Analytical Industries GPR-7500 AIS Trace PPM Hydrogen Sulfide Analyzer - Manual (Page 22)

Advanced Instruments, Inc.
22
The maximum zero offset of every transmitter is checked prior to shipment. However, due to the fact that the
factory sample system conditions differ from that of the user, no ZERO OFFSET adjustment is made at the
factory
Typical offset seen is less than 0.5-1 PPM. Therefore, for most applications, a Zero calibration is not
required. However, ZERO calibration option has been provided to allow the user to precisely measure H
2
S
concentration at the very low levels (less than 0.5 PPM). As described below, accomplishing either objective
places a degree of responsibility on the user.
Determining the true offset requires the user to wait (see Online Recovery Time section) until the analyzer
reading is no longer trending downward (best evidenced by a constant horizontal trend on an external
recording device.
The zero offset adjustment is limited to 5-20% of the most sensitive range of the analyzer. At factory,
analyzer is QC tested to confirm that the maximum offset is less than 5% of the most sensitive range
available. Should you observe a zero offset more than 5% of the lowest range, check sample system for any
possible leaks, integrity of the zero gas and assure that the analyzer has been given enough time to stabilize
on zero gas before initiating the
” ZERO CALIBRATION”.
Caution: If adequate time is not allowed for the analyzer to establish the true baseline and a ZERO
calibration is performed, the analyzer will, in all probability, display a negative reading in the sample mode
after a certain period of time. If a negative reading is seen, perform ZERO calibration again
Span Calibration
Involves periodically, see Intervals section below, checking and/or adjusting the electronics to the sensor’s
signal output at a given H
2
S standard. The frequency of calibration varies with the application, e.g., the
degree of accuracy required by the application and the quality assurance protocol of the user. However, the
interval between span calibrations should not exceed three (3) months.
Note: Regardless of the value of the standard used, the span calibration process takes approximately 10-15
minutes
Factors to Consider when calibrating
1. When it comes to the calibration of transmitter, circumstances vary widely from the ideal conditions that
exist at the factory to a variety of differing circumstances users encounter in the field. The following
describes the most common factors and reasons that influence the calibration procedures.
2. All electrochemical sensor based devices require periodic calibration, e.g. weekly intervals to a 3 month
maximum, to ensure accuracy and ascertain the integrity of the sensor
3. For optimum accuracy, calibrate the analyzer at or close to the temperature and pressure of the sample
gas
4. The priority users place on getting or keeping an analyzer online is “the” most significant factor involved
in calibration and troubleshooting issues
” the time it takes an analyzer/transmitter to come down to a
specific level after installation or calibration. A new sensor would require 30-60 minute settling time after
installation before commencing calibration. If a sensor has been in service, calibration can be performed
any selected time.
5. For optimum accuracy, the H
2
S concentration of a span gas should be approximate 50-90% of the full
scale range of analysis or one range above the analysis range, e.g. 50-80 PPM on the 0-100 PPM
range.
6. Use of span gas less than 50% of the full scale range of measurements will introduce an “expanding
error” as illustrated by Graph A in Example 1 in the Accuracy section above, close to the top end of the
range.
7. Prematurely initiating the SPAN CALIBRATION function (before the analyzer reading has stabilized)
can result in erroneous readings as follows:
Zero Calibration Procedure
Zero calibration should be carried out after the span calibration has been performed. Normally, zero
calibrations are performed when a new sensor is installed or changes are made in the sample system
connections.
Before performing a ZERO calibration, it is highly recommended to perform a factory default zero. This will
eliminate previous zero offset adjustment that might have been made. With factory default setting, if the zero