Innovate Motorsports ST-12 - Manual (Page 15)

15
must use thermocouple wire to connect the probe. If you do not, there will be an extra two-metal junction
where the Copper wire meets the Constantan wire of the probe. This extra junction will cause a large
error in the temperature readings.
Most Thermocouple probes are of the “grounded junction” type. This means that the “hot junction” is also
connected to the probe’s body. As this body is connected for example to the exhaust manifold, the
sensor wires are essentially grounded through that. The same is true if a home-made thermocouple
junction is used as described above by twisting the wires and if that wire-twist is connected to some
grounded engine part.
You can check if you have a grounded junction type by measuring between the probe body and one of
the Thermocouple wires. If you have continuity, you have a grounded junction.
5.2 Connecting an RPM signal
For RPM measurement you can either connect a tach signal to the CH1+ input or plug an inductive clamp
into the 3.5 mm stereo socket marked RPM. See chapter 6 for RPM measurement details.
5.2.1 Using the Variable Potentiometer
1. Turn the potentiometer all the way to the right. (until RPM light goes out, which is on the lower right-
hand side of the 7-segment)
2. Slowly turn the potentiometer to the left until the RPM light starts to flicker
3. Keep turning until the RPM light goes solidly on.
4. Turn the potentiometer an additional amount roughly equal to the distance between where the light 1st
came on and where it went solidly on.
5.2.2 RPM Measurement basics
Most RPM measurement methods use the ignition system of the car as a convenient source of RPM
dependent pulses. Other methods use a TDC sensor (one pulse per rotation), cam sensor, or fuel
injection pulses (number of pulses/rotation is dependent on the fuel-injection system). Some actually
measure the AC frequency created by the car's alternator.
Because the number of pulses per crank rotation is dependent on the ignition system and engine type, a
universal RPM measurement method must be adaptable to the different environments encountered. The
typical ignition system consists of an ignition coil, a coil driver that switches current to the coil on and off,
and a distributor. When current is switched on to the coil, the coil stores energy in its magnetic field.
When the current is switched off, that energy gets discharged at a very high voltage pulse on the coil’s
secondary winding, creating a spark.
A capacitive discharge ignition system (CDI) uses a capacitor to store the spark energy. The capacitor is
charged to about 400V and then rapidly discharged over the ignition coil's primary winding. The coil thus
only acts as transformer and does not store energy (and can therefore be smaller). The advantage of a
CDI system is a very high and fast rising spark voltage (less susceptible to spark fouling). The weakness
of the CDI system is the very short duration spark, which might not be long enough to ignite the mixture.
Multispark ignition systems try to overcome the inherent weakness by creating multiple spark pulses over
some degrees of crank rotation to increase the likelihood of igniting the mixture. The distributor switches
the spark voltage to the appropriate spark plug.