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Ref. No. [UMCES] CBL 2016-014
ACT VS16-05
25
Individual response slopes and intercepts of instrument measurement versus reference DO
for each of the laboratory trials at a fixed temperature and salinity level are summarized in Table 4.
Minor differences were observed in response slopes across temperature or salinity. The mean of
the slopes decreased from 1.02 to 0.98 from the 0 to 35 salinity trials.
Table 4.
Summary of regression statistics for the Hach HL4 versus reference sample response curves for
each of the nine laboratory trials.
Test ID
R2
y Intercept
Slope
LT15S00
1.000
-0.073
1.040
LT15S10
1.000
-0.112
1.023
LT15S35
1.000
-0.084
0.973
LT04S00
1.000
-0.104
1.047
LT04S10
1.000
-0.016
0.985
LT04S35
nd
nd
nd
LT30S00
0.999
-0.196
0.972
LT30S10
1.000
-0.146
0.986
LT30S35
1.000
-0.086
0.987
Results of the 56 day long-term stability and thermal stress challenge for the Hach HL4
LDO are shown in figure K. The instrument was maintained in a well circulated tank and oxygen
content manipulated by alternately varying water temperature set point between 15 and 25
o
C
several times per week of deployment. The time series of instrument readings at 15 min intervals
is plotted against discrete values for Winkler reference samples (
top panel
) along with the time
series of the difference between instrument and reference measurements (
bottom panel
). The
instrument stopped logging 7 days after deployment. The overall mean of differences between the
initial 18 comparative measurements (out of a potential total of 156) was -0.177 (±0.086) mg/L.
Results for a functional sensor response time assessment of the Hach HL4 are shown in
figure L. The top panel depicts the time series of 15s instrument reads during transfers between
adjacent high (9.6 mg/L) and low (2.0 mg/L) DO water baths, maintained commonly at 15
o
C. The
bottom panel (
lower left
) depicts results fit with a 3 parameter exponential decay function: DO
rel
=
DO
relMin
+ a
e
-bt
and indicated τ calculated from fit. Data for low DO to high DO transitions (
lower
right
) were treated similarly but normalized to steady state value in subsequent high DO tank and
subsequently fit with an analogous 3 parameter exponential rise function: DO
rel
= DO
relMin
+ a(1-
e
-
bt
) with indicated τ being directly calculated from fit. The calculated τ
90
was 27 s during high to
low transitions and 26 s for low to high transitions covering a DO range of approximately 8 mg/L
at a constant 15
o
C.