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7.3 Calibration and maintenance
1 How do I know when I need to calibrate my sensors?
The simple answer is that frequent calibration will give you better data. The more meticulous you are with
calibration, the better data you will gather. If you are uncertain whether you need to calibrate, check your sensors
against a known sample. If the reading is within the accuracy specification and/or your accuracy expectations, there
is no need to calibrate.
Experience and your program’s accuracy expectations will help determine calibration frequency for the various
sensors. If, for instance, your reservoir discharge is hovering near the regulatory minimum for dissolved oxygen, you
should pay special attention to DO calibration frequency and technique. On the other hand, if a conductivity accuracy
of +/- 10% is OK, you needn’t calibrate conductivity very often. (4.2)
2 How often should you calibrate your multiprobe?
That depends on a number of factors, including the nature of the waters being monitored and your expectations for
accuracy. We suggest that you start by calibrating once per week and shorten or lengthen that interval as the data
suggest. (4.2)
3 How do I choose calibration standards?
For best results, choose a calibration standard whose value is close to what you expect to see in the field. For
example, calibrate with a 1413 μS specific conductance standard if you expect to see specific conductances between
500 and 1000 μS in the field. Don’t calibrate with a sea water standard. And if your waters tend toward the acidic,
calibrate with a 4-buffer instead of a 10-buffer.
If you are moving your multiprobe across wide ranges of water conditions, you may wish to recalibrate to match the
new situations. For instance, if you are measuring a clear lake during the morning and a high-sediment stream in the
afternoon, you might consider recalibrating at noon with a high-range turbidity standard. (4.3)
4 What is an SRF?
Suppose that a typical c onductivity sensor reports 100 μA in a 1413 μS/cm standard. If your particular 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 click 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.
Each sensor calibration’s Sensor Response Factor (SRF) is automatically logged into the Cal Record with the details of
that calibration. (4.5)
5 Do I have to calibrate temperature?
No; the temperature sensor is so stable that it needs no calibration. (D.6)
6 What is the basic calibration procedure?
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 known calibration situation. The general procedure is shown below. (4.2)
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.
3) Rinse sensors thoroughly (more than once may be required) with DI (deionized) water, especially if you have
been using other calibration solutions. Shake the Scuba so the DI can vigorously remove traces of old calibration
solutions and cleaning agents – repeat if necessary.
4) Rinse the sensors twice with a small quantity of your calibration standard. Discard the used calibration standard
because it is probably contaminated with DI water.
5) Immerse the sensor in the calibration standard. This is usually accomplished by securing your Scuba with the
sensors pointing up, screwing the cup onto the Scuba, and filling the cup with your calibration standard. Make