LI-COR LI7200RS, Инструкция по эксплуатации

Страница: 108 / 226

Section 4. User Calibration
Considerations for Setting the Secondary
Span
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 most common reason for doing this would be after a change in the
chopper housing temperature set point. The two gas concentrations used for the
span and secondary span should be as far apart as possible. For example, at the fact-
ory, this is typically done for CO
2
using 200 ppm and 3000 ppm. At a minimum,
the pair should be at least 500 ppm apart, and bracket your intended operating
range (e.g. 300 ppm and 800 ppm). For H
2
O, you are constrained by the tem-
perature of the air. You would normally choose a very low dewpoint, such as 5 °C,
and something close to (just below) ambient. If the air temperature is 15 °C or less,
you should probably avoid doing a secondary water vapor span.
Secondary CO
2
Span
1
Zero the CO
2
reading (see Zero CO2 on
page 4-6).
2
Span the CO
2
reading at a concentration below
your normal operating range (something in the
200-300 ppm range, see
Span CO2 on the pre-
vious page).
3 Flow a CO
2
concentration that is at or above
your upper operating range (above 800 ppm, for
example).
4 Click on the Span 2 tab, and when stable, click
Span 2 CO2
.
NOTE: You can reverse the con-
centrations if you wish. The normal span can be the high concentration, and the
secondary the low concentration. Also, multiple secondary spans can follow a nor-
mal span. Simply make sure that the concentration difference between the normal
span and any secondary span is large.
4-8 LI-7200RS Enclosed CO₂/H₂O Analyzer