Teledyne 306WA - Analog trace oxygen analyzer - Manual (Page 43)

A-5
T T T T Trace Oxyg race Oxyg race Oxyg race Oxyg race Oxygen Anal en Anal en Anal en Anal en Analyz yz yz yz yzer er er er er Appendix Appendix Appendix Appendix Appendix
TELEDYNE ELECTRONIC TECHNOLOGIES
Analytical Instruments
Calibration Considerations Calibration Considerations Calibration Considerations Calibration Considerations Calibration Considerations
In order to calibrate the Model 366, all that is required is to center the
float of the flow tube in the target, add a known quantity of oxygen (0–100
PPM) to the sample gas and adjust the span according to the example on
page 2.4. It should not be necessary to compensate for changes in altitude or
ambient temperature.
The flow tube is a mass flow device and therefore automatically com-
pensates for altitude changes should the instrument be moved to a higher
elevation (in this instance a higher span setting will be required since the
sensing element is sensitive to the partial pressure of oxygen).
The ambient temperature does not affect the flow rate through the
analyzer since the flow tube is located in the same temperature controlled
compartment as the cell assembly.
Furthermore, it should be noted that the only time the flow rate is
critical is when calibration is being accomplished using the internal calibra-
tor. When span gases are used and when sample gas is being analyzed the
flow can vary ±10–20% and more without changing the reading. It is best,
however, to keep the sample flow so that the float in the flow tube is at or
near the center of the target. Otherwise, a different humidifier setting may be
required.
If positioning the float in the center of the target is in error by plus or
minus one-quarter of the float’s diameter, an error of approximately ±1.5%
of reading
*
will be produced.
There are instances when it will be necessary to reset or check the exit
flow of sample gas and set or reset the position of the target on the flow tube:
• When a different background gas is being analyzed.
• When the target has been accidently repositioned on the flow tube.
The vent flow should be measured either with a volume displacement
flow device (e.g. a “bubble-o-meter”) or a calibrated rotometer with correc-
tion factors for ambient temperature and pressure, and sample gas density
and viscosity. When using a volume displacement flow device it will be
necessary to correct the 150 cc/min flow rate for ambient temperature and
pressure. It will also be necessary, using either type of flow measuring
device, to compensate for the increase in flowrate due to humidifying the
sample gas. (The flow tube inside the analyzer is measuring the dry
dry dry dry dry gas
flowrate.)
*
If the actual oxygen concentration were, for example, 8.0 ppm, the
resultant reading would be 7.9 (float high) or 8.1 (float low).