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Equipment for measuring low air velocity
Agricultural Meteorology, 12 (1973) 281-296 © Elsevier Scientific Publishing Company, Amsterdam-Printed in The Netherlands
EQUIPMENT FOR MEASURING LOW AIR VELOCITY R ROER and M. KJOLSVIK
Department of Agricultural Structures, The Agricultural University, As (Norway) (Accepted for publication October 2, 1973)
ABSTRACT Roer, P. and Kj/51svik, M., 1973. Equipment for measuring low air velocity~ Agric. Meteorol., 12: 281-296. This publication comprises detailed information for the construction of an inexpensive low velocity thermo-anemometer as well as experience gained from its application. It includes a description of necessary instruments involved and some important requirements related to them. The method of calibration is described. The lower velocity limit for the particular type of sensor is established as well as its directional characteristics, etc. High levels of reproducibility have been obtained even at extremely low velocities. The lower velocity limit was down to 3 cm sec ~ for any separate reading performed. At a reduced heating current disturbances caused by natural convection have not been observed even below 3 cm sec -~ .
INTRODUCTION Our demand for adequate e q u i p m e n t to measure low air velocities is c o n n e c t e d with research on the climate in store-rooms for fresh agricultural produce. We selected the t h e r m o - a n e m o m e t e r as being the m o s t suitable for our demands. It records the t e m p e r a t u r e difference b e t w e e n a heated and u n h e a t e d t h e r m o c o u p l e as a f u n c t i o n of air velocity. This t y p e of a n e m o m e t e r was first developed by Den O u d e n (1958) at TNO, Delft, and further elaborated at The Technical University o f N o r w a y , T r o n d h e i m . A detailed description of the c o n s t r u c t i o n o f the sensors and the experience gained f r o m using t h e m is included. This m a y be of interest in biological research w h e n measuring e x t r e m e l y low air velocities. BASIC PRINCIPLE OF OPERATION Fig. 1 represents the principle o f the transducer. The t w o externally identical spherical bodies, b o t h enclosing a t h e r m o c o u p l e , are equally exposed to the air m o v e m e n t . When one o f the spheres is heated by constant current, leaving the other at a m b i e n t air temperature, the difference in temperature b e t w e e n the t w o b e c o m e s a measure o f air velocity.
282
P. ROER AND M. KJOLSVIK
--I> I> --I> 1> --1> I>v
E
--l>
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constant heating current
Fig.1. Transducer principle. It is of great importance that the amount of energy (determined by W = 1"2R) is kept constant within very limited range. Constantan was chosen for the heating filament, whose temperature coefficient of resistance is as low as 2.10 -s per degree. This enables the flow of energy to remain constant if supplied by a suitable source of current. When the loss of heat through radiation and conduction is minimized, the temperature of the heated sphere almost exlusively depends on its loss of heat through convection. The e.m.f, output will consequently appear a reliable function of velocity. According to Den Ouden (1958), the influence of air temperature was clearly negligible within the practical range.
DETAILSOF CONSTRUCTION The outer sphere diameter was set at 4.76 mm (3/16") and inner diameter at 3.97 mm (5/32 ") to provide a conveniently sized sphere for easy mounting of the coil and thermocouple. Pressing the separate hollow hemisphere required adequate tools. The equipment consisted of a tool and hardened die, see Fig.2. The cavities form the exterior of the sphere, while the 3.99 into (5/32 ") diameter ball corresponds with its inner diameter, To ensure accuracy in form, balls from ball-bearings were employed. A ball with diameter 3/16" was silver soldered to the end of a short rod and subsequently edged. This tool was employed to form the two cavities. The simple tool for pressing the hemisphere was a 5/32" ball silver soldered to a short rod. The hemispheres were made of annealed copper sheet which is easily shaped and has high thermal conductivity to assure uniform surface temperature. Discs with diameter of 7 mm were punched and pressed into hemispheres of the 3/32" ball in a drill press. The hemisphere was then moved to the sharp-edged cavity and filed down to die level.
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
283
V//////////////~V////////////////////AV//////////////A
'l -l.Omm
O I
O sm"
Fig.2. Die and tool for pressing the hemispheres. Where the heating coil was to be mounted, a hole with a diameter of 0.3 mm was drilled to let the coil ends through. A slot was also filed in the edge of one of the hemispheres to let the thermocouple wires through (Fig.3).
G) Fig. 3. Hemispheres and heating coil. The heating coil was an 0.1-mm enamel covered constantan wire. In order to keep the temperature of the coil wire to a minimum the length was set at 60 cm. The coil could be mounted in the sphere without difficulty if appropriately wound. The winding was done on a 2-mm drdl shank, leaving the coil with a 2-ram hole through the center. Care was always taken to cut the coil wire into equal lengths and to keep the protruding coil ends at an equal distance from the sphere.
284
P, ROER AND M. KJOLSVIK
The distance between the centers of the two spheres was set at 37 mm for our particular purposes. Generally, the distance might be set much lower, e.g., 15-20 mm (cf. p.290). Ordinary copper constantan thermocouples were chosen. The wire diameters were 0.1 and 0.15 mm for constantan and copper, respectively, both enamel covered, giving them approximately the same mechanical properties. Wires of this thickness limit the mechanical strength of the head of the device. It is, nevertheless, robust enough to.take normal field equipment handling. On the other hand, thin wires represent two advantages. Firstly, they reduce the heat conduction from the heated sphere. Secondly, if the heat loss is limited to the sphere itself, a decreasing directional sensitivity of the sensor must be expected. As mentioned above, the thermocouple junctions were placed 37 mm apart. Care was taken to make each junction physically as small as possible. To ensure that the thermocouple junction was centred in the coil this junction was coated with epoxy cement before mounting. Direct thermal contact between the thermocouple and the coil was thus avoided. This method contributes toward higher uniformity. It was a practical advantage to use a wire which may be soldered without first having the insulating coating removed. The hemispheres were all filled with epoxy cement and glued together with the coil and the thermocouple junctions. To ensure they were accurately positioned they were left in a jig during hardening, as shown in Fig.4. rl ~
Fig.4. The jig. The frame was made of 1 mm uncoated copper wire after having been drawn to increase its rigidity. For minimum air disturbance a thin wire was expected to have a lower shading effect. On the other hand, the frame must be firm enough to keep the sensor in position. The head was mounted on a 6-ram acrylic tube. The top end was blocked by a bushing and drilled to insert the frame ends. Screened cables and high quality plugs with gold over silver coatings were chosen to avoid signal disturbances.
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
285
The spheres were finally painted black to reduce their sensitivity to dust deposits (Den Ouden, 1958). The complete device is shown in Fig.5.
Cu
----4
Const.Cu-1 I
-1 cm Fig.5. The complete sensor. INSTRUMENTATION When several sensors are to be operated simultaneously they may be conveniently plugged into the same switchboard. This enables the operator to select the sensors in any required succession b y means of a manual or automatic selector. During the calibration a scanner adjustable to certain time intervals was used, in this case a 2.4-sec interval was chosen. It is essential that all the operated anemometers have their heating coils connected in series. Small deviations due to instability or insufficient resettability of the supply will therefore not affect the comparability of the sensors. The heating current will be identical for all sensors and they will consequently be fed by the same energy 12R as far as R remains constant. This arrangement was made on the panel provided with terminals for connecting the current source. To ensure a heating current with high long-term stability a constant DC source was chosen which had the following specifications when set at 30 mA: temperature dependence less than 3.5" 10 -3 mA or 120 p.p.m. ° c - l ; long-term stability better than 10 -2 mA or approximately 330 p.p.m.
286
P. ROER AND M. KJOLSVIK
The heating circuit also p e r m a n e n t l y covered a milliammeter to ensure precise setting and control of the current. During the calibration (referred to in the following) a digital millivoltmeter with resolution 1 /xV was applied. The instrument was supplied with BCD-output and the signal was recorded on punched paper tape. The c o m p l e t e arrangement is shown in Fig.6.
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l:ig.6, Schematic layout of instruments: l = power supply; 2 = panel; 3 = scanner; 4 = digital ,uV-meter: 5 = adapter with timer; 6 = paper punch driver: 7 = paper punch; 8 = constant current source; 9 = ma-nleter. In most cases a c o m p e n s a t i o n recorder would be the most convenient instrument preferably with resolution d o w n to 5~ V. CALIBRATION The calibration was carried out at the Norwegian Building Research Institute. Fig.7 and 8 show photographs of the low velocity wind tunnel at the institute. Defined air velocities in the test section were set by the corresponding pressure drop over an orifice. Five different standard orifices were used to cover the whole range. For each setting corrections were made for r o o m temperature, atmospheric pressure and R e y n o l d s number. 1
Q = 3.47.10 -6 • o~. d 2 (Ap/y) ~
[ m 3 sec -1 ]
= f(Re): d = orifice diameter ( m m ) , Ap = pressure drop (mm water column): Y = f(P, t).
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
Fig.7. The wind tunnel.
l:ig.8. Wind tunnel working section and sensor arrangement.
287
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This wind tunnel was particularly well suited to our purpose. Maximum obtainable air velocity in the working section was about 5 m sec -1 and the lowest one set during calibration was 1.5 cm sec -1. Even this extremely low velocity proved to be fairly resettable. In some low velocity situations the air movement was made visible by introducing smoke into the test section. The sensors proved to have slightly different characteristics and needed individual calibration. Typical calibration curves are represented in Fig.9 and more complete for the six calibrated sensors in Table I. ~V
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300 t,00500 cmls
CALIBRATIONS AT DIFFERENT HEATING CURRENTS As seen from Fig.9 the calibration has partly been carried out at different heating currents ranging from 10 to 50 mA. Calibration at low heating current was expected to give a lower velocity limit than that of a higher current due to less influence from natural convection. On the other hand, the disadvantage of reducing the heating current below a certain limit is the increasing requirement for resolution of the recording or indicating instrument. At a higher current than 50 mA the temperature of the heated sphere may reach approximately 40 degrees above that of the ambient air. At a given air temperature of 20°-25°C the heated sphere will reach a temperature of 6 0 ° - 6 5 ° C in the lower velocity range. This temperature approaches the limit to which epoxy cement may be exposed. ERRORS
DUE
TO HEATING
CURRENT
DEVIATION
Variations in the heating current may derive from deficient regulation of the current
290
P. ROER AND M. KJOLSVIK
source or from inaccurate adjustment. Errors deriving from deviations in the heating current are shown in Fig. 10. The curve is relevant to 40 c m sec -1 and is not much different for other velocities within this range. Calculated deviations from 30 mA in our case were less than -+0.2-0.3% at room temperature fluctuations of 10 degrees. With the additional inaccuracy of resetting and milliammeter deviation (0.1%) the total error of records is calculated to approximately -+2%.
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0 0
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current
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Fig.10. Errors in velocity measurement due to deviations in heating current. THE LOWER VELOCITY LIMIT
The possibility of measuring extremely low velocities was of greatest interest to us. Efforts were made, therefore, to establish the lower velocity limit. Based on the range of instruments at our disposal, we regarded the lower limit as being determined mainly by natural convection. Three conditions were suspected of influencing natural convection, namely: (1) the size of the sphere; (2) the temperature of the heated sphere above that of the ambient air; (3) orientation of the sensor with respect to the field of gravity. The size of the sphere was kept as small as possible to minimize the natural convection. The heating current was set at 10, 20 and 30 mA for velocities less than 40 cm sec -~ It was expected that a lower heating current might extend the lower velocity range. The arrangement of the sensors is shown in Fig.8 and in Table 1. Only horizontal air flow was possible. Three of the sensors were mounted with the heated sphere topmost and the other three with that at the bottom. In the lower-velocity range, 10 mA was expected to be the most suitable heating current. This proved, however, contrary to our records. For the bottom sphere position, we have recorded a halt at 3 cm sec -~ which remains unexplained (Table I). A regular rise is recorded for both 20 and 30 mA for velocities declining to 3 cm sec -1 and in most cases also below this level. A possible different effect of the two currents is the fact that for 20 mA, acceptable values are read even as far down as 1.5 cm sec -1 This indicates that the lower velocity limit of the sensors does not exceed 3 cm sec -~ at 30 mA. At 20 mA, the influence of natural convection has not been detectable even below this level (Table I).
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
291
As far as the arrangement of the sensors is concerned, a slight deviation in favour of the topmost sphere position can be traced. This is, however, unlikely to be of practical importance. THE EFFECT OF D I F F E R E N T DISTANCES BETWEEN THE SPHERES
As mentioned above, the heat loss from the heated sphere is limited to convection. Loss through radiation to the surroundings and to the unheated sphere, in addition to heat conductance through the thermocouple wire, are disturbing factors. Heat transference to the unheated sphere both through radiation and conductance, will depend on the distance between the two spheres. Two distances between the centers were tested in addition to the general one, i.e., approximately 25 and 12 mm, which correspond, respectively, to 2/3 and 1/3 of the original 37 ram. Results of this test are presented in Fig.11 and 12 and in Table II. Sensors No.49, 50 and 51 were first calibrated according to normal procedure. The distance between the spheres for No.49 and 50 were then reduced by shortening the constantan wire between them. The frame was subsequently bent to give uniform surroundings. The three sensors were then recalibrated in their previous positions, using the unchanged sensor No.51 for reference. The two calibration curves for the reference sensor were near identical and consequently highly reproducible. The deviation due to 1/3 reduction of the original distance between the spheres was not significant (Fig. 11). The deviation due to 2/3 reduction appears from Fig. 12. Respectively, the two sets of curves should represent characteristics of equal reliability for practical purposes. ~V
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"-4-, 9O 70 60 50
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40 30 3
6 78910
20
30
40
60
80 100
200
300 t,O0 cm/s
Fig.1 1. Deviations due to a 1/3 reduction in the distance (a) between the two spheres. Sensor No.49. All values are means of 8 readings.
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EQUIPMENT FOR MEASURING LOW AIR VELOCITY
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40
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80 100
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300 ~00 crn/s
Fig.12. Deviations due to a 2/3 reduction in (a). Sensor No.50. All values are means of 8 readings.
Still another condition deserves mentioning, although it remains to be investigated: The distance between the spheres may be expected to influence the directional characteristics of the sensor. This may be derived from mutual shading of the spheres when the direction of the air flow deviates from the normal of the axis. THE TIME CONSTANT For measuring mean velocities the time constant should not be too small. On the other hand, a large time constant will hardly affect our usage of the sensor. A time constant determination was of interest.
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Fig. 13. Signal decline after initiating air flow at 20 and 40 cm sec-~ T20 = 45 sec; T4o = 39 sec.
294
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From instant velocity start towards equilibrium the signal was expected to follow an approximately exponential function represented by the formula:
(
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f(t) = k l - e -
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f(T) = k . 0 . 6 3
where T is the time constant and k the final value. The time constant was determined at air velocities of 20 and 40 cm sec -1 and represented in Fig. 13. T was subsequently determined from the plotted curves which deviate very little from true exponentials. The time constants were read at 39 and 45 sec for 40 and 20 cm sec -1, respectively.
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Fig.14. Definitions of angles between flow direction and sensor.
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120
±180
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
295
DIRECTIONAL CHARACTERISTICS Settling this question was i m p o r t a n t in our case. T o investigate the extent o f directional dependence, the sensor was turned slowly in the air flow, changing b o t h pitch angle 0 and yaw angle a as defined in Fig.14. The most favourable position will be w h e n b o t h angles are 0 . 0 was varied b e t w e e n - 9 0 °, -+ 180 ° and +90 °, with -+ 180 ° as the most unfavourable position. The rotation was p e r f o r m e d at 1/30 r.p.m, giving ap]~roximately one reading per degree o f rotation. As will appear from Fig.15 the rotation represented only covers 180 °. TABLE III Deviations in air velocity measurements for different combinations of both 0 and c~ 0
~x: 90 ° v (cm/sec)
180 ° 165 ° 150 ° 135 ° 120' 105 ° 90 ° 05 -90 ° -105 ° -120 ° -135 ° -150 ° -165 ° -180 °
14.3
60 ° %
71.5
30°
0°
v (cm/sec)
%
v (cm/sec)
%
v (cm/sec)
%
15.8 17.3 17.7 17.7 17.6 17.6 17.6 17.7 17.5 17.5 17.5 17.5 17.5 16.9 15.7
79.0 86.5 88.5 88.5 88.0 88.0 88.0 88.5 87.5 87.5 87.5 87.5 87.5 84.5 78.5
15.5 17.7 19.2 19.6 19.8 20.0 20.0 20.0 20.0 19.8 19.6 19.6 19.0 17.7 15.7
77.5 88.5 96.0 98.0 99.0 100.0 100.0 100.0 100.0 99.0 98.0 98.0 95.0 88.5 78.5
12.9 15.0 18.3 19.4 19.6 19.2 19.2 20.0 20.0 20.0 19.8 19.4 18.1 16.0 12.8
64.5 75.0 91.5 97.0 98.0 96.0 96.0 100.0 100.0 100.0 99.0 97.0 90.5 80.0 64.0
Deviations from full value caused b y shading f r o m the frame in excess o f 10% cover a p p r o x i m a t e l y 20 ° on b o t h sides o f the most unfavourable position. R o t a t i o n at different values of a enables a three-dimensional representation of the directional characteristics. The figures are given in Table III. All values are given in cm sec -1 and percentage of full value at 0 = 0 and a = 0. Deviations in excess o f 10% of full value are limited to cases of e x t r e m e l y adverse sensor position in relation to the direction of the air flow. Such positions are easily avoidable in cases w h e n the main direction is known. ACKNOWLEDGEMENTS The present w o r k has been financed by The Norwegian Council for Agricultural Research and carried out under guidance of J. Apeland of the Storage Research C o m m i t t e e . The authors gratefully acknowledge his e n c o u r a g e m e n t to have the w o r k separately published.
296
P. ROER AND M. KJOLSVIK
We gratefully acknowledge the helpful discussion with E. Skaaret at The Technical University of Norway, concerning important details. We further wish to express our gratitude to S. Myklebost, at The Norwegian Building Research Institute, Oslo, for the opportunity of using the wind tunnel during calibration. Thanks are due to Department of Agricultural Structures for providing laboratory facilities. REFERENCE Den Ouden, H. Ph. L., 1958. The measurementof air velocities. T.N.O. Rept., 27:17 pp.
EQUIPMENT FOR MEASURING LOW AIR VELOCITY R ROER and M. KJOLSVIK
Department of Agricultural Structures, The Agricultural University, As (Norway) (Accepted for publication October 2, 1973)
ABSTRACT Roer, P. and Kj/51svik, M., 1973. Equipment for measuring low air velocity~ Agric. Meteorol., 12: 281-296. This publication comprises detailed information for the construction of an inexpensive low velocity thermo-anemometer as well as experience gained from its application. It includes a description of necessary instruments involved and some important requirements related to them. The method of calibration is described. The lower velocity limit for the particular type of sensor is established as well as its directional characteristics, etc. High levels of reproducibility have been obtained even at extremely low velocities. The lower velocity limit was down to 3 cm sec ~ for any separate reading performed. At a reduced heating current disturbances caused by natural convection have not been observed even below 3 cm sec -~ .
INTRODUCTION Our demand for adequate e q u i p m e n t to measure low air velocities is c o n n e c t e d with research on the climate in store-rooms for fresh agricultural produce. We selected the t h e r m o - a n e m o m e t e r as being the m o s t suitable for our demands. It records the t e m p e r a t u r e difference b e t w e e n a heated and u n h e a t e d t h e r m o c o u p l e as a f u n c t i o n of air velocity. This t y p e of a n e m o m e t e r was first developed by Den O u d e n (1958) at TNO, Delft, and further elaborated at The Technical University o f N o r w a y , T r o n d h e i m . A detailed description of the c o n s t r u c t i o n o f the sensors and the experience gained f r o m using t h e m is included. This m a y be of interest in biological research w h e n measuring e x t r e m e l y low air velocities. BASIC PRINCIPLE OF OPERATION Fig. 1 represents the principle o f the transducer. The t w o externally identical spherical bodies, b o t h enclosing a t h e r m o c o u p l e , are equally exposed to the air m o v e m e n t . When one o f the spheres is heated by constant current, leaving the other at a m b i e n t air temperature, the difference in temperature b e t w e e n the t w o b e c o m e s a measure o f air velocity.
282
P. ROER AND M. KJOLSVIK
--I> I> --I> 1> --1> I>v
E
--l>
I> 1>
constant heating current
Fig.1. Transducer principle. It is of great importance that the amount of energy (determined by W = 1"2R) is kept constant within very limited range. Constantan was chosen for the heating filament, whose temperature coefficient of resistance is as low as 2.10 -s per degree. This enables the flow of energy to remain constant if supplied by a suitable source of current. When the loss of heat through radiation and conduction is minimized, the temperature of the heated sphere almost exlusively depends on its loss of heat through convection. The e.m.f, output will consequently appear a reliable function of velocity. According to Den Ouden (1958), the influence of air temperature was clearly negligible within the practical range.
DETAILSOF CONSTRUCTION The outer sphere diameter was set at 4.76 mm (3/16") and inner diameter at 3.97 mm (5/32 ") to provide a conveniently sized sphere for easy mounting of the coil and thermocouple. Pressing the separate hollow hemisphere required adequate tools. The equipment consisted of a tool and hardened die, see Fig.2. The cavities form the exterior of the sphere, while the 3.99 into (5/32 ") diameter ball corresponds with its inner diameter, To ensure accuracy in form, balls from ball-bearings were employed. A ball with diameter 3/16" was silver soldered to the end of a short rod and subsequently edged. This tool was employed to form the two cavities. The simple tool for pressing the hemisphere was a 5/32" ball silver soldered to a short rod. The hemispheres were made of annealed copper sheet which is easily shaped and has high thermal conductivity to assure uniform surface temperature. Discs with diameter of 7 mm were punched and pressed into hemispheres of the 3/32" ball in a drill press. The hemisphere was then moved to the sharp-edged cavity and filed down to die level.
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
283
V//////////////~V////////////////////AV//////////////A
'l -l.Omm
O I
O sm"
Fig.2. Die and tool for pressing the hemispheres. Where the heating coil was to be mounted, a hole with a diameter of 0.3 mm was drilled to let the coil ends through. A slot was also filed in the edge of one of the hemispheres to let the thermocouple wires through (Fig.3).
G) Fig. 3. Hemispheres and heating coil. The heating coil was an 0.1-mm enamel covered constantan wire. In order to keep the temperature of the coil wire to a minimum the length was set at 60 cm. The coil could be mounted in the sphere without difficulty if appropriately wound. The winding was done on a 2-mm drdl shank, leaving the coil with a 2-ram hole through the center. Care was always taken to cut the coil wire into equal lengths and to keep the protruding coil ends at an equal distance from the sphere.
284
P, ROER AND M. KJOLSVIK
The distance between the centers of the two spheres was set at 37 mm for our particular purposes. Generally, the distance might be set much lower, e.g., 15-20 mm (cf. p.290). Ordinary copper constantan thermocouples were chosen. The wire diameters were 0.1 and 0.15 mm for constantan and copper, respectively, both enamel covered, giving them approximately the same mechanical properties. Wires of this thickness limit the mechanical strength of the head of the device. It is, nevertheless, robust enough to.take normal field equipment handling. On the other hand, thin wires represent two advantages. Firstly, they reduce the heat conduction from the heated sphere. Secondly, if the heat loss is limited to the sphere itself, a decreasing directional sensitivity of the sensor must be expected. As mentioned above, the thermocouple junctions were placed 37 mm apart. Care was taken to make each junction physically as small as possible. To ensure that the thermocouple junction was centred in the coil this junction was coated with epoxy cement before mounting. Direct thermal contact between the thermocouple and the coil was thus avoided. This method contributes toward higher uniformity. It was a practical advantage to use a wire which may be soldered without first having the insulating coating removed. The hemispheres were all filled with epoxy cement and glued together with the coil and the thermocouple junctions. To ensure they were accurately positioned they were left in a jig during hardening, as shown in Fig.4. rl ~
Fig.4. The jig. The frame was made of 1 mm uncoated copper wire after having been drawn to increase its rigidity. For minimum air disturbance a thin wire was expected to have a lower shading effect. On the other hand, the frame must be firm enough to keep the sensor in position. The head was mounted on a 6-ram acrylic tube. The top end was blocked by a bushing and drilled to insert the frame ends. Screened cables and high quality plugs with gold over silver coatings were chosen to avoid signal disturbances.
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
285
The spheres were finally painted black to reduce their sensitivity to dust deposits (Den Ouden, 1958). The complete device is shown in Fig.5.
Cu
----4
Const.Cu-1 I
-1 cm Fig.5. The complete sensor. INSTRUMENTATION When several sensors are to be operated simultaneously they may be conveniently plugged into the same switchboard. This enables the operator to select the sensors in any required succession b y means of a manual or automatic selector. During the calibration a scanner adjustable to certain time intervals was used, in this case a 2.4-sec interval was chosen. It is essential that all the operated anemometers have their heating coils connected in series. Small deviations due to instability or insufficient resettability of the supply will therefore not affect the comparability of the sensors. The heating current will be identical for all sensors and they will consequently be fed by the same energy 12R as far as R remains constant. This arrangement was made on the panel provided with terminals for connecting the current source. To ensure a heating current with high long-term stability a constant DC source was chosen which had the following specifications when set at 30 mA: temperature dependence less than 3.5" 10 -3 mA or 120 p.p.m. ° c - l ; long-term stability better than 10 -2 mA or approximately 330 p.p.m.
286
P. ROER AND M. KJOLSVIK
The heating circuit also p e r m a n e n t l y covered a milliammeter to ensure precise setting and control of the current. During the calibration (referred to in the following) a digital millivoltmeter with resolution 1 /xV was applied. The instrument was supplied with BCD-output and the signal was recorded on punched paper tape. The c o m p l e t e arrangement is shown in Fig.6.
,
l I
6
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i I'
I JI 9
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l:ig.6, Schematic layout of instruments: l = power supply; 2 = panel; 3 = scanner; 4 = digital ,uV-meter: 5 = adapter with timer; 6 = paper punch driver: 7 = paper punch; 8 = constant current source; 9 = ma-nleter. In most cases a c o m p e n s a t i o n recorder would be the most convenient instrument preferably with resolution d o w n to 5~ V. CALIBRATION The calibration was carried out at the Norwegian Building Research Institute. Fig.7 and 8 show photographs of the low velocity wind tunnel at the institute. Defined air velocities in the test section were set by the corresponding pressure drop over an orifice. Five different standard orifices were used to cover the whole range. For each setting corrections were made for r o o m temperature, atmospheric pressure and R e y n o l d s number. 1
Q = 3.47.10 -6 • o~. d 2 (Ap/y) ~
[ m 3 sec -1 ]
= f(Re): d = orifice diameter ( m m ) , Ap = pressure drop (mm water column): Y = f(P, t).
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
Fig.7. The wind tunnel.
l:ig.8. Wind tunnel working section and sensor arrangement.
287
288
P. ROER AND M. KJOLSVIK
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289
This wind tunnel was particularly well suited to our purpose. Maximum obtainable air velocity in the working section was about 5 m sec -1 and the lowest one set during calibration was 1.5 cm sec -1. Even this extremely low velocity proved to be fairly resettable. In some low velocity situations the air movement was made visible by introducing smoke into the test section. The sensors proved to have slightly different characteristics and needed individual calibration. Typical calibration curves are represented in Fig.9 and more complete for the six calibrated sensors in Table I. ~V
,o0 600 500 400 300
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30
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Fig.9. Calibration curves for sensor No.44. All values are means of 8 readings.
300 t,00500 cmls
CALIBRATIONS AT DIFFERENT HEATING CURRENTS As seen from Fig.9 the calibration has partly been carried out at different heating currents ranging from 10 to 50 mA. Calibration at low heating current was expected to give a lower velocity limit than that of a higher current due to less influence from natural convection. On the other hand, the disadvantage of reducing the heating current below a certain limit is the increasing requirement for resolution of the recording or indicating instrument. At a higher current than 50 mA the temperature of the heated sphere may reach approximately 40 degrees above that of the ambient air. At a given air temperature of 20°-25°C the heated sphere will reach a temperature of 6 0 ° - 6 5 ° C in the lower velocity range. This temperature approaches the limit to which epoxy cement may be exposed. ERRORS
DUE
TO HEATING
CURRENT
DEVIATION
Variations in the heating current may derive from deficient regulation of the current
290
P. ROER AND M. KJOLSVIK
source or from inaccurate adjustment. Errors deriving from deviations in the heating current are shown in Fig. 10. The curve is relevant to 40 c m sec -1 and is not much different for other velocities within this range. Calculated deviations from 30 mA in our case were less than -+0.2-0.3% at room temperature fluctuations of 10 degrees. With the additional inaccuracy of resetting and milliammeter deviation (0.1%) the total error of records is calculated to approximately -+2%.
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E
0 0
2 % Heating
current
error
Fig.10. Errors in velocity measurement due to deviations in heating current. THE LOWER VELOCITY LIMIT
The possibility of measuring extremely low velocities was of greatest interest to us. Efforts were made, therefore, to establish the lower velocity limit. Based on the range of instruments at our disposal, we regarded the lower limit as being determined mainly by natural convection. Three conditions were suspected of influencing natural convection, namely: (1) the size of the sphere; (2) the temperature of the heated sphere above that of the ambient air; (3) orientation of the sensor with respect to the field of gravity. The size of the sphere was kept as small as possible to minimize the natural convection. The heating current was set at 10, 20 and 30 mA for velocities less than 40 cm sec -~ It was expected that a lower heating current might extend the lower velocity range. The arrangement of the sensors is shown in Fig.8 and in Table 1. Only horizontal air flow was possible. Three of the sensors were mounted with the heated sphere topmost and the other three with that at the bottom. In the lower-velocity range, 10 mA was expected to be the most suitable heating current. This proved, however, contrary to our records. For the bottom sphere position, we have recorded a halt at 3 cm sec -~ which remains unexplained (Table I). A regular rise is recorded for both 20 and 30 mA for velocities declining to 3 cm sec -1 and in most cases also below this level. A possible different effect of the two currents is the fact that for 20 mA, acceptable values are read even as far down as 1.5 cm sec -1 This indicates that the lower velocity limit of the sensors does not exceed 3 cm sec -~ at 30 mA. At 20 mA, the influence of natural convection has not been detectable even below this level (Table I).
EQUIPMENT FOR MEASURING LOW AIR VELOCITY
291
As far as the arrangement of the sensors is concerned, a slight deviation in favour of the topmost sphere position can be traced. This is, however, unlikely to be of practical importance. THE EFFECT OF D I F F E R E N T DISTANCES BETWEEN THE SPHERES
As mentioned above, the heat loss from the heated sphere is limited to convection. Loss through radiation to the surroundings and to the unheated sphere, in addition to heat conductance through the thermocouple wire, are disturbing factors. Heat transference to the unheated sphere both through radiation and conductance, will depend on the distance between the two spheres. Two distances between the centers were tested in addition to the general one, i.e., approximately 25 and 12 mm, which correspond, respectively, to 2/3 and 1/3 of the original 37 ram. Results of this test are presented in Fig.11 and 12 and in Table II. Sensors No.49, 50 and 51 were first calibrated according to normal procedure. The distance between the spheres for No.49 and 50 were then reduced by shortening the constantan wire between them. The frame was subsequently bent to give uniform surroundings. The three sensors were then recalibrated in their previous positions, using the unchanged sensor No.51 for reference. The two calibration curves for the reference sensor were near identical and consequently highly reproducible. The deviation due to 1/3 reduction of the original distance between the spheres was not significant (Fig. 11). The deviation due to 2/3 reduction appears from Fig. 12. Respectively, the two sets of curves should represent characteristics of equal reliability for practical purposes. ~V
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. . . . .
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"-4-, 9O 70 60 50
"~20mA
40 30 3
6 78910
20
30
40
60
80 100
200
300 t,O0 cm/s
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292
P. ROER A N D M. KJOLSVIK
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,~V 700600 + 500/,00 300 200
SO - - - 60 50 40 30
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Still another condition deserves mentioning, although it remains to be investigated: The distance between the spheres may be expected to influence the directional characteristics of the sensor. This may be derived from mutual shading of the spheres when the direction of the air flow deviates from the normal of the axis. THE TIME CONSTANT For measuring mean velocities the time constant should not be too small. On the other hand, a large time constant will hardly affect our usage of the sensor. A time constant determination was of interest.
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294
P. ROER AND M. KJOLSVIK
From instant velocity start towards equilibrium the signal was expected to follow an approximately exponential function represented by the formula:
(
')
f(t) = k l - e -
~
f(T) = k . 0 . 6 3
where T is the time constant and k the final value. The time constant was determined at air velocities of 20 and 40 cm sec -1 and represented in Fig. 13. T was subsequently determined from the plotted curves which deviate very little from true exponentials. The time constants were read at 39 and 45 sec for 40 and 20 cm sec -1, respectively.
e=o -o;\
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120
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EQUIPMENT FOR MEASURING LOW AIR VELOCITY
295
DIRECTIONAL CHARACTERISTICS Settling this question was i m p o r t a n t in our case. T o investigate the extent o f directional dependence, the sensor was turned slowly in the air flow, changing b o t h pitch angle 0 and yaw angle a as defined in Fig.14. The most favourable position will be w h e n b o t h angles are 0 . 0 was varied b e t w e e n - 9 0 °, -+ 180 ° and +90 °, with -+ 180 ° as the most unfavourable position. The rotation was p e r f o r m e d at 1/30 r.p.m, giving ap]~roximately one reading per degree o f rotation. As will appear from Fig.15 the rotation represented only covers 180 °. TABLE III Deviations in air velocity measurements for different combinations of both 0 and c~ 0
~x: 90 ° v (cm/sec)
180 ° 165 ° 150 ° 135 ° 120' 105 ° 90 ° 05 -90 ° -105 ° -120 ° -135 ° -150 ° -165 ° -180 °
14.3
60 ° %
71.5
30°
0°
v (cm/sec)
%
v (cm/sec)
%
v (cm/sec)
%
15.8 17.3 17.7 17.7 17.6 17.6 17.6 17.7 17.5 17.5 17.5 17.5 17.5 16.9 15.7
79.0 86.5 88.5 88.5 88.0 88.0 88.0 88.5 87.5 87.5 87.5 87.5 87.5 84.5 78.5
15.5 17.7 19.2 19.6 19.8 20.0 20.0 20.0 20.0 19.8 19.6 19.6 19.0 17.7 15.7
77.5 88.5 96.0 98.0 99.0 100.0 100.0 100.0 100.0 99.0 98.0 98.0 95.0 88.5 78.5
12.9 15.0 18.3 19.4 19.6 19.2 19.2 20.0 20.0 20.0 19.8 19.4 18.1 16.0 12.8
64.5 75.0 91.5 97.0 98.0 96.0 96.0 100.0 100.0 100.0 99.0 97.0 90.5 80.0 64.0
Deviations from full value caused b y shading f r o m the frame in excess o f 10% cover a p p r o x i m a t e l y 20 ° on b o t h sides o f the most unfavourable position. R o t a t i o n at different values of a enables a three-dimensional representation of the directional characteristics. The figures are given in Table III. All values are given in cm sec -1 and percentage of full value at 0 = 0 and a = 0. Deviations in excess o f 10% of full value are limited to cases of e x t r e m e l y adverse sensor position in relation to the direction of the air flow. Such positions are easily avoidable in cases w h e n the main direction is known. ACKNOWLEDGEMENTS The present w o r k has been financed by The Norwegian Council for Agricultural Research and carried out under guidance of J. Apeland of the Storage Research C o m m i t t e e . The authors gratefully acknowledge his e n c o u r a g e m e n t to have the w o r k separately published.
296
P. ROER AND M. KJOLSVIK
We gratefully acknowledge the helpful discussion with E. Skaaret at The Technical University of Norway, concerning important details. We further wish to express our gratitude to S. Myklebost, at The Norwegian Building Research Institute, Oslo, for the opportunity of using the wind tunnel during calibration. Thanks are due to Department of Agricultural Structures for providing laboratory facilities. REFERENCE Den Ouden, H. Ph. L., 1958. The measurementof air velocities. T.N.O. Rept., 27:17 pp.