Analytical Industries GPR-7500 AIS Trace PPM Hydrogen Sulfide Analyzer - Manual (Page 21)

Advanced Instruments, Inc.
21
4. Advance the reverse shade cursor using the ARROW keys to highlight the desired MANUAL RANGE.
5. Press the ENTER key to select the highlighted menu option.
The following display appears with the range selected and oxygen concentration of the sample gas:
6. If the value of H2S goes above the full scale range selected, display will not shift to the next higher
range. Instead, when the H2S reading exceeds 125% of the upper limit of the current range, an OVER
RANGE warning will be displayed.
7. Once the OVER RANGE warning appears the user must advance the transmitter to the next higher
range.
8. NOTE: With H2S reading above 125% of the selected range, the mA signal output will increase but will
freeze at a maximum value of 24 mA. After the sample reading falls below the full scale range, the mA
signal will become normal.
Calibration of Transmitter
The electrochemical H
2
S sensors generate an electrical current that is linear or proportional to the H
2
S
concentration in a sample gas. In the absence of H
2
S the sensor exhibits an absolute zero, i.e., the sensor
does not generate a current output in the absence of H
2
S. Given the properties of linearity and an absolute
zero, a single point calibration is possible.
As described below, zero calibration is recommended only when the application (or user) demands optimum
accuracy of below 5% of the most sensitive or lowest range available on the transmitter. Span calibration, in
one of the forms described below, is necessary to adjust the analyzer sensitivity for accurate measurements
of H
2
S. As a rule of thumb, zero calibration should be carried out after span calibration.
Zero Calibration
Despite the absolute zero inherent to the electrochemical H
2
S sensors, the reality is that analyzers may
display H
2
S reading even when sampling a zero gas (H
2
S free gas) due to:
1. Contamination or questionable quality of the zero gas
2. Minor leakage in the sample line connections
3. Residual electrical current generated by the sensor
4. Tolerances in the electronic components
12 PPM
Manual Sampling
0-500 PPM Range
76 F 97 Kpa
LO1 0 PPM HI 40 PPM
65 PPM
OVERRANGE
Manual Sampling
0-500 PPM Range
76 F 97 Kpa
LO1 0 PPM HI 40 PPM