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Ref. No. [UMCES] CBL 2015-013
ACT VS15-06
respondents using more than one type. The greatest area of use among respondents was
academic research (76%) followed by state and federal resource management (40%). The
environmental applications were broad with coastal ocean greatest (74%), followed by estuarine
(64%), followed by open-ocean (41%), and followed by freshwater (38%). Correspondingly,
these environments included temperature ranges from -5 to 50
o
C, with medians of 5
o
C and
28
o
C when responses were binned into low and high ranges. Similarly, salinities ranged from
0 – 100, with low and high bin medians of 15 and 35, respectively. The range of pH measured
by the respondents in these applications was between 4.0 and 11.0, with low and high bin median
values of 7.0 and 8.3. Remote deployment was the most common method of use (74%),
followed by depth profiling (50%), then hand-held portable use (48%), then flow-through
systems (26%). Respondents used a variety of calibration procedures including commercial
buffers (68%), CO
2
chemistry (35%), seawater CRMs (23%), pH indicator dyes (18%), and
supplied by manufacturer (13%). The four areas where respondents expressed the greatest
concern over the use of in situ pH sensors were ruggedness (49%), calibration life (46%), level
of measurement uncertainty (43%), and reliability (41%). The complete needs and use
assessment reports can be found at:
http://www.act-us.info/Download/Customer_Needs_and_Use/pH/index.htmlINSTRUMENT TECHNOLOGY TESTED
The Submersible Autonomous Moored Instrument for pH, SAMI-pH, is Sunburst
Sensors’ highly accurate colorimetric reagent instrument for in situ pH measurements. The
SAMI-pH measures and logs pH in marine environments over long periods with ultra-low power
and reagent consumption, providing valuable time-series data to researchers around the world. It
can power and log data from up to three external instruments such as PAR, dissolved oxygen,
chlorophyll fluorometer, or CTD. One of these instruments can be RS232. The SAMI-pH can
be deployed to depths of up to 600 m. The SAMI-pH’s log their own data, but can be configured
to communicate with the Seabird Inductive Modems or external data loggers. The SAMI-pH
measures pH
T
(total hydrogen scale) in the marine pH range of 7-9. It does not suffer from the
drift that plagues most electrode based pH probes. The client software parses the data at the time
of download into a text file that contains the configuration, data records and status records. The
user can then open this raw file and the software will calculate the pH based on the salinity value
entered by the user of through an attached CTD.
No calibration of the SAMI-pH was performed by ACT staff. Sunburst helped with the
post processing of the data after being supplied with only our final salinity time-series. For the
freshwater Great Lakes field deployment test, two SAMI units were tested. One contained the
normal MetaCresol Purple dye indicator and the other contained Phenol Red as the dye indicator.
The use of Phenol Red as the dye indicator for freshwater applications is still considered
experimental.
pH SCALES
Four pH scales are commonly used to describe the acidity of an aqueous solution: (1) the
free hydrogen ion concentration scale, (2) the total hydrogen ion concentration scale, (3) an H
+
concentration scale termed the ‘seawater scale’ that is numerically quite similar to the total
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