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Ref. No. [UMCES] CBL 2016-014
ACT VS16-05
6
Oxygen) technology. Hach LDO was made commercially available in 2003 in an application
designed for the process industries and was repackaged to integrate into Hydrolab sondes in 2005.
The Hach LDO method (Hach method 10360) has gained wide popularity and is codified as an
accepted method in 40 CFR 136.
The Hach LDO sensor leverages properties of a platinum-based oxygen quenching
luminophore that is incorporated into a sensor cap that is attached to the end of a sensor that
contains blue and red light-emitting diodes (LEDs) and a photodetector. In simplified terms, the
luminophore is excited by light from the blue LED and red emission is subsequently evaluated.
The characteristics of the red emission are related to oxygen concentration at the cap/water
interface. The red LED provides an internal reference for the optical and electronic signal paths.
Temperature and salinity are required parameters for the generation of LDO parameters
from a Hydrolab sonde. Temperature values come from the thermistor (temperature sensor) that is
included on every Hydrolab sonde. Salinity values come from the conductivity sensor that is
required with every Hydrolab sonde that has an LDO sensor. Salinity is not needed for mg/L DO
reporting, but is required for % saturation (% sat) calculation and during user calibration when
using the % sat method. The calculations are managed in the sensor and sonde electronics.
Accuracy specifications for the Hach LDO sensor used in Hydrolab sondes are stated as:
± 0.1 mg/L at <8 mg/L
± 0.2 mg/L at >8 mg/L
± 10% reading >20 mg/L
For this performance verification a HL4 sonde was tested in all the Laboratory based trials.
Both the HL4 and DS5X were tested in the three field deployments, but only the DS5X has an anti-
fouling wiping system designed to provide extended deployment periods. Only the HL4 was tested
in the field profiling application in the Great Lakes.
PERFORMANCE EVALUTION TEST PLAN
Laboratory Tests
Laboratory tests of accuracy, precision, response time, and stability were conducted at
Moss Landing Marine Lab. All tests were run under ambient pressure (logged hourly from a
barometer at the laboratory) and involved the comparison of dissolved oxygen concentration
reported by the instrument versus Winkler titration values of water samples taken from the test
baths. All tests were run in thermally controlled tanks at specific temperature, salinity, and DO
concentrations. Tanks were well mixed with four submersible Aquatic Ecosystem Model 5 pumps
with flow rates of 25 L/min. Temperatures were controlled to within approximately 0.2
o
C of set
point using Thermo Digital One Neslab RTE 17 circulating thermostats flowing through closed
coils distributed within the tank. Four RBR temperature loggers were deployed within the tank to
verify actual temperature to better than 0.02
o
C. Salinity was varied by addition of commercial
salts (Instant Ocean) to Type 1 deionized water. Salinity was verified at the beginning and end of
each test condition by analysis on a calibrated CTD
.
Dissolved oxygen concentrations were
controlled by use of compressed gases of known oxygen concentration sparging through diffusers