I.R.I.S. FLAME MONITORING SYSTEM P522 - Manual (Page 21)

IRIS MODEL P522 APPLICATION MANUAL
Page 21
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