The only way to ensure that a sample reaches the sensor is to employ active sampling. This is essential in our comparison of flammability analyzers and catalytic sensors for process applications. So let's focus on an important feature: failsafe performance.
Drawn-sample systems typically use a vacuum pump to pull vapors from the process and across the face of the sensor. For vapors with dew points above ambient temperature, the entire sample train must be heated to prevent drop-out or clogging due to condensation.
Even a drawn-sample system can not assure that catalytic sensors will measure the vapors correctly, or at all. Flame arrestors clogged by sample particulate or moisture will block passage of the sample to the sensing element.
Sensing elements which become poisoned, coated or depleted will not respond to sample vapors. Nor will they provide any indication that they have failed. Elements which are still active but have lost some sensitivity over time will produce false safe readings.
Flammability analyzers are fail-safe. A fault relay de-energizes whenever any of the following occur:
- controller electrical failure
- loss of system power
- loss of heat
- loss of flow through the flame cell
- downscale readings caused by loss of flame or fuel
The best analyzer design should be failsafe: it will provide malfunction alarm for any and all faults. For greatest safety, the malfunction alarms should shut down the process.