Most flammable vapor analyzers respond differently to different vapors. Whenever the process solvent is changed, the analyzer must be either recalibrated or reprogrammed to ensure that it’s measurement of the new solvent vapor is still accurate. This creates a challenge when trying to measure a mixture of solvent vapors, especially when using narrow-banded infrared sensors. Now let's compare FTA vs IR for process applications.
Flammability Analyzers have the unique ability to measure most common process solvent vapors, including mixtures, to within a few percent of their lower flammable limit, without recalibrating.
It measures the amount of heat given off by a pilot flame as it burns in an explosion-proof measuring chamber. The small, well-regulated flame heats the tip of a temperature sensor suspended directly above it. The signal produced by the sensor when no flammable vapors are present drives the LFL indicator up to 0% LFL. This failsafe technique is known as a "live" zero because a weakening or loss of flame caused by lack of fuel will generate a downscale malfunction alarm.
- When a flammable sample is drawn into the measurement chamber it is seen by the pilot flame as an additional source of fuel. This causes the temperature in the area of the pilot flame to increase. Since the meter knows that the increased temperature can only be caused by added fuel (the sample), it rises above zero in direct proportion to the flammability of the sample.
- The dynamics of the flame temperature analyzer give it highly uniform response factors for a wide variety of combustible gases.
Combustible gases absorb infrared radiation at certain characteristic wavelengths. A typical non-dispersive infrared (NDIR) detector passes a source of infrared energy through the sample and measures the energy received by one of two detectors. The active detector responds to wavelengths in the same band as the sample gas, while the other detector measures a reference to compensate for changes within the instrument.
When specific combustible gases are present, they absorb some of the infrared energy and produce a signal in the active detector relative to the reference detector. Energy absorbed by the combustible gas for a given wavelength varies exponentially with the particular gas's absorptivity, the concentration, and the path length. This means that infrared detectors must be specifically calibrated for a particular gas, and can have very high variations in response factors and linearity for other gases.
- Infrared detectors are usually limited to detecting a single combustible gas. Like catalytic-bead sensors, they are best suited to area monitoring applications.