Delta OHM D35ED-ALM, Manual

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6. The instrument tests the measurement stability. Wait for a few minutes for measurement
completion. In the meantime, don’t stay too close to the instrument to avoid altering the
measurement.
WAIT FOR STABILIZATION
OF CO
2
CONCENTRATION…
Please stay away to not
alter measures
Meas. [ppm] Avg [ppm]
472
- - - -
4
7. At the end, a message is shown indicating the calibration success or failure. Press any key to
return to the calibration submenu.
If a message appears declaring that calibration has failed, it means that the value measured
by the instrument during the procedure differs too much from the set reference value. In that
case repeat calibration checking the CO
2
reference value in the environment and making sure
to operate in a stable environment.
8.6.4 CO
2
SENSOR
A
UTOCALIBRATION
In HD35ED[G]14bNAB and HD35ED[G]1NAB models, the instrument can be set so as CO
2
cali-
bration is automatically performed at predetermined intervals.
In order for auto calibration to be effective, the CO
2
concentration in the environment where
the instrument is installed must assume a known value (referred to as environment back-
ground value ). For example, we can have that an instrument installed inside a public place
performs a weekly auto calibration when people are not present and CO
2
concentration is close
to the outdoor air value (if there is an adequate air change).
Fig. 8.6.3: example of background value for CO
2
auto calibration
The offset applied to the measurement by the auto calibration procedure can be limited to a
maximum value, so as to avoid erroneous calibrations when the measured value differs too
much from the estimated background value. The auto calibration procedure acts therefore in the
following way:
o
If the difference between the measured value and the background value is lower than the
maximum offset, an offset is applied to the measurement so that the measured value coin-
cides with the background value.
o
If the difference between the measured value and the background value is higher than the
maximum offset, only the maximum offset is added or subtracted so as to approach the
background value.
To set the auto calibration interval and the maximum offset and to activate auto calibration
see the menu item CO
2
autocalibration on page 33.
Background
value
Background
value
Weekly trend Weekly trend