LumaSense Technologies 3434i SF6 Leak Detector - Manual (Page 163)

Chapter 12
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BE6028-14 3434i SF6 Leak Detector LumaSense Technologies A/S
Page 163 of 191
Gas Concentrations – Two-point Calibration
During a two-point span calibration, the total signal in the cell (V
low
)
is measured first with a low concentration (C
low
) of span gas, and
then the total signal in the cell (V
high
) is measured with a high
concentration (C
high
) of span gas. This enables the Monitor to
compensate for any un-linearity in the relationship between the
signal measured in the cell and the concentration of the gas in the
cell (this is illustrated, for a gas A1, in Fig.12.14a ).
Here are some guidelines about the concentration of gas you should
use during a two-point span calibration:
· The Low Concentration should be 10 to 30% of the highest
concentration you expect to measure.
If the desired measurement range is mainly within the linear
range then the low concentration should be between 1000 and 10
000 times the detection limit.
If the desired measurement range is mainly above the linear
range then the low concentration should be more than 10 000
times the detection limit.
· The High Concentration should be 75% to 100% of the highest
concentration you expect to measure, but no higher than this.
Simple Test to Determine the Linearity
Let us assume that you wish to measure a gas over a dynamic range
from 100 ppm and 3000 ppm of a gas that has a detection limit of
0.3 ppm.
In this case, the range in which you want to measure is from 333
times to 10 000 times the detection limit. With our general
guidelines, you’re not able to decide whether you have to perform a
single- or two-point calibration. Therefore, we advise you to perform
the following test:
1. Perform a single-point calibration with a low concentration of the
gas. Use 10% of the highest expected concentration level. In this
case, use 300 ppm (1000 times the detection limit).
2. Perform a measurement with a high concentration of the gas.
Use, for example, 90% of the highest expected concentration
level. In this case, use 2700 ppm.
If the measured gas concentration differs by more than 5% from the
real concentration, then a two-point calibration with 2 gas
concentrations is recommended.