Owner's manual

Advanced Instruments Inc.
12
Zero Calibration
In theory
, the galvanic fuel cell type oxygen has an absolute zero meaning it produces no signal output when exposed to an
oxygen free sample gas. In reality
, expect the analyzer to generate an oxygen reading when sampling a zero gas due to:
¾ Contamination or quality of the zero gas
¾ Minor leakage in the sample line connections
¾ Residual oxygen dissolved in the sensor’s electrolyte
¾ Tolerances of the electronic components
The Zero Offset capability of the analyzer is limited to 50% of lowest most sensitive range available with the analyzer.
As part of our Quality Control Certification process, the zero capability of every ppm analyzer is qualified prior to shipment.
However, because the factory sample system conditions differ from that of the user, no ZERO OFFSET adjustment is made to
analyzer by the factory
Recommendations:
¾ ZERO CALIBRATION is recommended only
for online analyzers performing continuous analysis below 5% of the lowest
most sensitive range available with a ppm analyzer, e.g. analysis below 0.05 ppm on the 0-1 ppm range, 0.5 ppm on the
10 ppm range, or below 0.1% (1000 ppm) with a percent analyzer.
¾ Determining the true ZERO CALIBRATION adjustment requires approximately 24 hours to assure the galvanic fuel cell
sensor has consumed the oxygen that has dissolved into the electrolyte inside the sensor while exposed to air or
percentage levels of oxygen. After allowing the analyzer to stabilize with flowing zero gas (evidenced by a stable reading or
horizontal trend on an external recording device) perform the DEFAULT ZERO function before the ZERO CALIBRATION
function. For optimum accuracy, utilize as much of the actual sample system as possible.
¾ Always calibrate at the same temperature and pressure of the sample gas stream.
¾ Caution: Prematurely initiating the ZERO CALIBRATION function can result in negative readings near zero.
¾ ZERO CALIBRATION should precede SPAN CALIBRATION.
¾ If a ZERO CALIBRATION adjustment is made during initial installation, it is normally not required again until the sample
system connections are modified or a new oxygen sensor is installed. Therefore the DEFAULT ZERO function is
recommended only when performing a ZERO CALIBRATION and during troubleshooting and should not be repeated before
routine subsequent SPAN CALIBRATION
.
¾ If a ZERO CALIBRATION adjustment has NOT been made as described above, perform the DEFAULT ZERO and DEFAULT
SPAN functions when troubleshooting an analyzer and before SPAN CALIBRATION.
¾ ZERO CALIBRATION is not practical and not recommended for portable analyzers or measurements on higher ranges.
However, satisfying these users that the zero offset is acceptable for their application without the 24 hour wait can be
accomplished by introducing a zero gas (or sample gas with a low ppm oxygen concentration) to the analyzer. Unless the
zero gas is contaminated or there is a significant leak in the sample connections, the analyzer should read less than 100
ppm oxygen within 10 minutes after being placed on zero gas thereby indicating it is operating normally.
Span Calibration
Span Calibration involves adjusting the transmitter electronics to the sensor’s signal output at a given oxygen standard.
Maximum drift from calibration temperature is approximately 0.11% of reading per °C. The frequency of calibration varies with
the application conditions, the degree of accuracy required by the application and the quality requirements of the user.
However, the interval between span calibrations should not exceed three (3) months.
Note: Regardless of the oxygen concentration of the standard used, the span calibration process takes approximately 10
minutes, however, the time required to bring a ppm analyzer back on-line can vary depending on a combination of factors and
assumes exposure to a zero/purge/sample gas** with an oxygen content below the stated thresholds immediately after span
calibration: