S
TARTUP AND
C
ALIBRATION
748213-P Rosemount Analytical June 1997
3-5
Model 755R Oxygen Analyzer
3.5.2 C
OMPUTING
A
DJUSTED
S
ETTINGS FOR
Z
ERO AND
S
PAN
C
ONTROLS
During instrument calibration, adjusted values may be required in setting the ZERO
and SPAN controls to correct for the magnetic susceptibility of the background gas.
The quantities are defined as follows:
BGGst = Oxygen equivalent of background gas in standard gas (see Table 3-2)
BGGs = Oxygen equivalent of background gas sample (see Table 3-2)
OP = operating pressure. Unless special pressure corrections are to be made,
the zero standard, span standard and sample gases must all be admitted at the
same pressure.
Use the following equation to compute the adjusted settings for the ZERO and SPAN
controls:
Adjusted % O
2
for standard gas =
Where:
A = true % O
2
of standard gas
B = BGGs
C = BGGst
Example:
Background gas in sample is CO2, oxygen equivalent = -0.623%
Zero gas is 100% N2
Span standard gas is air: 21% O2, 79% N2
Background gas in zero and span standard gases is N2, oxygen equivalent = -
0.358%
With N2 zero standard gas flowing, ZERO control is adjusted so digital display
reads:
With air flowing, SPAN control is adjusted so the digital display reads:
(A)[100 + (B-C)] - 100 [B-C]
100
0[100+(-0.623-(-0.358))] - 100[-0.623-(-0.358)]
100
=
0.265% O
2
21[100 - 0.265) - 100 (-0.265)
100
= 21.209% O
2
≅
21.21
