Two independent, selective measuring principles are used in the ULTRAMAT 23.

Infrared measurement

An IR source (7) at 600 °C emits an infrared radiation which is modulated with 8 1/3 Hz by a chopper (5). After passing the sample cell (4), the intensity of the radiation is measured by the detector (11, 12).

The represented detector is composed of layers filled with the component to be analyzed. The energy of the middles of the IR bands of the measured gases is mainly absorbed in the first layer. The second layer absorbs the edge energy which is tuned on high selectivity in the third layer via an aperture.

When passing through the layers the radiation absorption results in different pressure increases and so to a flow via the capillary hole. The microflow sensor there generates a signal which is nearly independent of interferences from components at the band edges.

Note

The sample gases have to enter the analyzer dustfree. Avoid condensate in the sample cells. Therefore an appropriated gas preparation is required depending of the applications.

The ambient air of the analyzer should be, in a large extent, free of high concentrations of the component to be measured.

ULTRAMAT 23, mode of operation of infrared channel (example with three-layer detector)

Automatic calibration with air

The ULTRAMAT 23 can be calibrated e.g. with ambient air. During this AUTOCAL (adjustable between 0 and 24 hours), the chamber is purged with air. The detector then generates the largest signal U0 (no pre-absorption in the sample chamber). This signal is used as the reference for the zero calibration and serves simultaneously as the initial value for calculation of the full-scale value in the manner described below.

The absorption in the sample chamber increases along with the concentration of the measured component. As a result of this pre-absorption, the radiation energy measured in the detector decreases, and thus also the signal voltage. The mathematical relationship between the concentration of the measured component and the signal voltage corresponds to a good approximation in the single-beam procedure of the ULTRAMAT 23 to an exponential function of form:

U = U ₀ · e ^-kc

c Concentration

k Device-specific constant

U₀ Basic signal with zero gas (sample gas without measured component)

U Detector signal

Changes in the radiated power, contamination of the sample chamber, or ageing of detector components have the same effect on both U₀ and U, and result in:

U’ = U’₀· e^-kc

Apart from being dependent on the concentration c, the measured voltage therefore changes continuously with increased ageing of the radiator or with persistent contamination.

The mentioned influences of contamination and ageing will have a negligible effect on the measurement as long as U’ remains within a certain tolerance range which is monitored by the analyzer.

The analyzers carry out automatic calibration of the zero point with ambient air every 1, 2, 3 ... 24 hours as desired. Calibration with a calibration gas is unnecessary since the calibration curve is calculated with the new U’₀ value.

ULTRAMAT 23, mode of operation of oxygen measuring cell

Oxygen measurement

The oxygen sensor operates according to the principle of a fuel cell. The oxygen is converted at the boundary layer between cathode and electrolyte; the resulting current is proportional to the concentration of oxygen.

This sensor version with an acid based electrolyte is less sensitive to cross interferences, especially CO₂, CO, CH₄ and H₂ than other sensor types.

Essential characteristics

• Practically maintenance-free as a result of AUTOCAL with ambient air (or with N₂, only with units without oxygen sensor); both the zero and the span are calibrated in the process
• Calibration with calibration gas is only necessary every 12 months, depending on application
• Two measuring ranges can be set per component within defined limits;
  all measuring ranges linearized;
  autoranging with range identification
• Automatic correction of variations in atmospheric pressure
• Gas flow monitoring;
  Low-flow alarm at  1 l/min
  (with viton sample gas path only)
• Maintenance request alert
• Two limits can be freely configured for each component, for upward or downward violation.