AMI Analyzer Manual
Analyzer Description
22
There are two ways of doing this. You can use a span gas, a mixed gas containing a certain level of oxygen
in a background gas that ideally matches your sample, but is normally nitrogen, or you can use air. The
advantage of using air is that its value really is 20.9% and it doesn’t change; the disadvantage is that it
means that the sensor is exposed to air and so it will take a while for the reading to come down again
afterwards. Typically, at room temperature, an AMI analyzer will take about twenty minutes or less to
come down to below 10ppm after a one minute exposure to air.
The advantage of using a span gas is that you can calibrate it to a value close to the range of interest – often
people use 80ppm oxygen in nitrogen as the span gas. You may have political reasons for having to do this.
The sensor recovers from this level of oxygen immediately so there is no down time before the analyzer is
working properly again. The disadvantage is that span gases sometimes are incorrectly made, and they can
be contaminated by improper handling.
Calibrating with a span gas
Fist put a suitable regulator on the span gas tank, and “bleed” it as described below. This step is essential,
since otherwise the air in the regulator will contaminate the gas in the tank. Connect the regulator to the
analyzer with a flexible line such as that provided by AMI, and leak check the connection with “Snoop™” or
similar leak detection fluid. Purge the line for several minutes with a small flow of gas prior to doing this,
and leave the gas flowing while you make the connection to the analyzer. This prevents a slug of air from
giving you excessively high readings when you start spanning the analyzer.
Make sure the analyzer is seeing a low oxygen level gas – you want the analyzer to go UP to the span gas,
not down to it, particularly not from air. Otherwise it will take a very long time to get a good calibration.
When all is assembled, turn the flow selector valve on the front of the analyzer to the SPAN position. Span
gas is now flowing into the analyzer, and you should see the reading move to the span gas value. Assuming
it stabilizes somewhere reasonably close, press the UP or DOWN button until the reading on the LCD shows
what the span gas bottle says the value should be. Let go of the buttons, and after a second or two the
analyzer will store its calibration value. Return the flow selector valve to the SAMPLE position.
Calibrating with air
You can either connect a compressed air line – from the plant air, not a bottle of compressed air – to the
span port or you can leave the span port open. If you use compressed air, go through the same procedure
as above, only adjust the oxygen reading to 20.9%. Bottles of compressed air are frequently actually a
nitrogen/oxygen mixture and may not contain exactly 20.9% oxygen.
If you do not have compressed air, turn the flow selector valve to OFF, and unscrew the cell cap on the front
of the analyzer. Blow some air under the sensor by waving a hat or some such at it. Adjust the reading as
before to 20.9% and then turn the flow selector valve back to SAMPLE. Then screw the cell cap back on the
analyzer.
