Analytical Industries GPR-1500 A Series Trace PPM Oxygen Analyzer - Manual (Page 13)

Advanced Instruments, Inc
13
Sample System
The standard GPR-1500-A transmitter is supplied without a sample conditioning system thereby giving users
the option of adding their own or purchasing a factory designed sample conditioning system, see section 2
QC Certification for optional equipment ordered. Whatever the choice, the sample must be properly
conditioned before introducing it to the sensor to ensure an accurate measurement.
Users interested in adding their own sample conditioning system should consult the factory. Advanced
Instruments Inc. offers a full range of sample handling, conditioning and expertise to meet your application
requirements. Contact us at 909-392-6900 or e-mail us at
[email protected]
.
Calibration & Accuracy Overview
Single Point Calibration: As previously
described the galvanic type 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 must be the
same.
Temperature: The rate at which 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 and
a network of resisters offsets this effect with an accuracy of +5% or better over a wide operating temperature
range e.g., 5-45
o C can be obtained thus the signal output remains virtually independent of ambient
temperature. There is extremely low error in measurement if the calibration and sampling are performed at
similar temperatures (within +/- 5 ºC. Conversely, a temperature variation of 10 ºC may produce an error of <
2% of full scale.
Accuracy: In light of the above parameters, the overall accuracy of an analyzer is affected by two types of
errors: 1) 'percent of reading errors', illustrated by Graph A below, is contributed by the temperature
compensation
circuit (tolerance in the thermistor value, variation in temperature coefficient of the thermistor,
tolerances in resistors values and the accuracy in the measuring devices, e.g., LCD display and 2) 'percent
of full scale errors', illustrated by Graph B, such as1-2% offset errors in readout and calibration devices.
Other errors are 'spanned out' during calibration, especially when analyzer is calibrated close to the top end
of the measuring range.
Graph C illustrates these 'worse case' specifications that are typically used to develop an overall accuracy
statement of < 1% of full scale at constant temperature or < 5% over the operating temperature range. The
QC testing error is typically < 0.5% prior to shipment of analyzer from the factory.