Calibration, Calibration

Gases and Regulators Regulators are crucial to obtaining and maintaining quality calibration gases to the analysers and to preserving their third- party certifi cation. To this end the amount of care needed very much depends on the levels of pollutant present, the emission limit to be met and the reactivity of the gases.

However, good practices will always stand one in good stead. The range to be calibrated and the pollutant level very much determine the calibration level required, and the SSP is a good place to start. If the pre-testing site investigation has been completed appropriately then the testers will have a good idea of what to expect. Experience of a wide range of processes is also useful for spotting peculiarities, as is of course, listening to your client. They will probably know more about their process than you! This latter point is getting more diffi cult though as now emission tests are often reduced to a “maintenance” task where the communication between the company and the tester may be more limited.

When emission testing was more research orientated then the information exchange was enhanced. Nonetheless asking questions of the client/tester should be encouraged from both sides. It is also worth establishing whether this is a routine compliance test or a special set of tests for research purposes. Over the past few years I have commissioned emissions trials for testing various devices and fuels for research purposes. In a number of cases, despite good communication with the business development leader, a team that was prepared for only routine compliance tests was deployed. A detailed SSP is usually impossible in research situations and hence written and verbal communication are crucial. Likewise, the report will need to be more fl exible and will often be more detailed than a few page summary of the results in respect to IED limits.

There is never a good reason for measuring above a calibration point and relying on the linearity of an analyser, even one with QAL 1 certifi cation, as it cannot be guaranteed to be accurate after it has been banged around in a van to site. Why else do you calibrate?

There are reasons why specifi c calibration points are key, and usually these are the compliance limit values. It is expected by regulators that there will be occasional spikes and the odd level will be higher, hence the requirement to have instruments certifi ed (QAL 1) higher than emission limit values set by the EU directives. However, these spikes are supposed to transitory and provided these are explainable the key calibration should encompass the data being collected. In an ideal sense, this should be 70-80% of the range (or certifi ed range) of the analyser, as this is often where manufacturers will have concentrated their efforts to ensure the best linearity. The bottom end of a range is always poorer and more subject to irregularities. The limit value only becomes of key interest if the emissions are right on the limit and hence the uncertainties of the measurement need to be factored into the results. Typically, the regulator will not prosecute where a limit value is breached but the limit is within the uncertainty of the measurement

(provided that this is a reasonable estimate). However, it does frequently result in the regulator taking more interest in your client, your data being scrutinised more carefully, and the client incurring additional costs reviewing data and sometimes engaging a third party to investigate the issues.

Calibration is about checking the zero level, the maximum level and ideally an intermediate level if possible. However, the calibration can be used to identify many other issues whilst monitoring on site, and it is often a good indicator that something is beginning to go wrong. It should be remembered that in this context calibration is very different to QAL 2 or QAL 3 checks on fi xed installations and although some of the principles apply the methodology may be very different.

Calibration falls into two broad areas: a) Checking of the monitoring system integrity b) Calibration of the equipment to provide calibrated data

Checking system integrity nearly always includes, or should include, a leak check. It is important that the check is performed sensibly and not designed by deed or accident to hide problems. As discussed earlier in the section on probes introduction of gases to the probe provides a method to investigate if any leaks are present; it is useful if there is a thorough understanding of the sample handling system to gain the best from these procedures.

If we consider a sampling system, the positioning of the sample pump can affect the type of leak check conducted. Any position upstream of a sample pump will inevitably leak air into the system and will dilute the sample, and this can be picked up by detecting oxygen when applying a zero gas, whereas on the downstream side of a pump the sample would be leaking out of the system. This may mean there is insuffi cient pressure of the sample to pass through fl ow devices in the analyser if there is a severe leak, but there won’t be any dilution occurring. However, the sample may still be compromised as different fl ows/ pressures to those used during calibration may be observed, and a leak check with zero gas (nitrogen) won’t show this leak. A check with a calibration gas properly applied should show it.

The latter fault is much more diffi cult to check for in simple systems. In complex CEMS analyser systems the sample fl ow may be checked later in the system and hence differences between the main sample fl ow and that delivered to the sample analyser manifold can be checked.

Why might a leak check gas up at the probe not show up issues? This is mainly due to over-pressurisation of the system (i.e. where the leak gas is effectively forced into the line rather than being sampled), and therefore only a slight overfl ow of gases must ever be used. If there is a problem with absorption of gases on to the walls of the lines, probes or fi lters due to contamination by dust, then using a high pressure may temporarily mask the problem by fully absorbing the active sites and then only the gas concentration expected will be observed. This will occur with lower fl ows too but a slow response of the analyser is a key indicator of adsorption/absorption problems being present. Slow response to gases can be due to many issues, sometimes a sticking control valve in the analyser, or a


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