Analytical-industries GPR-1200 MS ATEX Portable PPB Oxygen Analyzer Manuel d'utilisateur télécharger pdf (Page 11)

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 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 +2% of full scale at constant temperature or +5% over the operating temperature range. QC testing

is typically <+0.5% prior to shipment.

1 2 ... 6 7 8 9 10 11 12 13 14 15 16 ... 31 32

Pas de commentaire

Analytical-industries GPR-1200 MS ATEX Portable PPB Oxygen Analyzer Manuel d'utilisateur Page 11