ASH Replacement - Tec Trek Web Site

Tip of the Week

Purchasing many ASH Sensors?
Nothing lasts forever!


The above question has been asked often throughout the industry and can be best answered by evaluating the devices in questions and adopting measurements to prevent them from failing.  

Before we can answer the question, we first must address the problems of sensor failure, specifically catalytic bead sensors.  

For the most part, sensor failure occurs when the devices cannot be spanned to achieve the desired concentration of the span gas that we calibrate them with.  The reasons for this failure are many and we will try to cover the causes and hopefully provide you with steps to get more life out of the units.  

The manufactures of the sensors recognize the limitations of the units and that is why most sensors only come with a 12-24 month warranty.  Due to the fact that these units are powered up 24 hours a day 7 days a week, steps must be taken to ensure reliable function.  The 90-day calibration ritual was originally a manufactures requirement that was adopted by standards such as API and CFR.  This procedure is done to compensate for sensitivity loss that occurs over time.  Because of their operating criteria and method of detection, the beads on the sensor will not last forever.  

The method of detection is referred to as diffusion, which will cause the gas in question to actually burn on the active bead of the sensor.  To do this the beads must be heated during normal operations, which cause the sensor to operate at higher temperatures usually around 225-290 degrees F. Operation at this constant temperature over a period of time has an effect on the lifetime of the sensor.  But the biggest cause of failure to a bead sensor is caused by contamination.  

Contamination of the beads occur when a foreign substance comes into contact or coats the beads which reduce the amount of exposure the beads have with the hydrocarbon gas being detected.  The majority of the sensors that fail are located in the Quarters/Galley or Sleeper Buildings.  This trend confuses people because of the fact that these areas are not exposed to the outside environment and are for the most part considered clean.  What they don’t know is that the methods that are used to keep these areas clean is the prime cause of sensor damage.  Cleaning agents that contain any type of chlorine or bleach will cause the sensors to become less sensitive.  The cleaning agents will coat or dissolve the catalyst coating on the active bead causing the sensor not to adequately measure the amount of hydrocarbon gas present.  Other chemicals such as Silicone will also reduce sensitivity.  

Another way that bead sensors fail is caused by calibration and exposure to high concentrations of hydrocarbon gas.  As gas is exposed to bead sensors, the temperature of the bead increases which is how we measure concentration levels.  The additional heat  produced will cause physical changes on the connecting wires on the bead, the continual heating and cooling of the bead over time will cause the wire to become bridle and break.  This is why we use a low concentration of calibration gas; we do not want to expose the bead to a higher concentration than is necessary.  If high concentrations are exposed to the sensor for any extended period, the sensor will quickly come to its end.  The higher the gas concentration, the more heat is produced reducing the life of the sensor.

Here are some helpful hints:
Do not use higher concentrations of calibration gas, use the manufactures recommended mix of gas.
While cleaning the Quarters/Sleeper, temporarily place a clean, contaminant free plastic bag over the sensor until the cleaning agent has dissipated. Do not forget to remove the bag; you may want to put the affected sensors in bypass as a reminder.
Avoid getting the sensors wet, the rapid decrease in operating temperature will fracture the bead wiring.
Avoid exposing the sensors to high gas concentrations for extended periods to perform a function test.
Use the appropriate regulator on the span gas during calibration and testing, generally 1.5-2.0 LPM. Check with manufacture recommendations on your particular unit.
Keep replacement spare sensors in a sealed bag.


With the cost of a catalytic bead filament ranging from 105.00-185.00 and the entire combustible sensor assembly costing about 180.00-500.00, the cost of replacement gets expensive.  

IR (Infrared) Combustible Detectors are not subject to contamination related issues.

For Technicians, Production Operators and Inspection Personnel:  
Please keep the following information in mind; bead sensors do not have the ability to give indication that they have been contaminated. Contamination will not be known until it is time to calibrate the unit and by then it may be to late.  Poison resistant sensors are available for most models, and if you want the ultimate in contamination free operation you may want to consider point IR detectors.  These IR detectors are immune to poisoning and will last a long time.

Always remember to purge your sensor with zero air prior to setting the zero function, this will prevent you from setting a false zero due to ambient gas in the area.


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-01A), Basic Technical Training (FG-01)

For more information regarding training schedules and course curriculum, please visit our Web Site at www.tectrek.org

or Contact Todd Olivier at (337) 298-7959.

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Email: todd@tectrek.org