Understanding Span Factor

The “spanning” of a gas analyzer is a well-known process for seasoned measurement technicians. In the days before advanced electronic controls, the process of spanning involved using a small screwdriver to make adjustments to the analyzer’s multi-turn spanning potentiometer. These days, spanning is done at the push of a button during routine calibration.

The question is “does everyone understand what they are really doing and why?” We can tell you that based on some of our technical support calls, the answer is “No!”

The SPAN FACTOR for an AMI gas analyzer is a numerical factor to compensate for small degradation of the electrochemical sensor output as the sensor ages. This aging is common, expected for this type of sensor technology and is the primary reason that analyzers must be calibrated periodically.

During regular calibration, the measurement technician makes small adjustments to the analyzer reading to match the calibration gas that they are using. This process of increasing the reading and calibrating the analyzer results in an increase of the Analyzer’s SPAN FACTOR, which is stored in the analyzer’s memory.

The current SPAN FACTOR stored in an AMI analyzer can be displayed at any time by pushing the up arrow on the front control pad. When a brand new sensor is installed in an AMI Oxygen or Hydrogen Sulfide Analyzer and it is properly calibrated, the SPAN FACTOR will be in the range of 400 to 600.

The span factor, rather than the age of the sensor, is an excellent way to measure the health of the electrochemical sensor in your AMI analyzer. If the span factor has reached a value around 980, it is getting close to time to replace the sensor. You should plan for a sensor replacement at the next regular calibration.

Now it is important to remember that if the AMI analyzer is never calibrated, the SPAN FACTOR will not change. The SPAN FACTOR only changes when the analyzer is calibrated.

One other key point is that if the span factor is ever below 400, someone has incorrectly adjusted the span factor by incorrectly calibrating the analyzer. The analyzer must be recalibrated with certified span gas. It is critical that all the steps outlined by AMI for gas analyzer calibration are followed exactly.


  • The SPAN FACTOR is to compensate for sensor age
  • The SPAN FACTOR changes when the analyzer is calibrated
  • To display the current SPAN FACTOR of any AMI Analyzer, push the UP ARROW BUTTON
  • New sensor SPAN FACTOR is in the range of 400 to 600
  • When the SPAN FACTOR reaches around 980, replace the sensor on the next calibration