QUESTION
Our G22 analysers are calibrated using an 8% span gas and will 0 with cal gas, when we test the upper range with pure oxygen some of the units show 100% with no issue however several units only show a maximum reading of between 60% – 79%. We have inter-changed the cells between the units, and the respective units readings remain the same, showing that the cells have the appropriate range and are all functioning.
ANSWER
When carrying out a calibration on the G22, it is possible to inadvertently create a negative offset by incorrect zero pot adjustment which has a knock on affect on the rest of the device output. If span calibration is carried out with a low concentration gas, any higher concentration will report incorrectly.
I suggest you apply a zero gas to the device and adjusts the zero pot until there are 4 zeros displayed, if a '-' sign followed by 3 zeros which alternates with 'neg' is shown, the adjustment is too negative and will need clockwise rotation until 4 zeros are seen. Following this would be span calibration (important span follows zero), apply the span gas and adjust the span pot until the correct reading is seen.