IRIS P522 manual Viewing Head Angle of View

IRIS MODEL P522 APPLICATION MANUAL

Signal sources other than flame, such as incandes- cent or fluorescent light, can cause false readings that should be understood.

For example: fluorescent light is nonsinusoidal in character, meaning that there is a sharp peak when the fluores- cent tube fires on the positive AC power cycle with a less amplitude excursion on the negative cycle, resulting in a radical voltage swing in the amplifiers employed in the viewing head. This sharp peak causes the first-stage amplifiers (AGC circuit for photovoltaic photodetectors) to react violently, resulting in down- stream signal perturbations that are not normal. This can be even worse if a strobe light is used as a signal source.

An incandescent light source is sinusoidal in char- acter, resulting in a less “disturbing” signal for the viewing head, but still does not resemble a true flame source because of the large AC/DC component present. The element in an incandescent light pro- duces radiation in step with the AC power cycle, ef- fectively turning on and off 120 times per second. There is a certain amount of thermal inertia, so that the element doesn’t turn off completely, but the large ratio of ON to OFF (AC/DC) still prevails. A more realistic, simulated signal source can be created by employing an AC to DC power supply with super- imposed ripple feeding an incandescent lamp. If the 120 Hz AC ripple is about 10 per cent, then this sig- nal source can be used to more closely resemble a flame source – keeping in mind that the simulated flicker is a single frequency of 120 Hz.

Consideration should be given to the effects of pre- senting a single-frequency, simulated flame source to the downstream, four-position filter.

For example: if an incandescent light source is used, powered by the 60 Hz line, switching from LL to L position will not affect the 120 Hz signal at all, because of the cutoff frequency of 36 Hz and 71 Hz respectively, providing the downstream stages of amplification are not saturated (sinusoidal 120 Hz signal amplified to re-

semble a squarewave caused by waveform clipping of the operational amplifiers). If this signal is saturated, then switching from LL to L may indeed cause a higher reading in the signal processor, caused by the squarewave type waveform presented to the filter. For this reason, it is impor- tant not to judge the filter performance when using this type of signal source.

In general, it is appropriate to use signal sources pow- ered by the 60 Hz line for viewing head cursory check- ing, but obviously not for definitive performance. No detrimental effects will result from using signals that saturate the viewing head amplifiers.

VIEWING HEAD - ANGLE OF VIEW

The angle of view of the IRIS S509 viewing head has been specified at 2.8 degrees. A description of what this means is given in this manual (see 2nd paragraph, section DETERMINING SIGHT PIPE SIZE).

The angle of view is a function of the focal length of the lens and the size of the photodetector element. The equation for angle of view of a camera lens is:

2F tan θ/2 = d

where F = effective focal length of lens

θ= angle of view

d = diagonal of negative so that the angle θ is:

θ= 2 tan-1d/2F

The diagonal of the negative is used for photographic reasons related to the size of the negative, but a bet- ter measure for the purposes of this explanation would be the diameter of a circle. The photodetec- tor element used for the S509 is 2.0 x 2.5 mm, which is nearly square, and the longer side is used for the angle-of-view calculation (2.8 degrees).

The PbS element used in the S512 is 2.0 x 2.0 mm square. If you take the area of this element (4 mm2) and find a circle of the same area, the diameter will be 2.2568 mm. This was derived as follows:

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