Technical Manual Page 53 CODEL
OPS.080 Issue : C Rev. : Date : 16/7/08 Doc. i/d : 0080/6 Ref. : 080040
7.3.3. Comparative Testing
Legislation often require that continuous emissions monitoring systems are audited at regular intervals (usually
once or twice per year) against an independent approved test system. These approved test systems are always
extractive analysers and measurements must normally be carried out by organisations independent of the
CEMS manufacturer or the plant operator.
If the analyser is working correctly and the comparative testing is carried out to the prescribed procedure, it is
rare for such exercises to be other than a routine confirmation of performance. However, to ensure that any
discrepancies can be easily resolved, it is recommended that the following precautions be taken:
a) Check what level of agreement is required by the relevant legislation. Most legislation is realistic
about what can be expected in comparing a system that provides an average of a significant
percentage of the whole duct or stack and one that averages a number of discrete
measurements from different positions at different times.
b) Be aware of the specified procedure for the comparative test and ensure that appropriately
qualified personnel using certified equipment and test gases carry it out. Do not accept the
validity of quick, single-point checks with uncertified analysers.
c) Ensure that, when comparing measurements made by the GCEM4000 and the audit system,
the measurements are on the same basis, i.e.
they have been recorded simultaneously and during a period when plant operation is
stable
the measurement averages are equivalent (e.g. 10-minute rolling average
measurements from the Model 4000 should be compared with 10-minute averages from
the audit system and not measurements averaged over significantly different time
periods).
all measurements are normalised/corrected to the same standard conditions of
temperature, pressure, water vapour (dry or wet) and excess oxygen (or CO
2
).
d) If the reasons for any discrepancies are not apparent it is strongly recommended that CODEL or
its local service agent be consulted before any attempt is made to recalibrate the Model 4000
analyser. Re-calibrating to a reference measurement that is subsequently found to be unreliable
creates many unnecessary difficulties. DO NOT RECALIBRATE UNLESS ABSOLUTELY
CERTAIN THAT IT IS NECESSARY!
7.4. Re-calibration
The laws of physics determine that the scale shape (i.e. the relationship between gas
concentration and analyser output) is absolutely fixed for a defined measurement waveband.
Only serious faults, (e.g. a leaking gas cell) that would be detected and registered as a data
invalid condition by the analyser itself, can modify this relationship.
If re-calibration cannot wait until the next scheduled automatic zero calibration (Section 7.3.1. Automatic Zero
Calibration) a manual zero calibration may be implemented (Section 7.3.2. Manual Zero Verification).
Any discrepancy that is now outside the combined measurement uncertainties of the test gas (or comparative
measurement) and the analyser, can only be due to either a significant fault in the analyser itself, inaccuracy in
the test gas or inappropriate comparative testing (see Section 9. Fault Finding)
Consider and check all other possibilities before assuming an analyser fault. Analyser faults are rare, are usually
detected by the analyser itself (initiating an automatic data invalid state) and likely to produce very large errors.
When convinced that any discrepancy is due to an analyser problem contact CODEL or the local service agent
but first consult Section 9. Fault Finding to determine what information to provide.
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