Allow the sensor to stabilize. A few minutes is adequate for a
new probe or new membrane. If recalibrating a probe with an
old membrane, several minutes may be required for stabiliza-
tion.
Determine the atmospheric temperature and the barometric
pressure and salinity. Using the charts on pages 5-7 of this
instruction manual, determine the saturation value for this
temperature, pressure and salinity. This 100% saturation value
is equal to the millivolt output reading of the sensor (for series
DO6400 and DO7400), or equal to the milliamp output of the
sensor (for series DO6441, DO6442, DO7441, DO7442).
When the output is stable, indicating that temperature equal-
izing has taken place, measure the level of saturation. For
example, using the DO6400/T electrode, a reading of 36.0 mV
could be a typical saturated output reading. For the DO6442/T
probe, a saturated reading in air could provide a calibration
reading of 12.0 mA.
Most customers will find that this single point reading is
satisfactory. The assumption is that the millivolt probes series
DO6400 and DO7400 will read 0.0 mV when there is 0.00
ppm O2. For series DO6441, DO6442, DO7441, DO7442, the
assumption is the milliamp output of the sensor will be 4.25
mA. The output of all of these sensors are linear. For custom-
ers routinely measuring 3 or more ppm, the assumptions will
provide excellent results. Using these 2 points, a line may be
drawn between these 2 points. All subsequent readings will
fall on this line, or for higher values, beyond the extended line
to higher levels of O2 saturation at higher pressures.
Calibration
First, ensure that the sensor is properly connected to the
instrumentation circuitry. A visual display must be available to
read the sensor output. For sensors that have been installed
in service, gently wipe the membrane using a soft cloth to
remove accumulated biological debris. The Probe may be
calibrated in air, or in air-saturated water. Please calibrate out
of direct sunlight, which may effect accurate calibration. The
simplest method is to calibrate in air. Simply hold sensor in air
and take reading (wait for reading to equilibrate). If calibrat-
ing in air saturated water, ensure that the water is saturated,
by agitating the water sample. Place the probe just below the
surface of the water level.
PRODUCT INSTRUCTION SHEET
Should you expect readings below 2 ppm, you should
perform a 2 point calibration. Prepare a solution of satu-
rated sodium sulfite (Na2SO3) in water. 17g Sodium sulfite
in 125mL DI water is more than enough. Sodium sulfite may
not reach a true zero output on your display but it should
read less than 2 mV for series DO6400 and DO7400, and
less than 5mA for series DO6441, DO6442, DO7441, DO7442
after 5 minutes.
Calibration should be performed periodically to ensure the
best accuracy. The application will determine the interval of
calibration. The user is responsible to understand how often
calibration will be performed for that application.
Page 4 of 8 Form: InstrDO6400-B [Rev: 2016-02-19]
Calculating DO in % Saturatio or ppm/
mg/L
Calculating Dissolved oxygen (DO):
% DO for mV sensors only (note that % DO us independent
of temperature):
1) Take Air reading in mV or mA and record value
2) Measure unknown sample of water mV or mA
3) Unknown/Air x 100% = % DO
a. Example: mV Sensor in air = 40mV, unknown sample
water = 20mV
20/40 x 100% = 50% Saturation
% DO for mA sensors only (note that % DO us independent of
temperature):
ppm or mg/L (temperature, salinity and pressure
dependent)
1) Take Air reading in mV or mA and record value
2) Take readings for: temperature, salinity, pressure
(pressure only if calibration and measurement are at different
elevations) and record values
3) If fresh water then salinity = 0, use chart on page 6,
column “0”, otherwise find salinity column and use that value
a. Example 1: Air = 20mA, Unknown water = 12mA,
temperature = 25°C, salinity = 0
i. 0.5 x 8.24ppm = 12mA (12mA is 50% of 4-20mA scale
(you cannot use 12/20mA) = 4.12ppm)
b. Example 2: Air = 40mV, Unknown water = 20mV,
temperature = 25°C, salinity = 0
i. 20/40 x 8.24ppm = 4.12ppm
c. Example 2: Air = 40mV, Unknown water = 20mV,
temperature = 25°C, salinity = 30ppt
i. 20/40 x 6.95ppm = 3.48ppm
