User calibration has two operations and each may be performed individually, however for a complete
user calibration both must be completed.
Factory Calibration
When the instrument is factory calibrated, a stable gas curve is generated (see curve 1 on Graph 1 -
User calibration explained). This curve is then used to determine the gas concentration based on the
infrared signal strength after being absorbed by the gas.
Zero Calibration
A zero calibration is used to correct the entire curve for the infrared source and filter variations
caused by aging and induced drift due to dirt and other contaminants. If done correctly, there is often
no need to complete a span calibration, as the new curve will follow closely to the factory calibration
curve (curve 1 on Graph 1 - User calibration explained).
The zero calibration is very sensitive and a rushed or poor calibration, (such as the target gas still
being present), will result in a zero error; see point A on curve 2 of Graph 1 - User calibration
explained. This also produces an error throughout the remainder of the curve proportional to signal
strength, but the effect on the span is significant, see point B on Graph 1 - User calibration explained.
Note: To perform an accurate user calibration it is critical that a good user zero has been
performed. QED recommend that this be done in nitrogen in order to guarantee that none of the
gas of interest is present.
Note: The Zero calibration is very sensitive and even 100% instruments will detect in the 0 to
100ppm range even though they do not display to this resolution. Please refer to Graph 2 - Typical
zero gas purge times.
Span Calibration
A span calibration is used to optimise the analyser at the span calibration concentration (see point C
on Graph 1 - User calibration explained) for the current operational conditions. It corrects the span
point but leaves the zero unadjusted (this will be left at the last user zero if this has been performed)
and should be done at the concentration of interest in the particular application.
If the user zero is poor and the span calibration is good, it will correct the gas curve for the point of
interest, but other points on the curve could be incorrect, see curve 3 on Graph 1 - User calibration
explained.
Typical Calibration Purge Times
Typical zero calibration although displaying zero needs to be given time to settle. We recommend
commencing the calibration at least five minutes after the display concentration stabilises.
To Next Page
Loading...
