AMI Analyzer Manual
Analyzer Description
21
Sample tubing and components
Oxygen diffuses through plastics, to a greater or lesser extent. Standard blue poly tubing will diffuse about
1ppm per foot into 1SCFH at 70°F. Silicone tubing is very much worse. In general, use stainless steel tubing,
or at least copper tubing, not plastic. Use high quality compression fittings such as “Swagelock™” or
“Gyrolock™” and high quality stainless steel filters, valves or regulators if necessary. Make sure that they
are assembled correctly and perform a thorough leak test on the sample system before use.
The analyzer expects to see sample pressures between about 1 psig and 150psig. Higher pressures will
overwhelm the input valve and make it hard to control the flow. If your pressure is higher than this use a
regulator with a stainless steel diaphragm to bring the pressure down appropriately. The analyzer is not
very much affected by changes in flow rate and the internal flow meter is adequate for setting the flow.
Don’t use a high precision external flowmeter in front of the analyzer because oxygen will diffuse in through
its O rings.
The span gas pressure should also be held below 100psig, and normally would be controlled to 10psig. AMI
recommends that you use a length of their flexible tubing with O ring seals on its fittings so that you don’t
damage the analyzer fittings by making and breaking them too often.
Leaks
If you suspect a leak due to high oxygen readings, change the flow rate and see how long it takes for the
oxygen reading to change. A higher flow rate will dilute the effects of a leak so that the reading will drop as
you increase the flow (this is a very good way of making sure that you do not in fact have a leak, by the
way). Turn the flow up from 1 SCFH to 2 SCFH using the front panel valve and see how long it is before the
reading changes. If it does so immediately, the leak is close to the analyzer. If it takes longer, the leak is
further away.
Exhaust
The exhaust port of the analyzer should be given at least a foot of tubing, but it should not be allowed to
become pressurized, or the readings will be affected. It should be vented outdoors or brought into a
scavenging system. If the latter, have the scavenging system suck in room air around the exhaust line so
that the pressure at the exhaust is atmospheric. A typical way of doing this is to have the scavenging
system suck on a 1” pipe, and have the ¼” exhaust line from the analyzer extend six inches or so into the
end of the pipe. If it is likely that a loss of flow could happen, use a “Back Diffusion Assembly” to prevent air
from getting back into the analyzer if this should happen.
Calibrating the analyzer
The oxygen sensor gradually gets used up over its life, and as it does so its sensitivity slowly drops until it is
at the end of its life, when the sensitivity drops much faster. Therefore you have to calibrate the sensor
every so often to make sure that your readings are correct.
