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4. Sensors and calibrations
4.1 The difference between sensors and parameters
A sensor is a basic element, like a thermistor or a pH glass electrode. Each sensor has one or more parameters. For instance, we
use a thermistor to measure both temperature °F and temperature °C – that’s one sensor with two parameters. A conductivity
sensor can be read as specific conductance
μ
S/cm, specific conductance mS/cm, total dissolved solids mg/l, and salinity (PSS) –
that’s one sensor with four parameters.
4.2 Basics of parameter calibrations
The Scuba never guesses parameter values, so you have to calibrate it from time to time by simply telling the instrument
what it should read in a calibration situation for which the correct parameter value is known. Here’s the general
procedure; instructions for specific parameters will follow beginning with 4.5:
1
Clean the sensor and perform any necessary sensor-specific maintenance.
2
Select a calibration standard whose value is close to the values you expect to see in the field. For best
results, use fresh calibration solutions, and discard once they have been used.
3
Rinse sensors thoroughly (more than once may be required) with DI (deionized) water, especially if you
have been using other calibration solutions. Pour the water into the calibration cup, position the
“stopper” side of the lid on top and shake the Scuba vigorously to remove traces of old calibration
solutions – repeat if necessary.
4
Rinse the sensors twice with a small quantity of your calibration standard. Discard the used calibration
standard.
5
Immerse the sensor in the calibration standard. This is usually accomplished by pouring the standard into
the Scuba’s calibration cup once it has been screwed onto the Scuba housing. Secure your Scuba with the
sensors pointing up and fill the calibration cup with your calibration standard. Make sure the standard
covers the sensor entirely, and that it also covers the thermistor for those parameters that are
temperature-compensated. For turbidity sensors and other fluorometers fill the cup to at least 1 ½ inches
above the sensor’s lens surface.
Select the parameter to be calibrated by clicking on the Scuba pull-down menu in the home page, then
clicking on
Calibrate
, and then clicking on the parameter you wish to calibrate. First, enter the calibration
value and press enter; when the reading has stabilized, press enter to calibrate. The Scuba will report the
resulting Sensor Response Factor (SRF); then press
Y
to accept the calibration,
N
to back up one step, or
Exit
to leave the sensor uncalibrated. (3.5.5)
