Appendix MIXCHEK
Teledyne
Analytical
Instruments
32
A.3 Helium/Air Chart
The helium/air chart allows the user to verify the proper
functioning and calibration of a helium and oxygen analyzer. This chart
provides the helium and oxygen percentages for a gas mixture
containing helium and air only. To use the chart, the user must have a
flowing source of helium and air. It is preferred to have a gas cylinder of
helium and a separate cylinder of compressed air. The gas cylinders
must be equipped with regulators, and control valves. Fittings, tubing
and connectors are also required. The cylinders need to share a common
manifold or delivery line so that the air and the helium can be mixed
together and supplied to the analyzer at a low flow rate and pressure. It
is best to have a separate control valve or gas flow restrictor between the
analyzer and the gas mixing system.
The procedure is to first calibrate the oxygen sensor with air at a
flow rate of 0.1 to 2 SCFH by setting the span to 20.9% oxygen. It is
important that the oxygen span be set accurately, and that the flow rate
does not exceed 2 SCFH during any phase of the testing.
After the oxygen section is calibrated, it will become the
standard for checking the helium section. The only other concern is that
the flow rate does not exceed 2SCFH.
Low flow rates do not typically cause an issue, but it should be
noted that the oxygen sensor does consume oxygen. If the gas is not
flowing the oxygen reading will drop as the sensor consumes the oxygen
around it.
To check the helium section, adjust the helium and the air to a
low flow rate. Connect the gas to the analyzer and allow the oxygen
reading to stabilize. Note the oxygen and helium readings. Use the chart
to look up the oxygen reading. The helium reading from the analyzer
should match the chart /-2% or better. Readjust the gas mixture
and repeat the process. Typically, it is adequate to check one high
helium reading, and one low reading on a
MIXCHEK
analyzer (these
analyzers use two point calibration). Helium concentrations of 10% and
90% are good targets to shoot for. The actual concentrations are not
important.
A more practical and portable helium and oxygen analyzer
validation system can be made by using two small gas cylinders of
mixed gas—one with a mixture of approximately 10% helium and air,