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Span Calibration
16116Operation.fm
Section 3
3.15.1 Preparation
Span calibration can be performed using a flow cell, in the flow stream, or in a
beaker with a stirring bar. Although all three of these methods are acceptable,
the flow cell setup is generally recognized as providing optimal conditions for
chlorine monitoring and thus, span calibration.
A “known concentration” of chlorine of at least 1.00 ppm is needed for the
calibration. If the anticipated typical concentration at the site is greater than
1.00 ppm, a concentration should be prepared which is representative of this
typical concentration. Use an appropriate laboratory method for determining
the exact concentration of this prepared sample.
Laboratory-precision procedures for determining the chlorine concentration in
a water sample are beyond the scope of this manual. Examples of typical
methods for precise measurement of chlorine concentration include
spectrophotometry, idometric, amperometric, titration, etc. For detail on these
and other methodologies, refer to reference books such as
Standard
Methods
.
3.15.2 Optimizing Span Calibration
Optimize the span calibration by taking into account specific conditions as
indicated in the guidelines below.
3.15.2.1 Check Maximum Range Setting
Ensure that the monitor’s “Maximum Range” has been set prior to performing
span calibration. For optimum accuracy, set the Maximum Range to a value
that is not greatly higher than the highest expected concentration. For
example, if the highest expected concentration for the site is 5 ppm, it is less
effective to set the meters Maximum Range to 20 ppm (see
).
3.15.2.2 Flow Cell Span Calibration
When using a flow cell, adjust the flow rate so that the fluid in the left column
of the flow cell rises to a level approximately ¼-inch from the top of the cell
overflow. This is typically consistent with 14 to 16 gph of flow rate.
3.15.2.3 Flow Stream Calibration
If spanning the probe in the actual flow stream, install the probe and verify that
the flow rate is at least 0.5 fps and not greater than 2.5 fps.
3.15.2.4 Beaker Calibration
Spanning the probe in a beaker can yield usable results when the following
steps are taken:
•
Use a concentration that is stable, and at least 1.00 ppm.
•
Cover the container to prevent chlorine from escaping into the
surrounding atmosphere.
•
Use a rotating magnetic stirring bar to keep the solution moving uniformly
around the tip of the probe. This will prevent the depletion of chlorine at
the sensor membrane.
3.15.2.5 Stabilization
Allow the displayed reading to stabilize for 30 minutes and then perform the
span calibration as indicated below.
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