Spectrasensors OXY5500, Operator'S Manual, Page: 134 / 140

Oxygen Analyzer
B
–
18 4900002239 rev. G 7-11-17
Signal Drifts due to Temperature Gradients
A further source of imprecise measurement is insufficient temperature
compensation. If temperature compensation is used, ensure that no
temperature gradients exist between the oxygen sensor and the temperature
sensors. If measurement is conducted without temperature compensation,
bear in mind that the OXY5500 only measures correctly if the sample
temperature is constant during measurement and the temperature is the same
as the entry at the beginning of the measurement. A temperature
measurement error of +/-0.3

C will result in a measurement error of about +/-
1% of reading. The temperature probe provided with the unit has excellent
precision, but large gas temperature gradients will result in an offset between
the oxygen probe and temperature probe. To avoid an offset, ensure that the
gas temperature has been stabilized prior to passing over the oxygen probe.
SCS systems provided by SpectraSensors are designed to ensure that this is
not a problem.
Signal Drift due to Photo-decomposition
The oxygen-sensitive material may be subject to photo-decomposition
resulting in a signal drift. Photo-decomposition takes place only during
illumination of the sensor tip and depends on the intensity of the excitation
light. Therefore, the excitation light should be minimized. Continuous
illumination of a OP-3 oxygen sensor over a period of 24 hours may lead to a
phase drift of up to + 0.4% of reading at 20

C. However, this effect of photo-
decomposition can even be minimized by changing the measuring mode to the
30-second or minute interval mode. In these modes, the software switches off
the excitation light after recording the data point and switches it on after the
interval chosen. Use the interval method whenever possible to increase the
operational life of the sensor. Refer to Table B–2 below.
Performance Improvement
To improve the performance over past measurements, check the calibration
values by using the calibration test gases for “0” (UHP Nitrogen 99.9999%) and
the Span test gas (100 ppm oxygen/N
2
). This can be completed by using a
3-way valve connected to the test gas enabling the user to switch back and
forth between bottles. This can assist in verifying proper operation.
Table B–2 Sensor drift at zero reading (0 ppb)
recording 3600, 50000 and 100000 data points
Drift per 3600
points
Drift per 50000
points
Drift per 100000
points
OP-3 <0.15% air-sat. <0.15% air-sat. <0.25% air-sat.
OP-6 <1 ppb <2 ppb < 3 ppb