Analytical Industries GPR-1600 Series Trace PPB Oxygen Analyzer - Manual (Page 13)

Advanced Instruments Inc.
13
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.
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.
Graph C illustrates these 'worse case' specifications that are typically used to develop an analyzer's overall accuracy statement
of < 1% of full scale at constant temperature or < 5% over the operating temperature range. QC testing is typically < 0.5%
prior to shipment.
Example 1: As illustrated by Graph A any error, play in the multi-turn span pot or the temperature compensation circuit,
during a span adjustment at 20.9% (air) of full scale range would be multiplied by a factor of 4.78 (100/20.9) if used for
measurements of 95-100% oxygen concentrations. Conversely, an error during a span adjustment at 100% of full scale range is
reduced proportionately for measurements of lower oxygen concentrations.
Refer to the Calibration section for additional details.