IRIS manual IRIS MODEL P522 APPLICATION MANUAL, Page

IRIS MODEL P522 APPLICATION MANUAL

you should definitely turn off the Si channel, which cannot contribute to the flame signal from the burner (when it is gas) but can only read background glow – which will certainly cause a discrimination problem.

An important aspect of using the S512 instead of the S509 has to do with linearity. The non-linearity of the Si channel due to the AGC circuit has already been explained. This AGC is used upstream of the 25-turn potentiometer, so you have no control over this action except for the viewing head sighting, or for using orificing to change the amount of signal reaching the photodetector. On the other hand, the PbS channel upstream from its potentiometer is lin- ear for low to medium brightness flames (i.e., no distortion of the flame signal occurs with regard to amplifier saturation (signal clipping) or nonlinear amplification (AGC) as with the Si channel).

However, saturation will occur with the PbS channel on high brightness flames. You can detect this by observing the digital display reading for boiler load conditions as they vary from low to high. If there is not much of a signal increase between low load and high load, then the PbS detector is saturating.

For example: if the burner flame signal on oil reads 1950 under low load condi- tions, and reads 2000 on high load con- ditions, you can be sure that the PbS de- tector is saturating.

To resolve this condition, reduce the signal bright- ness by orificing (see next section ORIFICING). The orifice disc (installed in the flange) is located immediately in front of the lens and will reduce the amount of signal radiation the same way a camera aperture reduces the amount of light.

As mentioned in the previous section on the S509, saturation can occur downstream from the gain ad- justment potentiometers because of the signal ampli- fication process. This type of saturation can be de- tected on the digital display by observing the signal count. The S509 will be completely saturated at a count of 2800 – 3000. Saturation begins (waveform just starts to clip) at a count of 2100. This threshold is somewhat dependent upon the flicker frequencies and their distribution, but for the lower frequencies (up to

200 Hz) the count of 2100 can be taken as the thresh- old. This amplifier type saturation usually will not occur with the S512 viewing head when reading ac- tual flames. It can occur when reading artificial sig- nal sources, such as incandescent and fluorescent lamps, because of the large AC/DC component. It should be noted that amplifier type saturation is not harmful per se, but saturation of the photodetector caused by high brightness flames can be.

You cannot control the nonlinearity caused by the AGC action of the Si channel on the S509, but with the Si channel turned off you can control the linear- ity of the PbS channel by observing the count on the digital display, and reducing the gain when neces- sary by orificing. Saturation of the PbS detector can only be corrected by orificing; turning down the gain adjustment potentiometer will not help, because it is the PbS cell upstream that is the cause.

Another nonlinearity of the PbS detector that should be noted has to do with the signal sensitivity and temperature. The PbS-type detector becomes less sensitive with increasing temperature.

For example: the sensitivity or signal will drop to one half with an increase in tem- perature from 25°C to 50°C. A further increase in sensitivity will occur above 50°C. This is the reason for the 60°C operating temperature limitation on the viewing head. This characteristic is in- herent in all PbS detectors, but should not cause a problem with respect to flame monitoring and discrimination. The sig- nal level will be stable once a tempera- ture equilibrium has been reached.

This sensitivity-versus-temperature characteristic is another reason why the PbS cell should not be ex- posed to high brightness flames. The radiation of the flame is focused on the PbS element, causing its tem- perature to rise, reducing the displayed count. This reduction in output with increased flame intensity is another form of saturation. Also, the exposure to el- evated temperatures over a period of time causes an aging process resulting in a change of the dark resis- tance of the PbS element, and a loss of sensitivity.

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