he full oven temperature. Reaching full temperature usually takes a minimum of 15-30 s. Early hot box heaters were insufficiently hot to reach these higher temperatures e Method 5 equipment was designed to operate at 120oC rather than the higher atures now required.


Annual Guide 2019 I SOURCE TESTING ASSOCIATION


g is frequently conducted with inappropriately sized probes for the duct. This is due to e range of duct sizes encountered and the lack of a wide enough selection of probes contractor.


ulate Sampling Probes is frequently conducted with inappropriately sized probes for the duct. This is due to


ulate Sampling Probes


e range of duct sizes encountered and the lack of a wide enough selection of probes contractor.


r problem is when the day is very cold and/or there is a very cold wind, and the probe t box have not had sufficient time to heat the filter assembly thoroughly and allow it to he full oven temperature. Reaching full temperature usually takes a minimum of 15-30 s. Early hot box heaters were insufficiently hot to reach these higher temperatures e Method 5 equipment was designed to operate at 120oC rather than the higher atures now required.


box have not had sufficient time to heat the filter assembly thoroughly and allow it to


The Isokinetic Sample Train


So far, we have focussed on flow measurement because it is essential to obtaining a g isokinetic sample. However, the rest of the equipment is important too. The sample no must be sharp edged with minimal cross-sectional area walls, so the edge presented to flow does not disturb the flow lines. The shank of an appropriately sized drill should be u to ensure that the nozzle tip is perfectly round, and any nicks should be carefully filed. nozzle should then be measured at least three times using a digital Vernier to ensure the nozzle is uniformly round. The probe tip and the pitot tube should never be used measure a duct as this risks the equipment being rammed into the walls opposite and pitot or nozzle damaged. Don’t laugh, I’ve seen it done!


A Method 5 Super Probe


oblem when conducting sampling from very small ducts is the cross-sectional area of be, pitot


ance of the flow lines. In that eventuality a custom probe and pitot assembly may o be used.


ance of the flow lines. In that eventuality a custom probe and pitot assembly may be used.


oblem when conducting sampling from very small ducts is the cross-sectional area of obe, pitot


The problem when conducting sampling from very small ducts is the cross-sectional area of the probe, pitot tube and thermocouples, they can cause restriction to the fl ow and disturbance of the fl ow lines. In that eventuality a custom probe and pitot assembly may need to be used.


A Method 5 Super Probe A Method 5 Super Probe


tube and thermocouples, tube and thermocouples,


they can cause they can cause


restriction to the flow and restriction to the flow and


Schematic of a US EPA Method 5 Sample Train Schematic of a US EPA Method 5 Sample Train


Nozzle Selection


A Custom Nozzle, Pitot and Thermocouple for Small Ducts A Custom Nozzle, Pitot and Thermocouple for Small Ducts


A Custom Nozzle, Pitot and Thermocouple for Small Ducts


Once the flow has been determined (and checked for turbulent and non-axial flow), correct nozzle needs to be selected. What determines the nozzle size? Besides the flo the stack, there are other requirements to consider, that the sample flow rate through absorbers is appropriate and that the pump on the equipment can pull the required flow a the points to be sampled. The nozzle size can be affected where there is an absorber in train such as XAD-2 for Dioxins/Furans, PAHS or PCBs. As the sample is taken, the b


Nozzle Selection


In the case shown above the nozzle and pitot tube are mounted in the 100mm duct, and a Method 5 probe is used as a suction tube. However even in this confi guration the particulates in the probe will need to be washed out and the probe washings will need to be added into the particulates or chemical analysis.


Does an excessive probe length affect the sampling? Not necessarily, but it can make deployment awkward, leading to compromises being made. Where a contractor has a probe that is of insuffi cient length to sample a larger duct or is unwilling to use additional ports on the other side of the stack then it may completely invalidate the work. The use of an inappropriately sized probe can be prevented by inspecting the SSP, but it can sometimes occur where a project is handed over to another team working out of a different offi ce and not enough care is taken to review the contract adequately.


The Isokinetic Sample Train


So far, we have focussed on fl ow measurement because it is essential to obtaining a good isokinetic sample. However, the rest of the equipment is important too. The sample nozzle must be sharp edged with minimal cross-sectional area walls, so the edge presented to the fl ow does not disturb the fl ow lines. The shank of an appropriately sized drill should be used to ensure that the nozzle tip is perfectly round, and any nicks should be carefully fi led. The nozzle should then be measured at least three times using a digital Vernier to ensure that the nozzle is uniformly round. The probe tip and the pitot tube should never be used to measure a duct as this risks the equipment being rammed into the walls opposite and the pitot or nozzle damaged. Don’t laugh, I’ve seen it done!


Once the fl ow has been determined (and checked for turbulent and non-axial fl ow), the correct nozzle needs to be selected. What determines the nozzle size? Besides the fl ow in the stack, there are other requirements to consider, that the sample fl ow rate through the absorbers is appropriate and that the pump on the equipment can pull the required fl ow at all the points to be sampled. The nozzle size can be affected where there is an absorber in the train such as XAD-2 for Dioxins/Furans, PAHS or PCBs. As the sample is taken, the back pressure increases in the train and hence the vacuum increases. This can increase to a dangerously high vacuum and the glass can implode or it could exceed the initial leak check. It is then possible at this higher vacuum used during the test that the sample train leaks and the test is invalidated. To ensure this does not occur, it is best to choose a smaller nozzle when the ideal size is bracketed by a choice of two sizes. In this way the problems described will generally be avoided. This may require a longer sample to be taken but a more valid sample will normally be produced. A lower fl ow rate also normally improves absorption of the gases when sampling for gaseous species combined with particulates.


Equipment Care, Leak Tests and


Inappropriate Repairs The fi lter holder should always be inspected for cracks especially if it is a glass holder, and it is advisable to load fi lters in a clean area. It has been proven to be a good use of time to set up a vacuum manifold to leak test the fi lter assembly when it is loaded in a laboratory. This ensures that the fi lter can be tightened under vacuum prior to use and it then rarely leaks when tested as a full assembly on site. Also, loading fi lters in a laboratory is easier and less prone to contamination than on site. The holder will also need to be released under vacuum typically (this latter point does not apply to wingnut type fi lter housings).


7


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