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Ref. No. [UMCES] CBL 2015-010
ACT VS15-03
measurements is better than 0.01 pH for seawater (
S
≥ 20), but actual laboratory based accuracy
and precision levels were quantified and reported for the brackish and freshwater environments.
Laboratory Test
Two thermally insulated, covered, 4.5 m
3 “
source-water tanks” were filled with 1 µm
filtered seawater, or a mixture of freshwater and filtered seawater. The two tanks were then
isolated so that each could be maintained at a specific temperature. The source-water tanks were
used to supply water into a third smaller "test tank" (capacity - .8 m
3
), where all instrument
performance measurements took place. One tank with source-water was used to continuously
flush the third test tank (containing the in-situ instruments); and water from the second source
tank was used to create a quick transition to a new temperature condition within the test-tank.
Test conditions within the source-water tank were set and equilibrated for several days prior to
delivery into the test tank. The large volume of equilibrated water in the source-water tank
allowed for a rapid transition (10-15 minutes) of temperature and salinity conditions in the test
tank. Temperature was maintained within the source water tank to ± 1
o
C using an AquaLogic
MT-3 circulating heat exchanger. Water in both of the tanks was mixed continuously with
several submerged bilge pumps. Evaporation and heat exchange through the water surface was
reduced to a minimum by using a covering on the surface of the water. The test tank was
instrumented with the test instruments, as well as three factory calibrated RBR temperature
recorders (accurate to 0.02
o
C) placed near the instruments to continuously measure actual
temperature conditions experienced by the test instruments. These data were used to help
evaluate fine scale variability within the test tank and to correct for temperature offsets that
might exist during pH measurement of discrete reference samples.
The test tank pH was also monitored continuously with two glass pH electrodes
(Metrohm ECOTRODE PLUS 6.0262.100) measured to 0.1 mv, and spaced across the span of
the test instruments. These data were used to create a continuous data record of pH within the
tank, and to confirm test conditions during acid/base additions. These pH data will not be used
as reference pH data to calculate instrument offsets. The pH probes were calibrated against the
dye estimated values obtained on test tank samples during acid-base additions (at the fixed
experimental T-S conditions) to get slope responses over a pH range of approximately 7.1 to 8.3.
In this way the electrodes did not experience any change in liquid junction potential from either
freshwater or saltwater buffers (Easley and Byrne 2012).
Each week testing was conducted at a set combination of temperature and salinity (T-S).
Nominal temperature conditions were set for 10, 20 and 30
o
C, and salinity conditions were set
for nominally 0, 20 and 35 psu. A week-long test was performed at each T-S combination.
After 4 to 6 days of testing at a stable T-S condition and ambient pH, pH was cycled over a
reasonable range using acid-base additions to the water of the test tank (7.5 to 8.5 for seawater
and 6.5 to 8.8 for freshwater). Two, raised - lowered pH cycles were conducted at each T-S
condition over the course of one day. Acid/base additions were done by first mixing known
quantities of acid/base into several liters of the current test solution and then adding this solution
into the test tank to facilitate mixing and rapid equilibration.
The sequencing of tests was to start with a fixed salinity and the tests were performed for
that salinity at the three different temperatures, starting at 10
o
C and increasing sequentially up to
30
o
C. In this way we were able to use the same source water for all three temperature
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