E3 Calibration Record (“Cal Log”)
Every Manta 2 has a dedicated data file called CAL.LOG. The CAL.LOG remembers every
calibration that your instrument has accepted. In this Calibration Record are the time and date of the
calibration, the parameter calibrated, the reading before the calibration was accepted, and the
reading after the calibrations was accepted. If you wished to know, for instance, the last time that
Conductivity was calibrated, the Calibration Record would tell you when the most recent Conductivity
calibration was accepted, the value of the calibration standard, and the instrument’s reading in the
standard before the calibration was made (to tell you exactly how much the instrument was changed
during calibration). This data cannot be altered within the Manta 2.
E4 Sensor Response Factor (SRF)
Also included in the Calibration Record is each calibration’s Sensor Response Factor (SRF).
Suppose that a typical Conductivity sensor reports 100 µA in a 1413 µS/cm standard. If your
Conductivity sensor reports 100 µA in that same calibration solution, then your SRF is 100% (some
parameters, such as pH, have a more complex SRF calculation, but the effect is the same). If your
response is 80 µA, your SRF would be 80%. When you press the OK button to accept a calibration,
the Manta 2 automatically accepts your calibration if the SRF is between 60% and 140%. If the SRF
falls outside that range, you will be cautioned to check your standard value, make sure the sensor is
clean, make sure the reading has stabilized, etc. But you can elect to accept any SRF.
E5 Temperature
The Temperature sensor is an electrical resistor (thermistor) whose resistance changes predictably
with temperature. The sensor is protected by a stainless-steel tube. Thermistors are very stable with
time, and so do not require calibration.
E6 Dissolved Oxygen, Clark Sensor
The Clark-type dissolved-oxygen sensor comprises a gold cathode and silver anode immersed in a
potassium-chloride electrolyte; the electrolyte is retained by a thin, oxygen-permeable membrane. If
a voltage of roughly 0.8 V is imposed between the cathode and anode, any oxygen gas at the
cathode is ionized, giving up electrons in the process. Those electrons are an electrical current
through the electrolyte, between the cathode and anode, the magnitude of which is related to the
amount of oxygen present in the air or water outside the membrane. The sensor output is corrected
for the temperature characteristics of the membrane, and for the temperature characteristics of
oxygen saturation in water.
Because the Clark sensor consumes oxygen, you must usually have a circulator to prevent oxygen
starvation at the outer surface of the membrane.
The Clark sensor has performed will in the field for decades, but is slowly being replaced by optical
dissolved-oxygen sensors. The latter have little calibration drift in the field, are not flow-sensitive (no
circulator needed), and do not require the occasional membrane changes that annoy Clark sensor
users.
Clark sensor maintenance is little more than replacing the membrane after cleaning the sensor and
refilling its electrolyte. There should be no bubbles in the electrolyte, and the membrane should be
taut with no wrinkles or holes. Eureka’s unique pop on membrane cap virtually eliminates wrinkles,
but care must be taken during the process:
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