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Tip of the Week
Winds of Change?
A recent call from an offshore technician prompted me to compose this newsletter, with the hopes that it will clear up calibration of gas detectors used in Air Conditioning Systems offshore.
After multiple failed calibration attempts, the technician assumed that the catalytic bead sensor was malfunctioning and decided to replace the sensor. After allowing burn-
The detector transmitter, was then replaced, and calibration was attempted again, and failed again. At that time I was contacted by the technician for assistance.
After a lengthy explanation of the problem and the steps he took, I asked him to shutdown the AC unit and try to calibrate. Success! But it had nothing to do with the parts he replaced earlier. The sensor works fine as long as the unit is not running, which is almost never.
This event brought up a few questions and issues related to detectors in AC Systems that I would like to share.
First of all, what was the detector doing there anyway?
If the confined space & AC unit were located in a hazardous area, then the AC unit should be classified and tagged as such. This also means that the AC unit must heat/cool re-
With the exception of my car, I have yet to see a AC unit that heats/cools air from the outside. If your AC unit has the ability to use air from the outside, then it is not rated for the area classification.
Secondly
Both catalytic bead & IR combustible detectors by design are heated for different reasons during normal operations.
For catalytic beads, before the diffusion process can take place to detect hydrocarbons, they must be at their designated operating temperature, which is one of the reasons we are required by the manufacturer to allow appropriate burn in time prior to calibration of a newly replaced sensor. It would be extremely difficult to maintain that operating temperature with air, specifically cold air, coming into contact with the active/reverence beads.
For IR combustible gas detectors, the housing is heated to maintain a constant temperature inside the detector to prevent condensation (fog) from developing on the optics and reflecting mirror inside the device. Imagine your safety glasses when you go from a hot to cold environment, this may also cause the IR detector to go into fault when the AC unit cycles on/off.
And finally, but probably the most important issue concerns wind volume.
Catalytic bead and IR detectors can not effectively sample air for hydrocarbons in high air volume areas, generally less 300 FPM, check with your system manufacturer for the maximum wind velocity specifications and limitations. It is similar to why you never see a smoke detector, used to detect a fire, inside of an AC duct system. We have a special device to detect smoke inside air conditioning systems known as a duct detector for that purpose. The method for detecting hydrocarbons in an AC system is similar. This is the reason why the above calibration attempts failed. If the detector will not calibrate with the AC unit on, how is it possible for it to detect gas with that type of air volume exposed to it?
Should there exist a requirement to monitor for hydrocarbons in a high air volume area, the manufacturers have accessories for their individual gas detectors.
The two that I am familiar with utilize an enclosure and sampling tube the extends into the full length of the AC duct, ensure that the holes of the sampling tube face into the wind direction. The other method consist of an aspirating pump that pulls a sample of the air from inside the duct system to the detector for sampling. Of the two, I would recommend the sampling tube, its less expensive and has no additional hardware/moving parts, which is one less item that can fail.
Also, ASH Detectors located in the AC System are in some way like Fire Detection Duct Detectors, they are of the variety of the seen and un-
If you require more information regarding the terms and explanations above, contact me by email or phone for assistance.
The above information is covered in the following Tec Trek class segments: Awareness Training (FG-
For more information regarding training schedules and course curriculum, please visit our Web Site at www.tectrek.org or Contact Todd Olivier at (337) 298-
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Email: todd@tectrek.org