ACT pH Sensor Verification Protocols, PV12-01
7
tank. Temperature will be maintained within the source water tank to ± 1
o
C using an
AquaLogic MT-3 circulating heat exchanger. A secondary temperature regulating system may
be added to the test tank to try to maintain the temperature to better than 0.5
o
C. Water in the
tanks will be mixed continuously with several submerged bilge pumps. Evaporation and heat
exchange through the water surface will be reduced to a minimum by using a covering on the
surface of the water. The test tank will be instrumented with the test instruments, as well as a
SeaBird CTD 19 (calibrated with a Guideline Portasal that is calibrated with IAPSO standards)
and three factory calibrated RBR temperature recorders (accurate to 0.02
o
C) placed near the
instruments to continuously measure actual temperature and salinity conditions experienced by
the test instruments. These data will be used to help evaluate fine scale variability within the test
and to correct for temperature offsets that might exist during pH measurement of discrete
reference samples.
The test tank pH will also be 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 will be 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. The pH probes will be calibrated each day by altering the pH of the test
water (set at the experimental T-S conditions) and measuring the pH of the water using the dye
technique to define the slope response (Easley and Byrne 2012). In this way the electrodes will
not experience any change in liquid junction potential from either freshwater or saltwater buffers.
Each week we will conduct testing at a set combination of temperature and salinity (T-
S). Nominal temperature conditions will be set for 10, 20 and 30
o
C, and nominal salinity
conditions will be set for nominally 0, 20 and 35 psu. A week-long test will be performed at
each T-S combination. After 4 or 5 days of testing at a stable T-S condition and ambient pH, we
will cycle pH 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 will be
conducted at each T-S condition over the course of one day. Acid/base additions will be 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 will be to start with a fixed salinity and perform the tests for that
salinity at the three different temperatures, starting at 30
o
C and decreasing sequentially down to
10
o
C. In this way we will be able to use the same source water for all three temperature
conditions. Each test is scheduled to last 1 week, except when delays are needed to
accommodate work schedules. For each new T-S test condition, the test tank will be flushed and
filled with new source water while keeping all test instruments submerged and recording
continuously. Each new batch of source water will be filtered through a 1 µm cartridge prior to
use in the tests.
6.2.
Reference Sampling
During the stable temperature and salinity period we will collect and analyze reference
samples at 4 timepoints each day. In addition, on one day during the stable cycle we will collect
a burst sampling of five independent measurements taken 5 minutes apart at one of the
timepoints to evaluate the repeatability and uncertainty of the reference measurement with
respect to the variability observed within the test tank. Reference samples will be drawn directly
