LI-COR LI-7500, Instruction Manual

Page: 85 / 155

Section 4
Calibration 4-3
Checking the Zero - Considerations
The zero is checked with dry CO
2
-free air within the optical path of the analyzer. A suitable
source of air for setting the zero can be generated with chemical scrubbers (such as soda lime
for removing CO
2
and magnesium perchlorate or Drierite
®
for removing water), or obtained
from a cylinder of zero grade gas. When using chemical scrubbers, it is important to make
sure that the chemicals are fresh, and that air goes through the chemicals in the right order;
soda lime first, desiccant last (if the desiccant is Drierite
®
, allow time for the CO
2
to "wash
out" of it). When using a cylinder, it is important to make sure that the air in the cylinder
really is CO
2
-free since a typical cylinder of standard grade nitrogen might have as much as
20 ppm of CO
2
in it. Compressed cylinders may be at pressures of several thousand pounds
per square inch; before using them for calibration, they should be fitted with a regulator to
reduce the pressure down to a range of around thirty pounds per square inch.
Checking the Span - Considerations
For checking the CO
2
span, use a cylinder of CO
2
in air with a concentration that is at the
higher end of the range of concentrations which may be encountered during measurements.
For example, a 500 to 1000 PPM cylinder of CO
2
in air, which has been verified to be
accurate to within at least 1% would be a suitable choice for many applications. Be cautious,
as the stated value of the calibration cylinder may be significantly different from the actual
gas concentration.