ACT pH Sensor Verification Protocols, PV12-01
5
Over 170 coastal resource managers, environmental health agency representatives,
manufacturers, and scientific researchers were surveyed regarding their needs, uses and current
practices of making pH measurements in the environment. Overall, 42 individuals responded to
the survey, all of which are using, or manufacturing, in situ pH sensors. The types of pH sensors
being used varied between potentiometric (45%), ion-selective field-effect transistor (32%), and
spectrophotometric (indicator dye) 20%, but with a quarter of 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 psu, 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:
.
4.1.
Definition of Performance Parameters being Evaluated
•
Accuracy (or Uncertainty)
–
uncertainty will be estimated by repeated comparisons
between instrument measurements and reference water sample pH determinations as
determined by an indicator dye. Standard buffers and Certified Reference Materials for
low and high ionic strength solutions will be supplied by the National Institute of
Standards and Technology (Kenneth Pratt, NIST) and Scripps Institution of
Oceanography (Andrew Dickson, SIO) to calibrate and monitor the accuracy of the
reference sample analysis.
•
Precision (or Repeatability)
–
Precision will be determined by repeated measures during
the pre and post laboratory calibration, and burst sampling during the test.
•
Stability
–
Stability will be determined by an evaluation of the difference in instrument
readings and reference samples over time a 2-3 month laboratory mesocosm test at
varying temperature and salinity conditions and during the two-month long field
deployments.
•
Reliability
–
Reliability of instruments will be determined by: 1) examining the percent of
useful data recovered versus percent of data expected, and 2) by performance evaluation
of pre- and post-measures of reference standards to quantify total drift during field
deployment periods. Comments on the physical condition of the instruments (e.g.,
physical damage, flooding, corrosion, battery failure, etc.) will also be recorded.
