Analytical Industries GPR-1900 Series Trace PPB Oxygen Analyzer - Manual (Page 7)

Advanced Instruments Inc.
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However, for optimal performance after exposing the sensor to air or elevated oxygen levels in terms of:
1) bringing the analyzer back online, and,
2) maximizing the service life of the sensor by isolating the sensor
Advanced Instruments recommends the user employ one of the following ‘bypass sample system’ designs illustrated below.
Preference is given to the illustration on the left because it is the surest and simplest approach.
The ‘bypass sample system’ is optimal, however, it is not required. For example, deleting the 3-way valve from the illustration
on the right enables the user to isolate the sensor but lengthens the time required to bring the analyzer online. The bypass
feature diverts the flow of sample gas containing high concentrations of oxygen around the sensor – which when the valves are
operated properly remains in an atmosphere containing a low concentration of oxygen, less than 100-200 ppm. The source of
the atmosphere with the low concentration of oxygen can be either be a zero gas or sample gas is not important - only the
oxygen concentration is important. Without the bypass feature, the sensor still can be isolated but not as easily and the best the
user can do when connecting a new gas line is start the flow of sample gas (to purge the air trapped inside the line) several
minutes before connecting to the analyzer.
Calibration & Accuracy Overview
Single Point Calibration: As previously described
the galvanic oxygen sensor generates an electrical
current proportional to the oxygen concentration in
the sample gas. In the absence of oxygen the sensor
exhibits an absolute zero, e.g. the sensor does not
generate a current output in the absence of oxygen.
Given these linearity and absolute zero properties,
single point calibration is possible.
Pressure: Because sensors are sensitive to the
partial pressure of oxygen in the sample gas their
output is a function of the number of molecules of
oxygen 'per unit volume'. Readouts in percent are
permissible only when the total pressure of the
sample gas being analyzed remains constant. The
pressure of the sample gas and that of the calibration
gas(es) must be the same (reality < 1-2 psi).
Temperature: The rate oxygen molecules diffuse into the sensor is controlled by a Teflon membrane otherwise known as an
'oxygen diffusion limiting barrier' and all diffusion processes are temperature sensitive, the fact the sensor's electrical output
will vary with temperature is normal. This variation is relatively constant 2.5% per ºC. A temperature compensation circuit
employing a thermistor offsets this effect with an accuracy of +5% or better and generates an output function that is
independent of temperature. There is no error if the calibration and sampling are performed at the same temperature or if the
measurement is made immediately after calibration. Lastly, small temperature variations of 10-15º produce < 1% error.