Analytical-industries GPR-1600 Series Trace PPB Oxygen Analyzer Manuel d'utilisateur télécharger pdf (Page 35)

Advanced Instruments Inc.

Span Gas Selection:

The O2 concentration of a span gas should approximate 70-90% of the full scale range

dictated by the span gas, e.g. 80 ppm O2 on the 0-100 ppm range. For optimum accuracy,

the full scale range dictated by the span gas should be at least one range higher than the

intended analysis range. Both of the aforementioned recommendations reduce the error

induced by the tolerance of the electronic components; also, span gases with higher O2

concentrations are more reliable and less expensive.

Conversely, if the recommended span gas is not available and air calibration is not an

option, a span gas of the same full scale range and near the anticipated analysis level

(approximately 10% of full scale) is acceptable with the understanding the accuracy will

suffer slightly.

Use of span gas near 10% of the same full scale range for measurements at the higher end

of the range has the effect of “expanding the error” by moving upscale as illustrated by

Graph A and Example 1 in the Accuracy section above and is not recommended. Of course

the user can always elect at his discretion to accept an accuracy error of +

2-3% of full scale

range if no other span gas is available.

Type of Sensor:

Galvanic Fuel Cell Sensors: Analyzers utilizing these sensors can be calibrated with

ambient air (20.9% or 209,000 ppm O2) and have been proven capable of precise analysis

below 0.5 ppm (500 ppb).

Note: As described above, these oxygen sensors are capable of one point calibration.

Span Gas vs. Air

(Fuel Cell Sensors only):

Span gas calibration: Recommended for calibration of in-service analyzers, if the priority

is the fastest online recovery time.

Air calibration: Recommended for calibration an analyzer if a new oxygen sensor is being

installed or if the availability of span gas, the cost of span gas or the accuracy of a span gas

is an issue. An air calibrated analyzer can be used to reliably verify a “certified” span gas,

which has frequently been found to be inaccurate. For best results, select a recognized

name supplier.

Note: Galvanic Fuel Cell sensors can be calibrated (using air) and brought online (using

sample gas) without “purchased calibration gases” and without sacrificing accuracy -

provided the analyzer is given enough time to come down to the O2 concentration of the

sample gas.

Span Calibration

Adjustments:

Prematurely initiating the SPAN CALIBRATION function (when there is no intention of

performing a ZERO CALIBRATION) before the analyzer reading has stabilized can result in

erroneous readings as follows:

When purging an analyzer to lower ranges for span gas calibration: If the oxygen

reading reaches less than 2% of the intended calibration range, enter the value of the span

gas. If the oxygen reading is greater than 2% of the calibration range, add the O2 reading

to the value of the span gas (the impact of the offset on accuracy is minor but the addition

allows the oxygen sensor to continue to purge down and avoid negative readings).

Note: If ZERO CALIBRATION has been performed or the analyzer has been in service, the

analyzer reading should already be stable and below 2% of the calibration range.

When installing a new oxygen sensor and calibrating with air: Allow 5-10 minutes

for the sensor to equilibrate in ambient air from storage packaging. Failure to do so can

introduce a positive offset (electronic gain) that prevents the analyzer from displaying low

ppm O2 readings.

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Analytical-industries GPR-1600 Series Trace PPB Oxygen Analyzer Manuel d'utilisateur Page 35

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