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4.4 Calibration record (“Cal Log”)
Every Scuba has a dedicated data file called CAL.LOG. The CAL.LOG records every calibration that your instrument has
accepted. In this file are the time and date of the calibration, the parameter calibrated, the reading before the calibration
was accepted, the reading after the calibration was accepted, the SRF, and a few other details. 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 Scuba, so don’t try any funny business.
4.5 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
μ
A in a 1413
μ
S/cm standard. If your conductivity sensor reports 100
μ
A 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
μ
A, your SRF would be 80%. When you press the
OK
button to accept a
calibration, the Scuba 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.
4.6 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.
4.7 Dissolved oxygen
The optical dissolved-oxygen sensor comprises a blue-light source, a sensing surface, and a red-light receiver. The sensing
surface is an oxygen-active compound stabilized in an oxygen-
permeable polymer, usually silicone. When the sensing surface is
exposed to water (or air, for that matter), oxygen diffuses into the
sensing surface according to the amount (partial pressure) of oxygen in
the water. The oxygen-active compound fluoresces – that is, it absorbs
energy in the form of blue light and then emits energy as red light. In
each measurement cycle, the blue light is first turned on, and then
turned off. The red-light receiver measures the time it takes, after the
blue light is turned off, for the fluorescence to die off. This value is
proportional to dissolved oxygen.
The sensor output is corrected for the temperature and salinity of the water.
