Section 3
Operation 3-25
The span offset is the parameter that is set when you span the instrument. The span offset
value is usually close to 1.0. The actual span is a linear function of absorptance, so the span
offset is the intercept term of that relationship, and the span slope is the slope of the
relationship.
If your LI-820 is exhibiting erratic behavior such as no response to CO
2
, or erratic readings,
check the span offset and span slope values. As an example, if the span is inadvertently set at
the wrong time (with CO
2
-free air in the cell, for example), the span offset term can be near
zero, or very large (10, 1000, etc.), in which case the instrument will not function correctly.
The editable fields under the Manual tab allow you to manually set the two values to correct
this. Set the Span offset value to 1.0, and the Span slope to zero and click
App ly to return the
instrument to a more “normal” state of operation. You should then perform a zero and span
(or two point span) of the instrument.
Two Point Span (Advanced Tab)
The two point span function under the Advanced tab is used to set the span slope value using
two non-zero gases. This is performed at the factory, and only needs to be repeated if the
source and/or detector are replaced. To test the span slope after replacing the source and/or
detector, zero the instrument, and then measure one of your two known gases, using it to set
the span, if necessary. Measure your second known gas; if the reading exceeds the
specification, you can use the two point span function to correct the span slope and bring the
instrument back within factory specification.
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