AERCO KC Series Equipped with C-More Controller for Massachusetts Only - Manual (Page 62)

6.13 FLAME STRENGTH MEASURMENT
The flame sensor is a very important safety device. Its function is to sense the absence of a flame (over a
certain amount of time) and so shut off the flow of gas to avoid the build up of unburned fuel within the
burner assembly.
Defining Flame Strength
A flame sensor works by alternating a very small electrical DC signal through the flame sensor rod and the
flame base (ground). Electrical ions travel from the base, through the flame itself, and into the sensor
completing an electrical circuit. When the flame is extinguished, there is a disruption of this electrical
circuit, which the controller senses and so initiates shutdown procedures. This current is referred to by
AERCO as the flame strength (also known as flame current or flame signal).
CAUTION!
There is a very low AC current used to energise the flame sensor when power is applied to the system
(should be between 90 and 110 volts), so touching the energized sensor may cause a mild electric shock.
Determining Flame Strength
Reading the flame strength through the C-More Display is not a reliable way to determine if there is
acceptable flame strength. Only by using a Digital Multimeter sensitive enough to measure current in
microamps ( µA ) can the flame strength be properly measured. The meter must at least be capable of
measuring between 0.1 µA and 10.0 µA of DC current with a minimum measurement resolution of 0.1 µA.
Typically, the operating range of the flame strength will be between 3.4 µA and 12.0 µA. Anything below
the minimum indicates insufficient flame strength. A reading of 2 µA or less probably indicates dirty rods or
calibration issues and requires investigation. A reading of more than 8.5 µA is acceptable and should not
cause concern.
NOTE:
Ensure that the multimeter probes are in the common (ground) and µA connections and that the
multimeter fuse is not burned out.
Instructions for Measuring Flame Strength
Disconnect the flame rod connector from the flame rod. Next, connect the multimeter probes in series
between the Flame Rod and the Flame Rod Connector as shown in Figure 6.21. Polarity is not a concern
in this measurement, so you may connect the probes without regard to polarity and ignore whether the
measurement indicates a positive or negative reading.
Figure 6.21 – Connecting the Multimeter to Measure Flame strength