The following statements made about the operational conditions and restrictions of the catalytic bead sensor have been taken directly from the manufacturers operations manual.
HIGH GAS CONCENTRATION
In certain conditions the detectors may be required to operate in high gas concentrations for short periods. Test have shown that for periods up to about 2 minutes, 10 second bursts of 8%, 10%, and 80% methane air have had no adverse effect. Prolonged exposure can result in zero drift, which may be reversible by operation for a short period in air or low methane concentration (about 1%). Exposure to 40% concentration for longer periods will begin to destroy the detector surface altering the zero reading and reducing the sensitivity. Whenever an instrument is exposed to high concentrations of combustible gas the calibration should be rechecked as soon as possible.
POISONS AND INHIBITORS
The performance of the catalyst may be temporarily impaired by operation in the presence of substances, which are described as inhibitors. These are usually volatile substances containing halogens and the detectors may recover after a short period of operation in clean air.
Whenever the substance produces a permanent effect on the catalyst with a catastrophic reduction in sensitivity the detector is said to be poisoned; examples are silicone oils and greases, anti knock petrol additives, phosphate esters. Activated carbon filters will provide adequate protection from poisoning in the majority of cases.
Care should be taken in the choice of materials from which mounting blocks, gas sampling systems, pipe work, pumps, etc, are manufactured. Plastic materials or metals machined using cutting fluids may leave volatile residues trapped in the system. It is essential therefore that all component parts of a gas system are scrupulously clean and that plastic items do not operate at high temperatures so evolving inhibitors or poisoning agents.
SOME ADDITIONAL INFORMATION FROM CONSPEC ENGINEERS
A few examples of substances considered catalytic bead poison.
- Silicones: Silicone based grease, lubricants, floor wax
- Mercaptans: Used as odorant for natural gas
- Sulphur containing gas compounds: ex. Hydrogen Sulphide and Sulphur Dioxide
- Lead containing compounds: ex. Tetraethyl Lead