Analytical-industries GPR-1800 IS ATEX Trace PPM Oxygen Transmitter Manuel d'utilisateur télécharger pdf (Page 13)

Advanced Instruments, Inc

Sample System

The standard GPR-1800 IS 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.

The GPR-1500 IS is generally supplied with a minimum of a sample flow control valve and a flow meter.

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

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.

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 50 51

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Analytical-industries GPR-1800 IS ATEX Trace PPM Oxygen Transmitter Manuel d'utilisateur Page 13

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