User Calibration - Setting the Zero and Span

Select Calibration from the View menu to open the Calibration window. This is the area in which you set the zero and span of the LI-820.

The calibration window shows the date and time at which the LI-820 was last zeroed and/or spanned. When spanning the instrument, you can enter the value of your span gas in the “Span gas concentration (ppm)” field. The Manual and Advanced tabs offer additional calibration options, discussed below under Two Point Span.

It is recommended that you perform the zero calibration first, followed by the span calibration. To zero, flow a dry, CO₂-free gas through the LI-820, and make sure the optical cell is completely purged. Press the Zero button. The display will show ZERO, and the text in the Calibration window is greyed out. The zero will be set electronically, and the current date will be entered in the Last zeroed on field when completed.

To span, connect your span gas to the input air stream. Make sure the cell is purged, enter the value of the span gas, and click on Span.

The display will show SPAN, and the text in the Calibration window is greyed out. The span will be set electronically, and the current date will be entered in the Last spanned on field when completed.

Advanced Calibration (Manual Tab)

The Manual tab in the Calibration window displays current calibration values for Zero, Span Offset, and Span Slope. The zero value is the parameter that is set when you zero the instrument. This value is typically between 0.8 and 1.2, and reflects the ratio of the two detectors’ output when there is CO₂-free air in the cell.

The span offset is the parameter that is set when you span the instrument. The span offset value is usually close to 1.0. The actual span is a linear function of absorptance, so the span offset is the intercept term of that relationship, and the span slope is the slope of the relationship.

If your LI-820 is exhibiting erratic behavior such as no response to CO₂, or erratic readings, check the span offset and span slope values. As an example, if the span is inadvertently set at the wrong time (with CO₂-free air in the cell, for example), the span offset term can be near zero, or very large (10, 1000, etc.), in which case the instrument will not function correctly. The editable fields under the Manual tab allow you to manually set the two values to correct this. Set the Span offset value to 1.0, and the Span slope to zero and click Apply to return the instrument to a more “normal” state of operation. You should then perform a zero and span (or two point span) of the instrument.

Two Point Span(Advanced Tab)

The two point span function under the Advanced tab is used to set the span slope value using two non-zero gases. This is performed at the factory, and only needs to be repeated if the source and/or detector are replaced. To test the span slope after replacing the source and/or detector, zero the instrument, and then measure one of your two known gases, using it to set the span, if necessary. Measure your second known gas; if the reading exceeds the specification, you can use the two point span function to correct the span slope and bring the instrument back within factory specification.

The two point span function sets the span zero and span slope parameters. The two gas concentrations used to do this should be fairly well separated. One gas concentration should be fairly low – 600 ppm or less. The second gas concentration should be fairly high, and ideally, close to the upper range of concentration that you are interested in (e.g. 1500, 3000, 10000, etc.). If you are interested in a narrow range of concentrations, such as 300 to 500 ppm, then we do not recommend using 300 and 500 for a two point span; simply use one or the other for a single span. Some suggested gas concentration pairs for various ranges of interest are shown below; these are not absolute limits, but simply recommendations.

Upper Range (µmol/mol)	Suggested Two Point Concentrations (µmol/mol)
2000	<400, >1500
5000	<600, >2000
>20,000	<600, >5000

To perform the two point span, first make sure the instrument is well zeroed. Flow the first span gas (either high or low, the order doesn’t matter) through the cell, and enter the mole fraction in the “Span gas concentration” field. When the instrument’s reading stabilizes, click the Span 1 button, After about 30 seconds the instrument should be reading the specified concentration. The span slope will be zero, and the span offset will reflect the span result. Repeat for the second gas concentration, clicking the Span 2 button. After about 30 seconds, the span slope value will change, as will the span offset, but only slightly. At this point, the instrument should read either of the two span values correctly.