Purple Peeper

Some fired heaters, especially boilers, have a device called a purple peeper, which is simply an optical device that looks at a flame. If it does not detect light with a wavelength in the high-frequency (i.e., purple) end of the optical scale, it interprets this as a flame-out. The fuel-gas regulator is automatically shut. 

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See also in sourсe #XX -- [ ]

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