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• On a regular cycle, depending on the precision aimed for. To find out the appropriate time between
calibrations, we recommend beginning with a more frequent check of the calibration. The time
between calibrations can range between:
– several months for a measuring task in the Vol.% range
– days to weeks for a measuring task in the sub-Vol.% range
– directly before every measurement if highest accuracy is needed.
• When the situation of the measurement regarding pressure, temperature or gas flow changes
The goal of the calibration is that the measured concentration agrees with the given test gas concen-
tration. To obtain this, two calibration parameters that correspond to the offset/zero and the gain/span
of a linear equation are available. A two-point calibration requires two test gases. Both calibration pa-
rameters, offset and gain, are adjusted. The concentration of the test gases does not have to meet the
beginning and the end of the measuring range, a difference of ±10% is permitted.
The menu sequence is designed such that prior to a gain always an offset calibration
has to be done first. Usually, a single point calibration determining a new offset value is
sufficient to obtain a good calibration.
In the case of one point calibration, a test gas of any concentration in the measuring range
is feasible. For two point calibrations it is preferable to use the gas concentrations at the
lower and upper end of your measuring range.
7.1.1 Calibration gas purities and flooding time
Messkonzept uses gases with the following purities for calibration:
H
2
He N
2
Ar O
2
CO
2
CH
4
5.0 5.0 5.0 4.6 4.5 4.5 4.5
Table 7.1: Recommended calibration gas purities
These gas purities are selected such that the devices comply with the specifications for the smallest
measuring ranges feasible with the FTC320. Messkonzept recommends gases of the same purity for
calibration on-site. For differing individual requirements, opt for an appropriate gas purity.
To ensure high precision results after calibration, you have to make sure that the device is well flooded
with the calibration gas before starting the calibration sampling. In Messkonzept’s internal calibration
process for 60 l/h applications this flooding is performed for min. 30 minutes for most gas mixtures
(containing H
2
, N
2
, Ar, O
2
, CH
4
, ...) and for min. 60 minutes in Helium mixtures. Note that the supply
pipe volume needs to be flooded as well. In Messkonzept’s calibration process the volume before the
device is <100ml. If your calibration setup has a larger volume in front of the device, you might have
longer running-in times. Please contact us if you need advice.
7.1.2 Use of Substitute Gases
Instead of using toxic or explosive gases for calibration, substitute gases may be used. A substitute
gas has (at a certain concentration) the same thermal conductivity as the test gas it is substituting,
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