Primary Calibration - Ametek Thermox WDG Insitu User Manual

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4-8 | Thermox Series 2000 WDG Insitu Analyzer

Primary Calibration

Perform a primary calibration on the zirconium oxide oxygen sensor. You
should perform a primary calibration on your system instead of a regular
calibration (from the Calibrate key) only when:
you replace the zirconium oxide oxygen cell or thermocouple
your system fails a regular calibration due to a deteriorating cell
You should always attempt to perform a regular calibration before
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resorting to a primary calibration for all cases other than when you
install a new cell.
NOTE
The Primary Calibration correlates the actual cell response for the desired 695°C
cell set-point temperature with the actual furnace thermocouple feed back tem-
perature. This is done by applying two known concentrations of oxygen to the
analyzer during the primary calibration. The software algorithm then calculates
the actual cell temperature from the cell mV response and increases or decreas-
es the furnace temperature control set point accordingly.
Because of the age of the cell and its characteristics, the control temperatures
should be slightly different. However if you perform a primary calibration incor-
rectly, or if the cell really starts to deteriorate, the actual cell response to calibra-
tion gas will drop off. The primary calibration is a way to extend the life of the
cell. The furnace control temperature can be increased to reduce the atrophy and
extend the cell life as the cell ages and the output starts to drop off its original
factory cal furnace set point temperature. This can be seen in the distribution in
your spreadsheet since the calculated cell temperatures for some of the analyz-
ers are well below 695°C or above by a fair amount. If the cell response goes
out of the limits in Table 4-1, there will be a zero or span gas range error. At this
point, you should perform a primary calibration and then attempt a standard cali-
bration again. If you get an excess cal error during primary calibration, then the
furnace temperature set point cannot be increased any further to prevent dam-
age to the cell housing and/or furnace assembly.
Failure Limits for Standard Calibration
•
+/- 10 mVs of the anticipated mVs for the gas concentration applied (air 0
mV expected).
•
+/- 5 mVs of the anticipated mVs for the gas concentration applied (2% O2
applied, 48 mV expected at 695°C cell temp) as referenced to the actual
span gas mV reading.
Table 4-1.