West Systems Portable diffuse flux meter, Руководство пользователя

Страница: 60 / 104

8 LI-COR Carbon dioxide detector
PAG
E
60
w
w
w
.w
es
tsy
stems
.co
m
W
EST
Syst
em
s
LI-COR Carbon dioxide detector
The information discussed in this chapter applies to the carbon dioxide detector
model LI-COR LI-830 or LI-850.
For full specifications and instructions please refer to the LI-COR manual which
is attached to this handbook.
8.1 Calibration check
As explained previously in Chapter 4, the flux measurement is proportional to
the slope of the concentration curve versus time. The proportionality factor
depends on the volume/surface ratio of the accumulation chamber used for the
measurement, as well as, the barometric pressure and air temperature at the
moment of making the measurement.
The most important aspect to understand is that the flux is proportional to the
gradient of concentration over time: ppm/second. This aspect allows us to
simplify the control of the response of the gas analyzers.
Each time a measurement campaign is initiated the instrumental response of
the gas sensors must be verified and, if necessary, their calibration fine-tuned.
To simplify the explanation, see the following example:
Step 1: Verifying the zero
Inject a flow of nitrogen, or synthetic air, into the instrument. Read the carbon
dioxide concentration on the FluxManager app, as explained on chapter 3.
Obviously, it is important that the injected mixture does not contain carbon
dioxide.
The method for injecting standard gas mixtures is explained in detail in the
following pages.
Step 2: Verification of the span
Inject a standard mixture containing approximately 1% (10,000 ppm) of carbon
dioxide and check the response of the instrument.
Let's suppose that the check gave the following results:
Injecting a mixture at zero concentration of carbon dioxide the CO2 detector
returns a reading of 10 ppm.
Injecting a mixture containing a 10,000 ppm concentration of carbon dioxide
the CO2 detector returns a reading of 9,940 ppm.
At a variation of concentration set at 10,000 ppm the instrument has a slightly
different response: 9,930 ppm (=9,940-10 ppm). The evaluation error is of
about 70 ppm, which in percentage points over the span corresponds to 0.6%
less
The error in evaluating the increment in concentration manifests as a systematic
error in the evaluation of flux and, therefore, must be corrected by calibrating
the instruments when it is too high (> 5%).
When you need to calibrate the detector
The LI-COR is a very stable detector, the pressure compensation and the
thermal stabilization features allows the instrument to maintain it calibration for
long periods. The calibration is mandatory if you decide to change the optical
path or to clean it. In any case we advise calibrating it only if necessary.