Analytical Industries GPR-3100 Series Oxygen Purity Analyzer - Manual (Page 12)

Advanced Instruments Inc.
5 Operation
GPR-3100 Rev 2/05 5
Calibration and Accuracy
Single Point Calibration: As previously described the galvanic oxygen sensor generates an electrical
current sensor exhibiting 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.
Accuracy:
In light of the above parameters, the overall accuracy of an analyzer is affected by two types
of errors: 1) those producing 'percent of reading errors', illustrated by Graph A below, such as +5%
temperature compensation circuit, tolerances of range resistors and the 'play' in the potentiometer used
to make span adjustments and 2) those producing 'percent of full scale errors', illustrated by Graph B,
such as +1-2% linearity errors in readout devices, which are really minimal due to today's technology
and the fact that other errors are 'spanned out' during calibration.