Analytical Industries GPR-1800 IS ATEX Trace PPM Oxygen Transmitter - Manual (Page 44)

Advanced Instruments, Inc
44
Appendix B
Matching - LCD Display with 4-20mA Output
In rare instances the 4-20mA signal output may not agree with the reading displayed on the LCD. The
Output Zero and Output Span features enable the user to adjust the 4mA and 20 mA signal output matching
with the reading displayed by the LCD.
For optimum accuracy make two separate adjustments as follows:
1. OUTPUT ZERO feature: To adjust the 4mA signal output and requires zero gas.
2. OUTPUT SPAN feature: To adjust the 20mA signal output and requires span gas near full range.
Note: In the field or in the absence of the preferred gases, use the OUTPUT SPAN feature and adjust the
20mA signal output using the span gas available.
Procedure – regardless of type of adjustment:
1. When you select OUTPUT ZERO OR OUTPUT SPAN, the microprocessor defaults to 100% to start.
2. The “actual” 4-20mA signal output will be adjusted to the “theoretical” value of the LCD display.
3. Adjustment general rule:
a) If the actual 4-20mA value < the theoretical LCD value, the adjustment value will be > 100%.
b) If the actual 4-20mA value > the theoretical LCD value, the adjustment value will be < 100%.
4. Convert the “actual” reading of the LCD display to the “theoretical” 4-20mA as follows:
a) Divide the “actual” (% or percent) LCD reading by the value of the span gas available.
b) Multiply 16mA (20mA – 4mA) times the “result of a.”
c) Add 4mA plus the “result of b.” to obtain the “theoretical” 4-20mA signal output value.
5. Adjustment value: Divide the theoretical by the actual 4-20mA values and multiply by 100.
6. Enter the adjustment value via OUTPUT ZERO or OUTPUT SPAN routines described below.
Example:
Analyzer reading is 60 % oxygen (100 % range) on 84 % span gas, 4-20mA signal output at
PLC is 24mA
Solution:
a) Use OUTPUT SPAN feature to make the adjustment.
b) Adjustment will be < 100% (default value of OUTPUT SPAN feature).
c) 13.6mA is the “theoretical” 4-20mA converted from the “actual” reading of the LCD.
60 % divided by 84 % = 0.71 or 71%
16mA multiplied by 0.71 = 11.36mA
4mA plus 11.36mA = 15.36mA “theoretical” 4-20mA signal output value
d) 15.36mA divided by 24mA the “actual” 4-20mA value = 64.0 adjustment value
e) Enter 64.0 via OUTPUT SPAN procedure below.