YSI 63 - Manual (Page 31)

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this assumption. Once the slope and intercept to the plot of pH vs. mv are assigned at the new
temperature, the calculation of pH under the new temperature conditions is straightforward, and
is automatically carried out by the software.
Number of pH Calibration Points
When calibrating the YSI Model 63, you have the choice of 1-point 2-point, or 3-point
calibration. Perform a 2 or 3 point calibration at least once per day for accurate results.
Select the 1-point option only if you are adjusting a previous calibration. If a 2-point or 3-point
calibration has been performed previously (at least once per day), you can adjust the calibration
by carrying out a 1-point calibration at pH 7 (or pH 6.86). This calibration procedure adjusts
only the pH offset and leaves the previously-determined slope unaltered.
Select the 2-point option to calibrate the pH probe using only two calibration standards. In this
procedure, the pH sensor is calibrated using a pH 7 (or pH 6.86) buffer and one additional
buffer. A two point calibration procedure (as opposed to a 3-point procedure) can save time if
the pH of the sample is known to be either basic or acidic. For example, if the pH of a sample is
known to vary between 5.5 and 7, a two point calibration with pH 7 and pH 4 buffers is
appropriate. Three point calibration with an additional pH 10 buffer will not increase the
accuracy of this measurement since the pH is not within this higher range.
Select the 3-point option to calibrate the pH probe using three calibration solutions. In this
procedure, the pH sensor is calibrated with a pH 7 (or pH 6.86) buffer and two additional
buffers. The 3-point calibration method assures maximum accuracy when the pH of the media to
be monitored cannot be anticipated.
9.2 Conductivity
The conductivity cell utilizes four pure nickel electrodes for the measurement of solution
conductance. Two of the electrodes are current driven, and two are used to measure the voltage
drop. The measured voltage drop is then converted into a conductance value in milli-Siemens
(millimhos). To convert this value to a conductivity (specific conductance) value in milli-
Siemens per cm (mS/cm), the conductance is multiplied by the cell constant which has units of
reciprocal cm (cm
-1 ). The cell constant for the Model 63 conductivity cell is 5.0/cm + 4%. For
most applications, the cell constant is automatically determined (or confirmed) with each
deployment of the system when the calibration procedure is followed. Solutions with
conductivity’s of 1.00, 10.0, 50.0, and 100.0 mS/cm, which have been prepared in accordance
with recommendation 56-1981 of the Organisation Internationale de Métrologie Légale (OIML)
are available from YSI. The instrument output is in
µS/cm or mS/cm for both conductivity and
specific conductance. The multiplication of cell constant times conductance is carried out
automatically by the software.
Temperature Effect On Conductivity
The conductivity of solutions of ionic species is highly dependent on temperature, varying as much
as 3% for each change of one degree Celsius (temperature coefficient = 3%/C). In addition, the
temperature coefficient itself varies with the nature of the ionic species present.