Code of Federal Regulations Title 40. Protection of Environment § 40.1065.672 Drift correction

(1) Correct each recorded concentration, x_i, for continuous sampling or for batch sampling, x.̄

(2) Correct for drift using the following equation:

Where:

x_{idriftcorrected} = concentration corrected for drift.

x_refzero = reference concentration of the zero gas, which is usually zero unless known to be otherwise.

x_refspan = reference concentration of the span gas.

x_prespan = pre-test interval gas analyzer response to the span gas concentration.

x_postspan = post-test interval gas analyzer response to the span gas concentration.

x_i or x ̄= concentration recorded during test, before drift correction.

x_prezero = pre-test interval gas analyzer response to the zero gas concentration.

x_postzero = post-test interval gas analyzer response to the zero gas concentration.

Example:

x_refzero = 0 μmol/mol

x_refspan = 1800.0 μmol/mol

x_prespan = 1800.5 μmol/mol

x_postspan = 1695.8 μmol/mol

x_i or x ̄= 435.5 μmol/mol

x_prezero = 0.6 μmol/mol

x_postzero = -5.2 μmol/mol

x_{idriftcorrected} = 450.2 μmol/mol

(3) For any pre-test interval concentrations, use concentrations determined most recently before the test interval. For some test intervals, the most recent pre-zero or pre-span might have occurred before one or more previous test intervals.

(4) For any post-test interval concentrations, use concentrations determined most recently after the test interval. For some test intervals, the most recent post-zero or post-span might have occurred after one or more subsequent test intervals.

(5) If you do not record any pre-test interval analyzer response to the span gas concentration, x_prespan, set x_prespan equal to the reference concentration of the span gas:

x_prespan = x_refspan.

(6) If you do not record any pre-test interval analyzer response to the zero gas concentration, x_prezero, set x_prezero equal to the reference concentration of the zero gas:

x_prezero = x_refzero.

(7) Usually the reference concentration of the zero gas, x_refzero, is zero:  x_refzero = 0 μmol/mol. However, in some cases you might know that x_refzero has a non-zero concentration. For example, if you zero a CO₂ analyzer using ambient air, you may use the default ambient air concentration of CO₂, which is 375 μmol/mol. In this case, x_refzero = 375 μmol/mol. Note that when you zero an analyzer using a non-zero x_refzero, you must set the analyzer to output the actual x_refzero concentration. For example, if x_refzero = 375 μmol/mol, set the analyzer to output a value of 375 μmol/mol when the zero gas is flowing to the analyzer.