LI-COR LI-8100A - Two Point Span (Span 2 Tab); Considerations for Performing a Secondary Span

5-6
User Calibration
ii. Verify that the H
2
O concentration in the main window is still stable.
Allow 15-20 minutes for the H
2
O vapor concentration to stabilize near
your span point.
iii. Click on the Span H
2
O button.
iv. Click Yes to perform the calibration.
D. Click Close to exit the calibration screen.
Software for Apple iOS
A. Select Calibration from the Utilities menu.
B. Tap the Span button.
i. Enable H
2
O.
ii. Verify that the H
2
O concentration is stable. Allow 15-20 minutes for
the H
2
O vapor concentration to stabilize near your span point.
iii. Enter the span gas dew point (°C).
iv. Tap Send Command.
Two Point Span (Span 2 Tab)
If you find that after zeroing then spanning at one concentration, the instrument is
not within specifications at a different concentration, a secondary span may be in
order. The optical bench in the LI-8100A uses a span value that is a linear function
of absorptance, with two parameters; a slope and an offset. The normal span
function adjusts the offset and leaves the slope alone. Setting the secondary span
(Span 2) adjusts the slope, and then adjusts the offset value for the new slope value.
The slope and offset values can be viewed under the Manual tab in the Calibration
window. Span offset values (CO
2
Span and H
2
O Span) are typically 1.0 ± 0.1.
Span slope values (CO
2
Span2 and H
2
O Span 2) are typically 0 ± 0.1. When they
are exactly zero, then slope is a constant, and not a function of absorptance.
The CO
2
span slope value is first determined at the factory by setting the span at
200 ppm, and doing a secondary span close to 20,000 ppm. This value should
remain valid for some time. Things that are likely to make it change include
changing the source and/or detector, a dirty optical cell, and possibly when taking
the optical bench apart for cleaning.
Considerations for Performing a Secondary Span
1. Zero the instrument first, then do a normal span.
2. The span and secondary span concentrations should be as far apart as possible,
and they should bracket the concentration range of interest to you (e.g. 200
and 20,000, or 300 and 1500, etc.). “Far apart” means in absorptance, not
concentration. We recommend absorptance differences of at least 0.1 when
choosing concentrations got setting span and secondary span (Figure 5-1).