The flame temperature detector measures the heat given off by a flame as it burns combustible gas that diffuses into the flame from the sample. 

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 flame diffusion give this detector the most uniform response factors for a very wide variety of combustible gases, making this detector the most direct measurement of flammability. 

Because the sample diffuses into the flame, there is no sample capillary. This makes the sampling system much more reliable than other types.

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