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Notes
1000
500
< 200
p 100
7
f 50
3 Tap. ’
v 20 30
0 32 30
b 40 “/.o
0.5
e- 0.4
B
z- 0.3
P
E
6
z
0.2
0 Baker (Tap, 30.3YZ)
A EMS (Tap, 30.5’C)
20
f 0.1
10
200 500 1000 2000 5000 lE4 2E4 5E4 lE5
Fig. 9. Comparison of clod card data with the equation
of Pastemak and Gauvin (1960) for spheres (dashed line)
and the equation for a flat plate (Skelland 1974), represented
by the dotted line. The data of Glenn and Doty
(1992) for 32960 seawater are shown, but were not used in
obtaining Eq. 12. The points below iVRe = 1,000 are increased
by natural convection and were also omitted from
the correlation.
NRI?
rienced problems of variability of weight loss
under uniform conditions by clod cards from
the same batch. Referring to Fig. 2, the variability
of clod card weight loss in our tests can
be estimated by comparing the loss from
matched pairs of cards at the same radial position
on the arm (1, 2, 3, . . .) but on opposite
sides of the arm. For all of our runs with
EM Science plaster of paris at the different temperatures,
salinities, and speeds, the standard
deviation of the weight loss differences, as
[CA w/KLIe* - (A IV/ WJsideZ], was 0.0135,
while the overall average difference was 7.7 x
10m4 (n = 60), which is not significantly different
from zero (P = 0.05). Extreme variability
of weight loss within a batch of cards
caused no problems during our tests.
We also ran limited tests with plaster of paris
from Sargent-Welsh Chemical Co. (technical
grade) and from Baker Scientific Co. (reagent,
extra-fine powder ~44 pm, suitable for electrophoresis
gel). The Sargent-Welsh material
produced clod cards of the same density as the
EM Science material used throughout these
tests; a slight excess of water (375 ml : 500 g)
used with the Sargent-Welsh plaster also failed
to alter the final density. There was no significant
difference between the dissolution rates
of cards made with the Sargent-Welsh plaster
and those made with EM Science material when
tested under the same conditions. The cards
made with plaster from Baker Scientific are of
more interest because of their higher density:
6 6 10 12
Velocity, cm s -’
2 5 1U LO
Velocity, cm s -’
Fig. 10. Effect of clod card density on dissolution rates.
Cards made from reagent-grade plaster of paris from Baker
had densities 50% higher than cards made with material
from EMS. Upper plot illustrates differences in dissolution
rates between the two batches, while the slopes of the lines
in the lower plot of the same data, adjusted by the term
Wi/Ai, are not significantly different (P > 0.05). Points
below V = 2 cm s-l, influenced by free convection, were
omitted from the analysis.
1.8 g cm-3 as opposed to 1.2 for our standard
EM Science material. High density cards are
likely to be preferable for fieldwork because
they take longer to dissolve than lower density
cards of the same volume, and they tend to be
stronger and less likely to be accidentally broken
or chipped during handling. Figure 10 indicates
that Eq. 8 holds for both high- and lowdensity
cards when the proper values Of Ai/ Wi
(1.38 for EMS and 0.92 cm* g-l for Baker) are
used. Despite these results, one should use caution
in changing the source or grade of plaster
of paris; materials are occasionally added to
building-grade material to lower the solubility
and enhance water resistance.
In addition to the rotating arm method for
calibrating clod cards, we also wanted to pursue
Doty’s (1971) and Howerton and Boyd’s
(1992) suggestions for a free-convention meth-