He/O
2
Analyzer
| 7 |
Measuring principle
2. Measuring principle
The described measuring principle is valid only for mixes of air, oxygen and helium.
An electrochemical sensor is used to determine the oxygen content. The voltage at the
sensor’s output is proportional to the oxygen content in the analyzed mix. The sensor has a li-
mited service life and the proportionality of the dependence of voltage on the oxygen content
changes over time; therefore, it has to be regularly calibrated. It is possible to choose between
single-point, two-point and three-point calibration. Single-point calibration is fast, especially
if air is chosen as the calibrating mix. For greater accuracy of the measurement, two-point
calibration is used with two different mixes, typically air and pure oxygen. For strongly hypoxic
mixes, i.e. mixes containing less than approximately 15% oxygen, three-point calibration is
recommended. In this case, the third calibration gas should be a gas with zero oxygen content,
i.e. pure helium or argon.
Helium content is determined on the basis of measuring the speed of sound in the analyzed
mix. The speed of sound depends on the content of helium and oxygen, and the temperature
of the mix. The dependence of the speed of sound on pressure is small and can be disregarded
under normal atmospheric pressure.
At 0º C the speed of sound is approximately 970 m/s in pure helium, 330 m/s in air and
315 m/s in pure oxygen. Raising the temperature by one degree increases the speed of sound
by 0.175 %. The speed of sound in the mix is described by a non-linear function of temperature,
oxygen content and helium content.
The content of helium is determined by measuring the speed of sound, temperature of the
mix and the content of oxygen. When measuring the concentration of helium, it is therefore
necessary to have the oxygen sensor correctly calibrated or to know the oxygen content and
enter it into the instrument.
The speed of sound is measured directly as the time it takes for an acoustic impulse to travel
between two microphones. This measurement is performed alternately in both directions to
make it possible to eliminate the influence of the gas flow-rate in the probe on the calculation.
The acoustic impulses are heard as weak “clicks” from the probe in the helium-measuring mode.
Gas is delivered to the analyzer from a sampler connected to the compressed-gas tank
and the gas flow is controlled using a nozzle to provide the volume needed for the analysis.
