Teledyne 9110T - Nitrogen Oxides Analyzer - Manual (Page 212)

Model 9110T NOx Analyzer Calibration Procedures
Teledyne Analytical Instruments 212
9.4.3.2. Zero/Span Point Calibration Procedure
Note
If the ZERO or SPAN buttons are not displayed, the measurement made
during is out of the allowable range allowed for a reliable calibration. See
Section 12 for troubleshooting tips.