ACT pH Sensor Verification Protocols, PV12-01
6
5.
Summary of Verification Approach
The ACT verification of in situ pH sensors will occur in two separate test components.
The first component will involve extended laboratory/mesocosm tests conducted over 2-3
months, under well-controlled conditions designed to minimize biofouling while covering a
broad range of water temperatures, salinities, and pH. The second component will involve field-
based deployments of 2-3 month duration at three to four coastal ocean and freshwater sites. The
test approach and analytical methods for the laboratory test are outlined below. Details of the
field test will be determined following further discussions with the Technology Advisory
Committee and participating manufacturers after reviewing the results of the laboratory test.
ACT will convene another protocol development workshop for this purpose in the spring of
2013,
with the goal of initiating the field test in the summer of 2013.
6.
Laboratory Test Overview
The overall goal of the laboratory test is to determine the repeatability and uncertainty of
the in-situ pH instruments over week to month periods, under well-characterized conditions and
without the influence of bio-fouling. The laboratory test will be conducted at the Hawaii Institute
of Marine Biology (HIMB), Kaneohe, HI starting in November 2012. Laboratory tests will be
conducted in a single test tank that will accommodate all instruments. The test water will be
made from mixtures of 1 µm-filtered seawater and freshwater (de-chlorinated tap water). The
laboratory test will last 2-3 months, and will include periods of days at stable temperature,
salinity and pH followed by several rapid increases and decreases in pH over a day. Experiments
will be conducted at three salinities (nominally 35, 20 and 0 psu), and at three temperatures
(
approximately 10, 20 and 30
o
C) for each salinity. Temperature will be maintained within 1
o
C
for the duration of each test condition but monitored throughout the test to better than 0.05
o
C.
Instruments will be tested for 4-5 days at a stable T-S condition and ambient pH, followed by
rapid cycles in pH between 6.5 and 8.5 using acid-base additions over periods of hours. The
water in the test tank will be mixed using submerged bilge pumps. Temperature, salinity and pH
of the tank water will be continuously monitored at several points throughout the tank. The
chosen reference standard against which instrument values will be compared will be pH
measured on discrete water samples using a spectrophotometric approach with a purified pH
indicator dye.
6.1.
Laboratory Test Experimental Design
Two thermally insulated, covered, 4.5 m
3 “
source-water tanks” will be filled with 1 µm
filtered seawater, or a mixture of freshwater and filtered seawater. The two tanks will then be
isolated so that each can be maintained at a specific temperature. The source-water tanks will be
used to supply water into a third smaller "test tank" (capacity - 1 m
3
),
where all instrument
performance measurements will take place. One tank with source-water will be used to
continuous flush the third test tank (containing the in-situ instruments); and water from the
second source tank will be used to create a quick transition to a new temperature condition
within the test-tank. Test conditions within the source-water tank will be set and equilibrated for
several days prior to delivery into the test tank. The large volume of equilibrated water in the
source-water tank will allow for a rapid transition (10-15 minutes) of T-S conditions in the test
