Performance Verification Statement for the Sunburst SAMI-pH Sensor

Ref. No. [UMCES] CBL 2015-013

ACT VS15-06

EXECUTIVE SUMMARY

The Alliance for Coastal Technology (ACT) conducted a sensor verification study of in

situ pH sensors during 2013 and 2014 to characterize performance measures of accuracy and

reliability in a series of controlled laboratory studies and field mooring tests in diverse coastal

environments. A ten week long laboratory study was conducted at the Hawaii Institute of

Marine Biology and involved week long exposures at a full range of temperature and salinity

conditions. Tests were conducted at three fixed salinity levels (0.03, 22, 35) at each of three

fixed temperatures (10, 20, 30

C). Ambient pH in the test tank was allowed to vary naturally

over the first five days. On the sixth day the pH was rapidly modified using acid/base additions

to compare accuracy over an extended range and during rapid changes. On the seventh day the

temperature was rapidly shifted to the next test condition. On the tenth week a repeated seawater

trial was conducted for two days while the temperature was varied slowly over the 10 – 30

range. Four field-mooring tests were conducted to examine the ability of test instruments to

consistently track natural changes in pH over extended deployments of 4-8 weeks. Deployments

were conducted at: Moss Landing Harbor, CA; Kaneohe Bay, HI; Chesapeake Bay, MD; and

Lake Michigan, MI. Instrument performance was evaluated against reference samples collected

and analyzed on site by ACT staff using the spectrophotometric dye technique following the

methods of Yao and Byrne (2001) and Liu et al. (2011). A total of 263 reference samples were

collected during the laboratory tests and between 84 – 107 reference samples were collected for

each mooring test. This document presents the results of the Sunburst SAMI-pH which

measures pH

spectrophotometrically using a dye-based colorimetric reaction.

The SAMI-pH operated continuously throughout the entire ten week lab test and

generated 3140 useable measurements out of a total possibility of 3154 timepoints. The total

range of pH measured by the SAMI-pH was 6.273 to 8.441, compared to the range of our

discrete reference samples of 6.943 to 8.502.The SAMI-pH showed some differences in accuracy

across the various test solutions and pH ranges, as would be expected based on the specific set-

up of the instrument in terms of the indicator dye used and its known operational range. For the

complete Lab test, the mean of the differences between the SAMI-pH measurement and

reference pH was -0.017 ±0.970 (N=263), with a total range of -1.159 to 0.140. The larger

offsets only occurred during the rapid pH shifts (see Appendix 1) and comparisons for only the

stable testing conditions had a mean difference from reference measurements of -0.010 ±0.058

(N=193), with a total range of -0.048 to 0.063. Instrument measurements conducted at ten weeks

with the second seawater trial exhibited a similar level of accuracy with a mean difference from

reference of -0.010 ± 0.033 (N=8) compared to 0.014 ± 0.003 (N=27) for measurements in the

first week of the lab test.

At Moss Landing Harbor the field deployment test was conducted over 28 days with a

mean temperature and salinity of 16.6

C and 33. The measured pH range from our 84 discrete

reference samples was 7.933 – 8.077 compared to a range of 7.798 – 8.150 reported by the

SAMI-pH for 2564 observations conducted continuously at 15 minute intervals. The average

and standard deviation of the measurement difference between the SAMI-pH and reference

samples over the total deployment was 0.039 ±0.003 with a total range of -0.123 to 0.158. The

useable data return for the deployment was 100%.

At Kaneohe Bay the field deployment test was conducted over 88 days with a mean

temperature and salinity of 24.5

C and 34.4. The measured pH range from our 101 discrete