LI-COR LI-7500, Instruction Manual

Page: 84 / 155

Section 4
4-2 Calibration
How Stable are Zero and Span?
The analyzer's zero is primarily affected by temperature, and the state of the internal
chemicals. The internal chemicals should be changed annually (see Section 5, Maintenance).
The zero's response to temperature is relatively small (typically 0.1 or 0.2 ppm per °C for
CO
2
, or 0.01 mmol/mol/°C for H
2
O). Also, this drift is measured at the factory, and
subsequently compensated for in software (Equation 2-10), so the effective zero drift should
be quite small. Therefore, the zero should be quite stable over a several month period, but
you might want to check it after an extreme temperature change (>30 °C).
The analyzer's span is affected by temperature, pressure, and the state of the internal
chemicals. Temperature: A 10 °C change will typically change the H
2
O span by 1 to 2%.
For CO
2
at ambient concentrations, the span is very insensitive to temperature, however.
Pressure: A large pressure change (40 kPa) will affect the CO
2
and H
2
O spans by <1%, for
ambient CO
2
concentrations (
≈
400 ppm) and high humidities (20 mmol/mol). So, diurnal
pressure variations should not be a concern. Chemicals: Reduced internal chemical
effectiveness will affect the span, but the effect on the zero will be much more pronounced.
In summary, span stability is mostly a concern with H
2
O, when there are large temperature
changes.