OxyDetect Oxygen Deficiency Gas Detector
Appendix
05311001B / Revision 4
79
A5
CROSS INTERFERENCE OFFSETS
For a paramagnetic oxygen measurement, the composition of any typical background
gas in the gas sample will have a small effect on the analyser oxygen measurement.
The following table gives 4 examples of cross-interference errors (O
2
measurement
errors) in atmospheres which contain 100% of a specific background gas, for an
analyser which has been 'Lo' calibrated with N
2
(nitrogen) and 'Hi' calibrated with 21%
O
2
/ balance N
2
(air), at 20 °C temperature.
Background gas
Error
Background gas
Error
Argon
–0.22%
Krypton
–0.49%
Carbon dioxide
–0.26%
Helium
+0.29%
Note that the error is directly proportional to the concentration of the background gas
in the sample being measured, and in most cases can be ignored. Most gases will
slightly reduce the measured oxygen concentration further than the actual reduction
of oxygen levels caused by the displacement of oxygen in the atmosphere.
Example: If you are measuring 21% O
2
(oxygen) in air (typical room
atmosphere) and a leak of argon occurred reducing the oxygen levels by 3%
in the local area, then the paramagnetic cell would measure the direct
decrease in oxygen, plus a very small measurement offset caused by the
argon now being present.
Before: 21% oxygen, 78% nitrogen, 1% argon
After: 18% oxygen, 67% nitrogen,
15% argon
,*
*quoted percentages rounded to simplify example.
The actual oxygen measurement would incur an additional measurement
offset caused by the contribution of 14% argon gas. This can be calculated
as ((-0.22 / 100) x 14) = -0.031%. The argon will further reduce the oxygen
reading by -0.031%.
Therefore, the measured oxygen concentration would actually be reported
by the OxyDetect as 18.0 – 0.031 = 17.97% O
2
. In reality this measurement
error caused by background gas effects is within the stated accuracy of the
OxyDetect and below the resolution of the display, so though it should be
considered, it can usually be discounted in most applications.
