The graph below shows the moisture content of air versus its dewpoint. At a dewpoint temperature of 4ºC the moisture content is around 0.7-0.8% (v/v) for air.

Various designs are available to improve the effi ciency of a chiller and one enhancement is to use a two-stage chiller with the sample pump between the chiller compartments. This results in gross amounts of water being removed in chamber one and an enhanced dewpoint being achievable on the second chamber giving more uniform moisture removal across a range of different sites.

For some applications, semi-permeable membrane driers such as Nafi on membranes in PermapureTM

driers are sometimes

preferred but these suffer problems too, as the membranes can become clogged with microscopic dust and their effi ciency can vary depending on a temperature gradient down the membrane bundle. The ambient temperature if too low can also affect this temperature gradient and lead to unacceptable condensation as the gases enter the bundle. The dewpoint of the counter current drying gas can have an effect too. The driers can achieve very low dewpoints of the gases being dried, down to -60ºC to -70oC, but these dewpoints can be variable and hence constant dewpoints are not always achieved. This is unlikely to be a problem in routine applications.

Semi-permeable membrane driers can also be affected by hydrocarbons. Most of these problems can be engineered out by multiple levels of fi ltration and/or heating the inlet side of the drier to prevent condensation. These techniques have the signifi cant advantage of having no moving parts and are simple lightweight compact devices, but they do require a source of air or a vacuum to operate the counter current drying stream.

Dilution probes and dilution benches are generally unpopular in the UK, but are used extensively in the USA and in Germany. The dilution approach results in the diluting of the gas stream with a known amount of pure air and hence reduces the dewpoint of the gases such that condensation will not occur. In a dilution probe, this occurs in the probe system and typically uses unheated lines to bring the gases down to the analyser system, whilst with a dilution bench a more traditional probe and heated line are used and the bench is used to alter the characteristics of the gas stream such that the gas is effectively non-condensing. This has the advantage of allowing gases at low levels to be analysed using a drier system prior to the dilution bench whilst gases at high levels can be diluted and analysed, yielding a system which has a wide dynamic range.


The dilution probe system has the advantage of not requiring heated lines and hence is a lighter and more convenient sampling arrangement, but dilution probes contain glass components and are not suitable for temperature applications above 350ºC. There are some special probes that can be used up to 600ºC but these are usually expensive and made to order. Dilution probes are unsuitable when some components are present at a low level and others at high levels unless used in conjunction with other systems.

Both the dilution probe and bench are ideal for high level process emissions if combined with normal stack gas analysers or with lower levels they can be used with ambient air analysers. Dilution ratios of 10:1 to 400:1 are possible for dilution probes, whilst a dilution bench can be operated over a range of 10:1 to 200:1 off the shelf. The dilution bench and probe both potentially suffer issues of calibration and require a very pure gas stream to be used for the dilution as any impurities in the dilution gas will be seen in the analysis and will give erroneous results. In a system we had operating in the UK, the gas diluent stream required a high performance molecular sieve clean up system (used by the diving industry) to remove carbon monoxide from the diluent gas stream, or else this affected the results especially at 100:1 dilution and with ambient air levels of typically 1-5ppm. It shows that all problems can be solved but fi rst you need a good understanding of the problem.

The other issue which is commonly ignored is that the results from a dilution probe/bench are given on a wet gas basis and these needs to be corrected for in the subsequent data processing stage. Any mistakes in calibration will be magnifi ed by the dilution applied and hence much greater care in operation is required in the use of this equipment.

Connecting The Equipment Together and Moving the Heated Lines

Connecting the equipment together is typically another source of aggravation and can result in lost time on site. SwagelokTM run courses on how to make a perfect joint for those using their connectors. Now with the advent of YouTube you can watch

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