IRIS manual S511 VIEWING HEAD, S512 VIEWING HEAD, IRIS MODEL P522 APPLICATION MANUAL

IRIS MODEL P522 APPLICATION MANUAL

nals. The Si element is a photodiode that is oper- ated in the photovoltaic mode (the diode generates current when exposed to light or radiation). The sen- sitivity range of this device in this mode is quite large, from nanoamperes to milliamperes – so some means to prevent saturation in the first stage of amplifica- tion is required. (The PbS element, on the other hand, has a rather limited dynamic range exhibiting not more than a 15:1 ratio from dark to a bright signal.) In order to prevent saturation, an AGC type circuit is used, so that the DC component or “brightness” signal doesn’t saturate, causing a loss of the AC com- ponent or flicker signal. On a “bright” signal, this Si channel is limited, or compressed, so that there is a maximum signal reached that will not be exceeded, no matter how much brighter the signal is.

Another non-linear characteristic can be attributed to both the Si and PbS channels on “bright” signals exhibiting a large AC/DC component, such as an in- candescent light source. In this case, the amplifi- ers downstream of the first stage are in a clipping mode (i.e., approaching 120Hz squarewave) which, even though the signal is AC, has a limiting charac- teristic. This is indicated on the digital display with a count or signal strength of about 2900. (This is with the filter switch in the LL position; the display will read about 2100 with the filter switch in the H position.) Fortunately, flame signals do not exhibit this large AC/DC component, so the amplifiers downstream play a lesser role with this type of non- linearity (see section SIGNAL SOURCES).

However, if it is determined that there is a limiting action taking place (readings on the digital display of 2700 – 3000), and if this is interfering with the ON/ OFF ratio, causing a discrimination problem, it can be easily resolved by turning both the Si and PbS 25- turn POTs down. No effect will be noticed until about 10 turns counter clockwise, and we recommend 15 counter clockwise turns on the first try. The digital display must be used for this adjustment to detect when the signal pulls out of a limiting or compressing mode (i.e., when the signal goes below 2700).

S511 VIEWING HEAD

This viewing head uses a germanium photodiode that has a narrower spectral range (750 nm to 1900 nm)

than the lead sulfide cell used in the S509 and S512 viewing heads. It will detect all types of flames, including natural gas and propane.

Being a photodiode, it is operated in the photovol- taic mode – the same as the Si element in the S509. The same type AGC circuit is used, which is inher- ently nonlinear. If it appears that a limiting action needs correction, the same remedy can be used here as on the S509.

S512 VIEWING HEAD

The S512 viewing head employs a PbS-only photode- tector, and performs in a similar manner as the S509 two-colour model with the Si channel turned off (i.e., Si channel potentiometer turned full counter clockwise).

It has been our experience that, for many applica- tions using the S509, the Si channel is not used; usually, it is turned off. There are two reasons for this, the primary reason being the inherent nonlinearity caused by the AGC in the Si channel. The wide dynamic range of the Si photodetector re- quires control of the gain of the first amplifier stage. If this were not done, amplifier saturation of the sig- nal would occur on bright flame signals, resulting in the loss of the flicker signal.

The problem occurs because the AGC responds to the brightness of the flame.

For example: when a burner is on, the flame monitor is picking up a strong, bright signal from the root of the flame; when the burner goes out, the flame monitor is reading background flame from other burners. This background flame is less bright, causing the AGC cir- cuit to increase the gain of the amplifier with a consequential reduction in the dis- crimination ratio (the ratio between the count displayed with flame ON and the count displayed with flame OFF). The AGC nonlinearity is self-defeating, so to speak, and is the principle reason this Si element is usually turned off.

The other reason is that the Si photodetector will not see propane or natural gas. If this is the main fuel, then

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