Intro4u2u

Spectrometer layout

The FTIR spectrometer, model MB9100, is installed suspended in the system cabinet. Its main components are as follows: The interferometer with the electronics for controlling the spectrometer and communicating with an external computer, the IR-ray source, the transfer optics and the IR detector, and the cell.

Interferometer

The interferometer modulates the IR light along with the light from the laser and from the white-light source. The latter two are used for scaling the spectrum. The complete interferometer unit comprises the above light sources, their power supplies, the detectors for laser light and white light, the beam splitter, the retroreflectors, the transfer mirrors and the interferometer arm. The interferometer arm is distinguished by its twin-pendulum design, which contributes to the spectrometers robustness and freedom from over sensitivity.

IR-radiation source

The source of the IR radiation is a glowbar, a resistor with positive temperature characteristics, made from silicon carbide.

IR detector

The IR detector is a DTGS detector (deuterized triglyceride sulphate).

Cell

Connected to the spectrometer is the heated long-path cell. Via an optical transfer device located above the cell, the IR ray passes into the cell. In the cell, it is reflected several times by three mirrors, increasing the length of the optical path before leaving the cell again and arriving at the detector. The optical path length is fixed in the factory through the setting of the mirrors. It is normally 6.4 m.

Processing of measured values

The digitised detector signal is evaluated in terms of concentration of the different sample-gas components by a computer that forms one of the system components.

Oxygen Analyser Sample-gas feed

A proportion of the sample gas stream is diverted and piped to the oxygen Analyser from the sample-gas feed. To prevent condensation the sample gas is passed along the heated cell and into the oxygen analyser.

Oxygen sensor

The type KE-25 electrochemical oxygen sensor in the oxygen analyser operates on the same principle as a lead acid battery. The cathode of the electrochemical cell is made of gold and the anode of lead. A weak acid is used as an electrolyte. At the cathode, the oxygen entering from the sample gas, is absorbed electrochemically; a process through which electrons are used up. At the anode, lead releases electrons and is oxidised to form lead oxide. The current flowing through the outer circuit in this process is proportional to the diffusing oxygen. A porous barrier limits the diffusion from the gas phase so that a linear signal results corresponding to the concentration of oxygen. The surface of the lead anode regenerates itself continuously as the lead oxide dissolves in the electrolyte.

Removal of condensate

In order to remove the condensate, the sample gas is passed over a condensate trap. The condensate is expelled from the filter housing by means of a peristaltic pump.

Flow monitoring

Monitoring the flow upstream of the oxygen sensor ensures a minimum flow of 10 l/h. If the flow rate falls below this value, then a status signal Fault is output.

Discharge of the condensation heat

The condensation heat is discharged through the cooling device in the system cabinet. For this purpose, a cold-air pipe passes from the cooling device into the oxygen Analyser.

Processing of measured values

The analogue sensor signal is digitised, incorporated into the processing of measured values from the Advance Cemas FTIR, and displayed on the screen. The measured value for oxygen, common with all other measured values, relates to the moist flue gas.

Adjustment

The oxygen Analyser is automatically adjusted during the daily recording of the zero spectrum.