Technical Manual Page 2 CODEL
OPS.080 Issue : C Rev. : Date : 16/7/08 Doc. i/d : 0080/6 Ref. : 080040
1.2. GCEM4000 Basic Principles
The GCEM4000 analyser uses an in-situ probe set into a duct to measure the concentration of gases of interest.
Figure 2 illustrates the arrangement. The in-situ tube includes a section that allows the diffusion of flue gases
into the measurement zone or the dispersion of purge or calibration gas out of the tube and into the duct. This
section of the tube is the analysers’ measurement cell. The analyser is capable of simultaneous measurement
of up to six different gases (plus water vapour as a seventh measurement if required).
As with conventional cross-duct analysers, this probe configuration does not require the extraction of a sample
from the gas stream and makes its measurement by analysing the way in which infrared radiation, transmitted
through the measurement section of the probe, is modified by the gases present.
Figure 2 :GCEM4000 Analyser with 'in-situ' Probe
The transceiver unit containing the infrared source and detector system, required to measure the received light
energy after its passage through the gas, is totally isolated from the flue gas. There is, therefore, no contact
between the analyser electronics and the flue gas. Correctly installed and commissioned, this provides the
opportunity to achieve very low maintenance factors and totally eliminates any possibility of altering the
composition of the flue gases to be measured.
The remaining components of a GCEM4000 analyser are:
• The Gas Control Unit (GCU) that controls the input of zero and span calibration gases into the
analyser. It contains the necessary compressed air filtration and drying equipment to ensure high
quality air supply for the zero calibration and probe purge functions. The analyser power supply and
Station Control Unit (SCU) are also housed within the GCU. The function of the SCU is as an
emissions data processing unit, communications centre for the monitor and controller of the zero and
span calibration functions.
• The Central Data Controller (CDC) that accepts data from 1 to 16 SCUs and processes the data for
onward transmission to a remote pc or SCADA system.
• The Active Weather Cover utilises a Peltier heat pump to provide heating or cooling to maintain a
constant air temperature around the sensor head in a wide range of ambient temperatures.
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