meteorol., 6: 695-698. LETTAU, H. H., 1967. Problems of Micrometeorological Measurements. In: E. F. BRADLEY and O. T. DENMEAD(Editors), Tile Collection and Processing of Field Data. Wiley, New York, N.Y., pp.3-40. MACCREADY, P. B., 1966. Mean wind speeds in turbulence. J. Appl. Meteorol., 5: 219-225. RIDER, N. E., 1960. On the performance of sensitive cup anemometers. Meteorol. Mag., 89: 209-215.
Agr. Meteorol., 6 (1969) 185 193