Divesoft HE/02, Руководство пользователя

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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 limited 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.