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od of calibrating that could be used in the field in the absence of the rotating arm apparatus. In quiescent solutions or solutions with very low forced convection rates, the density difference between the (saturated) liquid-solid interface and the bulk solution drives a convection current which is the principal mechanism that moves solution away from the solid. The natural and forced convection terms are in general not directly additive (Pei 1965; Churchill 1977) in determining total convection or the dissolution rates. We suspended clod cards in quiescent solutions in ice chests under constant temperature conditions indoors. The ice chests supported dissolution rates roughly following Eq. 4 over 4-6 d (Fig. 11A); Fig. 11 B correlates the natural convection ice-chest data. The equation fitting the data is [ 1 - ( W,lWi)1’3]/8 = 2.81 ($)[(&)(+L.,,] 1.25 (10) with r = 0.994. Equations 8 and 10 can be combined to give an expression that will allow an equivalent integrated velocity to be determined from data obtained in the field. The group Sn = [ 1 - (~~ Wi)“]lB is obtained in natural convection clod card tests with water from the field test site. Weight loss data from cards exposed to water motion in the field is reduced to a corresponding form: Then the integrated calculated as Sf = [ 1 - ( Wr/ Wi)“]/8- water speed in the field is I/= 4.31(5=(y). (11) S’.25/Sn is similar to DF (Doty 197 1) but should be superior as an indicator of water motion in that it corrects for the effects of decreasing surface area during the tests and the greater effect of forced convection in producing dissolution relative to free convection (the 1.25 exponent for S& If field tests are conducted in water with lim- 12 '; 0.03 0 3 ? -y 0.02 c r' Y C a ~0~/6.,25~C,n=Z . 25 3.. 16.5-C Time, d 0.04 , 0 Tap 0 20 '/.. 0.01 0.01 0.02 (T/T~~)&/P) AC250 mol Ihr-' A . Fig. 11. Free convection data obtained by suspending clod cards in large volumes of quiescent solutions in ice chests (A), and (B) correlation of the data using Eq. 8 at T,, = 298°K (25°C). ited motion, say V < 2 cm s-l, Eq. 11 will give erroneous results because free convection forces can also produce dissolution in the field. If it is necessary to separate the forced and free convection effects, multiply the right-hand side of Eq. 11 by [ 1 - (SJSf)3]5/12. This is a rough correction derived from Churchill (1977) for heat transfer with assisting forced and free convection. Many of the applications for clod cards involve measurement of unsteady flow, ranging from wind waves to oscillatory tidal currents. The methods presented here will indicate an integrated average speed near the scalar arithmetic mean velocity of the water because [ 1 - ( Wj &)“I/8 is nearly linear with velocity (actually Ve8). For example, if the water velocity over the card varies linearly from V, at time zero to V, at time 8, Eq. 1 and 2 can be used with the mass transfer coefficient from Eq. 8 to derive the expression 0.03
1778 Notes Sample Points Along center Lines - 6.0 - 5.0 -4.0 h b E .2 -3.0 .z 2 zi -2.0 z - 1.0 ABCD 0 ABCD ABCD ABCD 0.056 0.072 0.095 Flow Rate (liters se’) Fig. 12. Water motion vs. flow rate in culture tanks. Clod cards were attached to metal bars inserted into tanks holding the cards in positions A-D indicated on insert drawing. Water source was freshwater at 30°C and weight loss of the cards was converted to water motion using the temperature-corrected curve for freshwater in Table 2. Line connects the mean value of water motion in each tank at each flow rate. ABCD 0.160 _- 0 ( ) l/3 l- 1-k: = o.74$-J(yf) x [ lv&;J$JACl~e. (12) The following is a numerical example: if speed varied linearly from 16 to 4 cm s-l (mean, 10 cm s-l) over a 24-h period in 25°C seawater with a salinity of 35Y&, then for a clod card with AJ Wi = 1.38 cm* g- l, the predicted fraction dissolved from Eq. 12 is A W/ Wi = 0.328. From the steady flow Eq. 8, this dissolution fraction would be expected for a constant speed of 9.87 cm s- l, a value differing from the average flow in this example by - 1.3%. As previously explained, significant errors can be expected for low rates of water motion because the cards cannot differentiate between dissolution caused by free or forced convection under such conditions. The usefulness of clod cards in measuring natural and induced water motion was tested. In addition to the reef water motion experiments cited by Glenn and Doty (1992), the cards were used to investigate the amount of water motion induced by the flow of water into culture tanks (Fig. 12). Four cards were placed in the positions shown in each tank in order to measure water motion near the top and the bottom of the water column and at the outer circumference and near the center standpipe of the tanks (Fig. 12, insert). Increased flow into the tank mainly increased the amount of water motion near the periphery of the tank, whereas water motion near the center remained low. The results point up the inefficiency of using volumetric water exchange as a means of inducing water motion even though it is a common practice in culture studies. In conclusion, contrary to Muus (1968) and Doty (197 l), we have not found problems of erratic variability in fractional weight loss under seemingly uniform conditions by CaSO, blocks from the same lots. The still-water calibration method recommended by Doty (197 1) and Doty et al. (1986) should not be used. As pointed out by Howerton and Boyd (1992) as
Page 1 and 2: 1768 Notes coming tide over a broad
Page 3 and 4: 1770 Notes r=0.15 (typ.) ‘F q- El
Page 5 and 6: 1772 Notes J 1.0 2 E 0.9 L 2 0.6 P
Page 7 and 8: 1774 Notes 0.6 A 25.5’ A 30.5. 0
Page 9: 1776 Notes 1000 500 < 200 p 100 7 f

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