Analytical Industries GPR-2500 A Oxygen Analyzer - Manual (Page 10)

10
Calibration & 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 transmitter 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 transmitter's overall
accuracy statement of +2% of full scale at constant temperature or +5% over the operating temperature range.
QC testing is typically <+0.5% prior to shipment.