Once the sensor is properly zeroed, a span calibration is required. The best accuracy can be achieved by using a mixture of the target gas balanced in the background environmental air. This can be difficult to achieve, and it is much more practical to purchase calibration gasses from suppliers. Calibration gasses can be purchased as low-pressure containers or high-pressure container. Highly pressurized gas containers require some sort of regulator assembly to safely remove the gas from the bottle. However, these bottles can be designed to be much smaller and hold much more volume than low pressure containers.

Another method for span calibration is cross calibration. This involves using one gas to calibrate sensors that detect a different gas. All sensors are subject to interference by other gasses. Using this fact, gasses that may be easier to handle can be used to calibrate sensors that detect more difficult gases to handle. For example, a sensor calibrated to detect 100% LEL hexane can be calibrated using 50% LEL methane gas. Hexane gas is liquid at room temperature and it is difficult to make an accurate mixture and keep this at a high pressure. When calibrating a sensor using cross calibration, the sensor is calibrated using a calibration gas, and then a response factor is applied to the sensor. For example, if a pentane sensor is calibrated using methane, the output for pentane will be half of what the output for methane is. Therefore, a response factor of 0.5 is applied. This method only works for linear sensors, such as catalytic sensors. There are some issues with this calibration method. Every sensor is different, and the response factors may not be identical. Catalytic sensors must be at the specified heater voltage, otherwise the response factors are not applicable. It is good practice to periodically check the calibration of the sensor with the actual target gas.