ECM pmCAN Module Instruction Manual page 11

Figure 2 contains a more descriptive diagram of the sensor and the external measurement
electronics. Note from the figure the following:
1. The dendrites that look like black fur in the sensor. Broken off pieces of dendrites will
carry charge across the electrodes (resulting in "IST" current). These pieces may remain
on the other electrode, or trigger another dendrite break and charge transfer, or may exit the
sensor. PM entering the sensor will contribute to the regrowth of broken and lost
dendrites.
2. There are three currents measured and reported: IST, ILK, and ITP. IST is the current that
is used to determine the density of PM in the exhaust. ILK is a leakage current from the
shield surrounding the lead to the high voltage electrode. ILK has a steady-state value but
will sometimes increase when the sensor is getting fouled (by PM or liquid). ITP is the
current used to create the 1000V measured on the input side of the transformer. ITP also
has a steady-state value and will increase when the sensor is getting fouled.
3. There are two separate grounds: the one shown as a triangle and the one that is hatched.
The triangular one is used for the IST and ILK circuitry. The hatched ground is the ground
of the power supply used to power pmCAN. Actually, there is a third separate ground, the
ground for the CAN output.
4. An electrical connection between the exhaust pipe the sensor is mounted in and the
ground of the power supply powering pmCAN is highly recommended because it often
lowers the background noise on the IST (and hence PM) measurement. This connection is
often the case with most vehicular applications, however, if the PM sensor is
mounted in a piece of exhaust pipe attached with a piece of silicon hose to the end of the
vehicle's tailpipe, you will have to run some braided line from the piece of
exhaust pipe to the vehicle's exhaust. In some dynamometer applications, pmCAN will be
powered by an external power supply. In these cases, you will have to run a piece of
braided line from the exhaust pipe the PM sensor is mounted in to the negative terminal of
the power supply powering pmCAN. When running on a set of chassis rolls, always
ground the vehicle and dynamometer to earth ground. (i.e. bleed off static electricity).
5. In some cases, connecting the negative terminal of pmCAN's power supply to pmCAN's
metal enclosure via a braided line will lower background noise on the IST measurement.
Since pmCAN's enclosure is anodized (and hence virtually non-conductive), you will have
to connect to a screw inserted into one of the two non-anodized screw holes on the back of
pmCAN in order to connect to pmCAN's metal enclosure.
6. PM can be positively, negatively, or neutrally charged. This is represented in Figure 2 by
the circles with "+", "-", or "N" in them inside the exhaust pipe and sensor.
7. There is some parasitic coupling through the circuitry creating the 1000V. This will cause
some coupling between the triangular and hatched grounds.
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