testing I always attempt to be present during the testing phase.

2. Set-up time: Where is the site and how long will it take to get there? Once there how long is left to start? Is a site briefi ng or induction possible that day, and is any set up that day possible? Limited setup is often possible if there is a work area in a van/laboratory etc., prior to the detailed induction if required. Anything that can be done today makes time for the unexpected tomorrow!

3. Do we have all the necessary safety kit, PPE, H&S forms and are they fi t for purpose?

4. How long is the job and have we planned in any contingencies which might extend the project? If asked to undertake further work by the client, could we do it whilst on our visit? This very much gives additional profi t to a project. However, safety aspects need to be considered and the SSP updated in that eventuality. Can we accommodate issues and vary our deployment when the inevitable happens?

5. How high is the stack, is everyone happy at that height and is the area safe to work for both our staff and the client’s staff? Does the team or the site have the necessary barriers/cones/ tape to secure the area? It is essential to ensure that moving traffi c is diverted away from access points to the stack and from under the shadow of the stack where objects could fall.

6. How do we get equipment to the sample port? What equipment needs to be on the stack or at a remote location with connections to the equipment at the stack sample port? How will we communicate effectively if the team is split between the sample port and the analysis equipment? Shouting and smoke signals are ineffective and polluting, whilst mobile phones suffer poor reception and are often banned from site.

7. Where is the electricity? What power ratings are available and is all the required equipment 110V, or battery operated? Is all the kit PAT tested and fi t for use and does it pass a visual inspection with respect to leads and connectors, i.e. are wire clamps on the outer cable sheath or on the inner insulated cables etc? Look for any damaged cables, broken plugs etc.; do not assume that because it is PAT tested it is still fi t for purpose!

8. If the work is to be conducted in buildings or in hot countries then the temperature limits of the equipment need to be considered. Sometimes the problem is not the excessive heat, but rather the change in temperature which leads to unacceptable drift and variations in performance. This is often the case in the use of CEMS equipment, especially that which has been certifi ed to QAL 1 or MCERTS, as the temperature limits are specifi ed over specifi c ranges. It is also expected by manufacturers that the equipment temperature will be kept relatively constant.

Duct Being Measured Probe being Used

Gas and Particulate Sampling Probes

The majority of testing is conducted with inappropriately sized probes for the duct. This is partially due to the wide range of duct sizes encountered, both for isokinetic and gas analysis and the lack of a wide enough selection of probes by the contractor. For Instance, it is ridiculous to use a 2.5m long probe whilst sampling a 100mm duct. Of course, one wouldn’t necessarily expect the probe to be the perfect size but 2.5m is just silly!

Does an excessive probe length affect the sampling? Not necessarily, but it can make deployment awkward and it can lead to compromises being made. However, the opposite can also be true, where a contractor has a probe which is of insuffi cient length to adequately sample a larger duct, or is unwilling to use additional ports on the other side of the stack. This short probe scenario clearly has an impact on the sampling and may completely invalidate the work depending on the scope. An inappropriately sized probe should never be deployed if suffi cient care is taken over inspecting the SSP, but it can occur where a project is sometimes handed over to another team working out of a different offi ce and not enough care is taken to review the contract adequately.

In extractive gas sampling the same things happen, often there is not suffi cient thought given to the probe arrangement to be used with a heated line. Probes very often fall into several varieties:

a) A simple open tube without heating connected to a heated fi lter. b) A heated probe tube connected to a heated fi lter. c) A sintered tipped probe connected to a heated line. d) A multi-holed mixing probe.

Which probe is used often depends on the process, and/or the species being monitored, but unfortunately often the probe is chosen simply by what falls to hand.

Filtration is always required as there are exceptionally few plants which have a process with zero dust. A simple open tube on an ambient air temperature duct is fi ne, but still requires a fi lter to protect the sample line. Failure to do this results in the contamination of the sample/heated line. Then when more complex species are monitored there is a perfect absorption medium (i.e. the fi ne dust) in the sample/heated line ready to absorb the more reactive components.

Simple sintered tip probes or open tube probes can be partially withdrawn from the stack to drop the temperature of the gases being sampled or a gas cooling spool piece added between the probe prior to reaching a fi lter housing, etc. so that the fi lter assembly is not damaged by excessively hot gases. It is always worth calculating or measuring the temperature drop to the fi lter especially if the fi lter is PTFE, or not specifi ed much above 180ºC. Overheating a fi lter can result in damage to the housing and unacceptable leaks and these can result in the sampling being voided, whilst too much cooling risks condensation.

Sintered probes also have a maximum temperature and need to be operated according to the manufacturer’s specifi cations. Likewise, it is dependent on the process and the wall thickness of the stack whether a heated sheath or a cooling spool piece is required.


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